Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005739 mitochondrion	IBA GO_REF:0000033	ACCEPT	Summary: Phylogenetic (IBA) assertion that OTC is active in mitochondria. Correct but less specific than the well-supported mitochondrial matrix localization; retained as accurate parent-level localization. Reason: OTC is a mitochondrial-matrix enzyme; the mitochondrion is the correct compartment at this level. The more specific matrix term is captured separately (GO:0005759).
GO:0006526 L-arginine biosynthetic process	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: IBA-transferred role in L-arginine biosynthesis. OTC produces citrulline, which is a precursor for argininosuccinate and thence arginine; the urea cycle is the biosynthetic route to arginine. This is a downstream pathway role rather than OTC's direct biosynthetic product. Reason: OTC's direct product is L-citrulline, not L-arginine; arginine biosynthesis is a downstream consequence of the urea cycle in which OTC participates. Biologically defensible at the pathway level but not the core function of the enzyme.
GO:0019240 L-citrulline biosynthetic process	IBA GO_REF:0000033	ACCEPT	Summary: IBA assertion that OTC is involved in L-citrulline biosynthesis. This is exactly the direct product of the OTC reaction and is strongly supported experimentally and by the UniProt pathway annotation. Reason: L-citrulline is the direct product of the reaction OTC catalyzes; UniProt records "Nitrogen metabolism; urea cycle; L-citrulline from L-ornithine and carbamoyl phosphate: step 1/1." Core biological process. Supporting Evidence: PMID:2556444 An arginine to glutamine mutation in residue 109 of human ornithine transcarbamylase completely abolishes enzymatic activity in Cos1 cells.
GO:0004585 ornithine carbamoyltransferase activity	IBA GO_REF:0000033	ACCEPT	Summary: Phylogenetic (IBA) assignment of the diagnostic molecular function. Concordant with experimental IDA/EXP annotations and UniProt catalytic activity. Core function. Reason: This is the defining enzymatic activity of OTC (EC 2.1.3.3), supported by multiple experimental annotations and UniProt.
GO:0004585 ornithine carbamoyltransferase activity	IEA GO_REF:0000120	ACCEPT	Summary: Automated (IEA, multi-method) assignment of the core ornithine carbamoyltransferase activity, backed by EC 2.1.3.3 / RHEA:19513 and ARBA. Consistent with the experimental core. Reason: Correct core molecular function; redundant with the experimental and IBA annotations of the same term.
GO:0005739 mitochondrion	IEA GO_REF:0000117	ACCEPT	Summary: ARBA machine-learning electronic annotation to mitochondrion. Correct compartment at the organelle level. Reason: OTC is a mitochondrial protein; the more specific matrix location is captured by GO:0005759.
GO:0005759 mitochondrial matrix	IEA GO_REF:0000044	ACCEPT	Summary: UniProt SubCell mapping (SL-0170) to mitochondrial matrix, matching the experimentally established localization of the mature enzyme. Reason: Mitochondrial matrix is the correct, experimentally supported location of mature OTC. Supporting Evidence: PMID:6372096 We have deduced the complete primary structure of the precursor of a human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC)
GO:0006520 amino acid metabolic process	IEA GO_REF:0000002	KEEP AS NON CORE	Summary: InterPro2GO electronic annotation to the broad amino acid metabolic process. Correct but very general; the specific urea-cycle and citrulline/arginine biosynthesis terms convey the actual role. Reason: True at a high level (OTC metabolizes ornithine and produces citrulline) but too general to be a core annotation; more specific terms are present.
GO:0016597 amino acid binding	IEA GO_REF:0000120	KEEP AS NON CORE	Summary: Electronic annotation for amino acid binding, reflecting OTC's binding of the amino acid substrate L-ornithine within catalysis. Subsumed by the catalytic molecular function. Reason: Ornithine binding is part of the catalytic mechanism captured by GO:0004585; substrate binding is not an independent core function.
GO:0016743 carboxyl- or carbamoyltransferase activity	IEA GO_REF:0000002	KEEP AS NON CORE	Summary: InterPro2GO annotation to the parent carbamoyltransferase activity class. Correct but less specific than the exact GO:0004585. Reason: This is a parent of the precise ornithine carbamoyltransferase activity term already annotated; retained as an accurate but general classification.
GO:0000050 urea cycle	IEA GO_REF:0000120	ACCEPT	Summary: Automated annotation placing OTC in the urea cycle, consistent with the experimental IDA annotation and UniProt pathway. Core process. Reason: OTC catalyzes the citrulline-forming step of the urea cycle; central core process.
GO:0001889 liver development	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer (from rat OTC, P00481) of a liver-development role. OTC is highly expressed in liver but there is no evidence it functions in liver morphogenesis/development; this is an over-annotation conflating tissue of expression with a developmental role. Reason: Tissue-restricted metabolic expression (liver, intestinal mucosa) is not evidence of participation in organ development. Transferred by Ensembl Compara from a paralog/ortholog experiment; not appropriate as a human OTC function.
GO:0005543 phospholipid binding	IEA GO_REF:0000107	REMOVE	Summary: Ensembl Compara ortholog-transfer annotation of phospholipid binding. OTC is a soluble matrix enzyme with no established phospholipid-binding function; not supported by structure or biochemistry. Reason: No evidence that human OTC binds phospholipids; a soluble matrix carbamoyltransferase. Electronic ortholog transfer without biological support.
GO:0005743 mitochondrial inner membrane	IEA GO_REF:0000107	REMOVE	Summary: Ensembl Compara ortholog-transfer localizing OTC to the mitochondrial inner membrane. This is the wrong compartment: mature OTC is a soluble matrix enzyme, not an inner-membrane protein. Reason: UniProt and experimental data place mature OTC in the mitochondrial matrix, not the inner membrane. The inner-membrane assignment is an incorrect electronic transfer. Supporting Evidence: PMID:6372096 We have deduced the complete primary structure of the precursor of a human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC)
GO:0007494 midgut development	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer of a midgut-development role. OTC is expressed in intestinal mucosa but is not known to drive midgut development; over-annotation conflating expression site with development. Reason: Expression in intestinal mucosa does not equate to a role in midgut morphogenesis; transferred electronically without biological support for human OTC.
GO:0009410 response to xenobiotic stimulus	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer (rat) of a "response to xenobiotic" process. Not a described function of human OTC; likely reflects a rat-specific expression-response experiment. Reason: No evidence for human OTC participating in a xenobiotic response as a core or peripheral function; electronic transfer from a rodent stimulus study.
GO:0010043 response to zinc ion	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer of a zinc-response process. Not an established human OTC function; over-annotation from rodent data. Reason: No biological support that human OTC functions in a response to zinc ion; transferred electronically.
GO:0019240 L-citrulline biosynthetic process	IEA GO_REF:0000120	ACCEPT	Summary: Automated (multi-method) annotation of L-citrulline biosynthesis, matching the direct product of OTC and the experimental annotations of the same term. Core process. Reason: Redundant with the experimental IDA and IBA annotations; correctly captures OTC's direct biosynthetic product.
GO:0031667 response to nutrient levels	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer of a "response to nutrient levels" process. While OTC activity is modulated by Lys-88 acetylation in response to nutrient signals (UniProt PTM), this ortholog-transferred process annotation is a peripheral, non-core physiological response rather than a demonstrated human OTC function at this term. Reason: Electronic ortholog transfer; any nutrient-responsive regulation of OTC is an upstream modulation of its activity, not a core process the enzyme executes. Not appropriate as a core annotation.
GO:0032868 response to insulin	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer of an insulin-response process. Not a described function of human OTC; over-annotation from rodent data. Reason: No biological support that human OTC participates in a response to insulin as a gene function; electronic transfer.
GO:0042301 phosphate ion binding	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer of phosphate ion binding. Inorganic phosphate is a product of the OTC reaction, so transient phosphate interaction occurs within catalysis, but as an independent molecular function this is a peripheral, over-annotated activity. Reason: Phosphate handling is part of the catalytic mechanism captured by GO:0004585; standalone "phosphate ion binding" transferred by Ensembl Compara is not a distinct core function.
GO:0042802 identical protein binding	IEA GO_REF:0000107	MODIFY	Summary: Ensembl Compara ortholog-transfer of identical protein binding. This reflects the biologically real fact that active OTC is a homotrimer; "identical protein binding" is an uninformative surrogate for the self-assembly / homotrimerization role. Reason: UniProt records the subunit structure as "Homotrimer"; the annotation should point to the structural/self-association role rather than generic identical protein binding. Non-core relative to catalysis. Proposed replacements: protein-containing complex binding
GO:0055081 monoatomic anion homeostasis	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer of a monoatomic anion homeostasis process. Not an established human OTC function; over-annotation from rodent data. Reason: No biological support that human OTC maintains monoatomic anion homeostasis; electronic ortholog transfer.
GO:0070781 response to biotin	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Ensembl Compara ortholog-transfer of a biotin-response process. Not a described function of human OTC; over-annotation from rodent data. Reason: No biological support that human OTC participates in a response to biotin; electronic transfer.
GO:0005739 mitochondrion	IDA GO_REF:0000052	ACCEPT	Summary: Curated immunofluorescence (HPA) localization of OTC to the mitochondrion. Consistent with the established mitochondrial-matrix localization. Reason: Direct imaging evidence for mitochondrial localization; correct compartment.
GO:0004585 ornithine carbamoyltransferase activity	EXP PMID:6372096 Structure and expression of a complementary DNA for the nucl...	ACCEPT	Summary: Experimental demonstration of OTC enzymatic activity from expression of cloned human OTC cDNA. Core molecular function. Reason: Expression of the cloned cDNA yielded catalytically active OTC, directly demonstrating the ornithine carbamoyltransferase activity. Supporting Evidence: PMID:6372096 enzymatic activity was also detected.
GO:0005759 mitochondrial matrix	EXP PMID:3895227 Arginine in the leader peptide is required for both import a...	ACCEPT	Summary: Experimental study of OTC precursor import establishing OTC as a mitochondrial-matrix enzyme. Core localization. Reason: OTC is imported into and functions in the mitochondrial matrix; the arginine-rich leader peptide directs matrix import and cleavage. Supporting Evidence: PMID:3895227 the human mitochondrial matrix enzyme, ornithine transcarbamoylase
GO:0005759 mitochondrial matrix	TAS Reactome:R-HSA-9956527	ACCEPT	Summary: Reactome (TAS) localization of OTC to the mitochondrial matrix within a reaction where OTC variants fail to synthesize L-citrulline. Correct compartment. Reason: Matrix localization is well established; the Reactome model correctly places OTC in the matrix.
GO:0005829 cytosol	TAS Reactome:R-HSA-9956513	MARK AS OVER ANNOTATED	Summary: Reactome (TAS) cytosol location arising from a model of OTC leader-sequence variants that are NOT targeted to mitochondria. This is a mistrafficking/precursor context, not the localization of the mature functional enzyme. Reason: The cytosolic location reflects mislocalized targeting-signal variants (Reactome "Leader sequence variants of OTC aren't targeted to the mitochondria"), not the steady-state matrix localization of wild-type OTC. Misleading as a general localization claim.
GO:0005829 cytosol	TAS Reactome:R-HSA-9956502	MARK AS OVER ANNOTATED	Summary: Reactome (TAS) cytosol location from a model of OTC precursor targeting to the mitochondrial matrix. The cytosol appears only as the transient location of the newly synthesized precursor before import, not the mature enzyme's compartment. Reason: Reflects the pre-import precursor stage of the import pathway, not the functional localization of mature OTC (mitochondrial matrix).
GO:0005829 cytosol	TAS Reactome:R-HSA-9959881	MARK AS OVER ANNOTATED	Summary: Reactome (TAS) cytosol location from an "OTC gene expression" reaction. Reflects the biosynthesis/expression step, not the localization of the functional enzyme. Reason: Derived from a gene-expression modeling reaction rather than the mature enzyme's compartment; not the functional localization of OTC.
GO:0005739 mitochondrion	HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an...	ACCEPT	Summary: High-throughput mitochondrial proteomics (MitoCoP) detecting OTC in the high-confidence mitochondrial proteome. Supports mitochondrial localization. Reason: Proteomic evidence corroborating mitochondrial localization; consistent with the matrix assignment. Supporting Evidence: PMID:34800366 mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP)
GO:0005759 mitochondrial matrix	TAS Reactome:R-HSA-8986181	ACCEPT	Summary: Reactome (TAS) matrix localization within the presequence-proteolysis (PITRM1) reaction. Correct compartment for OTC. Reason: Matrix localization is well supported; the Reactome presequence-processing model correctly places OTC in the matrix.
GO:0000050 urea cycle	IDA PMID:2556444 An arginine to glutamine mutation in residue 109 of human or...	ACCEPT	Summary: Direct experimental annotation placing OTC in the urea cycle, from functional analysis of OTC and its deficiency in ammonia detoxification. Core process. Reason: OTC catalyzes an essential step of the urea cycle; loss of activity impairs ureagenesis and causes hyperammonemia. Supporting Evidence: PMID:2556444 Ornithine transcarbamylase (OTC) is an important enzyme in the detoxification of ammonia to urea, and its deficiency is the most common inborn error of ureagenesis in humans.
GO:0004585 ornithine carbamoyltransferase activity	IDA PMID:2556444 An arginine to glutamine mutation in residue 109 of human or...	ACCEPT	Summary: Direct assay of OTC enzymatic activity (wild-type versus Arg109Gln mutant) confirming the ornithine carbamoyltransferase activity. Core molecular function. Reason: Specific activity of expressed wild-type OTC was measured directly and contrasted with an inactive mutant, demonstrating the catalytic function. Supporting Evidence: PMID:2556444 the specific activity of mutant OTC was 100-fold lower than that of wild type
GO:0004585 ornithine carbamoyltransferase activity	IDA PMID:8112735 Expression of four mutant human ornithine transcarbamylase g...	ACCEPT	Summary: Direct measurement of OTC activity for wild-type and mutant cDNAs expressed in Cos1 cells, confirming ornithine carbamoyltransferase activity. Core molecular function. Reason: Enzyme activities of expressed OTC variants were quantified directly (0-8.9% of normal for mutants), demonstrating the catalytic function. Supporting Evidence: PMID:8112735 Predicted OTC activities of mutant OTC cDNAs ranged from 0% to 8.9% of the normal level
GO:0005739 mitochondrion	IDA PMID:3472484 Targeting of nuclear-encoded proteins to the mitochondrial m...	ACCEPT	Summary: Curated (IDA) mitochondrial localization from work on targeting of nuclear-encoded proteins to the mitochondrial matrix. Correct compartment. Reason: OTC is a well-established nuclear-encoded protein targeted to mitochondria; the more specific matrix term is also annotated.
GO:0005739 mitochondrion	IDA PMID:6372096 Structure and expression of a complementary DNA for the nucl...	ACCEPT	Summary: Curated (IDA) mitochondrial localization from characterization of the OTC precursor as a nuclear-encoded mitochondrial-matrix enzyme. Correct compartment. Reason: OTC was characterized as a human mitochondrial matrix enzyme whose precursor is imported into mitochondria. Supporting Evidence: PMID:6372096 We have deduced the complete primary structure of the precursor of a human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC)
GO:0006593 L-ornithine catabolic process	IDA PMID:2556444 An arginine to glutamine mutation in residue 109 of human or...	KEEP AS NON CORE	Summary: Direct annotation of OTC's involvement in L-ornithine consumption. L-ornithine is the amino acid substrate consumed in the OTC reaction; this is a substrate-centric framing of the same catalytic function. Reason: Accurate (ornithine is consumed by OTC), but this is the substrate side of the citrulline-biosynthetic reaction rather than a distinct core process; OTC's core role is better captured by the citrulline-biosynthetic / urea cycle terms.
GO:0006593 L-ornithine catabolic process	IDA PMID:8112735 Expression of four mutant human ornithine transcarbamylase g...	KEEP AS NON CORE	Summary: Direct annotation of OTC's role in L-ornithine consumption, from expression analysis of OTC variants. Substrate-centric framing of the OTC reaction. Reason: Ornithine is the consumed substrate of OTC; retained as an accurate but non-core substrate-side description of the same catalytic activity.
GO:0019240 L-citrulline biosynthetic process	IDA PMID:2556444 An arginine to glutamine mutation in residue 109 of human or...	ACCEPT	Summary: Direct experimental annotation of OTC in L-citrulline biosynthesis (the product of the OTC reaction), from functional analysis of wild-type versus inactive mutant OTC. Core process. Reason: L-citrulline is the direct product of OTC; enzyme-activity assays of wild-type and mutant OTC directly demonstrate this biosynthetic role. Supporting Evidence: PMID:2556444 the specific activity of mutant OTC was 100-fold lower than that of wild type
GO:0019240 L-citrulline biosynthetic process	IDA PMID:8112735 Expression of four mutant human ornithine transcarbamylase g...	ACCEPT	Summary: Direct experimental annotation of OTC in L-citrulline biosynthesis from expression/activity analysis of OTC variants in Cos1 cells. Core process. Reason: Measured OTC activities of variants directly demonstrate the enzyme's L-citrulline-biosynthetic role. Supporting Evidence: PMID:8112735 Predicted OTC activities of mutant OTC cDNAs ranged from 0% to 8.9% of the normal level
GO:0097272 ammonium homeostasis	IMP PMID:2556444 An arginine to glutamine mutation in residue 109 of human or...	ACCEPT	Summary: IMP annotation linking OTC function to ammonium homeostasis: loss of OTC activity impairs ammonia detoxification (hyperammonemia). Physiologically central and well supported. Reason: OTC deficiency is the most common inborn error of ureagenesis and causes hyperammonemia, directly implicating OTC in ammonium homeostasis via the urea cycle. Supporting Evidence: PMID:2556444 Ornithine transcarbamylase (OTC) is an important enzyme in the detoxification of ammonia to urea, and its deficiency is the most common inborn error of ureagenesis in humans.
GO:0097272 ammonium homeostasis	IMP PMID:2575934 Structure of the ornithine transcarbamylase (OTC) gene and D...	ACCEPT	Summary: IMP annotation of OTC in ammonium homeostasis, from OTC gene/mutation analysis in OTC deficiency. Consistent with the urea-cycle role in ammonia detoxification. Reason: OTC deficiency (studied via gene structure and DNA diagnosis) manifests as a urea-cycle failure with hyperammonemia, supporting OTC's role in ammonium homeostasis.
GO:0005759 mitochondrial matrix	TAS Reactome:R-HSA-70560	ACCEPT	Summary: Reactome (TAS) matrix localization within the core catalytic reaction (carbamoyl phosphate + ornithine => citrulline + orthophosphate). Correct compartment for the functional enzyme. Reason: The catalytic reaction occurs in the matrix; Reactome correctly localizes OTC there.
GO:0005759 mitochondrial matrix	TAS Reactome:R-HSA-9956502	ACCEPT	Summary: Reactome (TAS) matrix localization from the OTC precursor targeting/processing reaction (post-import, matrix-processed form). Correct compartment for the mature enzyme. Reason: The processed OTC resides in the matrix; this Reactome annotation captures the correct mature-enzyme compartment.
GO:0005759 mitochondrial matrix	NAS PMID:6372096 Structure and expression of a complementary DNA for the nucl...	ACCEPT	Summary: Non-traceable author statement (NAS) that OTC is a mitochondrial-matrix enzyme, consistent with the experimental EXP/IDA matrix annotations. Correct compartment. Reason: Concordant with the well-supported matrix localization of OTC. Supporting Evidence: PMID:6372096 We have deduced the complete primary structure of the precursor of a human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC)

Core Functions

Ornithine carbamoyltransferase activity in the mitochondrial matrix, catalyzing the condensation of carbamoyl phosphate with L-ornithine to form L-citrulline and inorganic phosphate (EC 2.1.3.3), the citrulline-forming step of the urea cycle.

Molecular Function:

ornithine carbamoyltransferase activity

Directly Involved In:

urea cycle citrulline biosynthetic process

Cellular Locations:

mitochondrial matrix

Supporting Evidence:

PMID:2556444
Ornithine transcarbamylase (OTC) is an important enzyme in the detoxification of ammonia to urea, and its deficiency is the most common inborn error of ureagenesis in humans.
PMID:8112735
Predicted OTC activities of mutant OTC cDNAs ranged from 0% to 8.9% of the normal level

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

GO_REF:0000052

Gene Ontology annotation based on curation of immunofluorescence data

GO_REF:0000107

Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:2556444

An arginine to glutamine mutation in residue 109 of human ornithine transcarbamylase completely abolishes enzymatic activity in Cos1 cells.

PMID:2575934

Structure of the ornithine transcarbamylase (OTC) gene and DNA diagnosis of OTC deficiency.

PMID:3472484

Targeting of nuclear-encoded proteins to the mitochondrial matrix: implications for human genetic defects.

PMID:34800366

Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context.

PMID:3895227

Arginine in the leader peptide is required for both import and proteolytic cleavage of a mitochondrial precursor.

PMID:6372096

Structure and expression of a complementary DNA for the nuclear coded precursor of human mitochondrial ornithine transcarbamylase.

PMID:8112735

Expression of four mutant human ornithine transcarbamylase genes in cultured Cos 1 cells relates to clinical phenotypes.

Reactome:R-HSA-70560

carbamoyl phosphate + ornithine => citrulline + orthophosphate

Reactome:R-HSA-8986181

PITRM1 proteolyzes mitochondrial targeting peptides (presequences)

Reactome:R-HSA-9956502

OTC precursor is targeted to the mitochondrial matrix and processed

Reactome:R-HSA-9956513

Leader sequence variants of OTC aren't targeted to the mitochondria

Reactome:R-HSA-9956527

OTC variants don't synthesize L-citrulline

Reactome:R-HSA-9959881

OTC gene expression

Deep Research

Falcon

(OTC-deep-research-falcon.md)

Comprehensive Research Report: Human Ornithine Transcarbamylase (OTC) Falcon Edison Scientific Literature 23 citations 1 artifacts 2026-07-05T13:05:56.505604

Edison artifact artifact-00 (text/markdown)

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You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Comprehensive Research Report: Human Ornithine Transcarbamylase (OTC)

Gene: OTC | UniProt: P00480 | EC: 2.1.3.3 | Organism: Homo sapiens

1. Gene Identity and Genomic Organization

The human OTC gene is located on the short arm of the X chromosome at position Xp21.1, spans approximately 73 kb, and contains 10 exons and 9 introns (diazmunoz2010molecularandbiochemical pages 2-3, arranz2007estimationofthe pages 1-2, couchet2021ornithinetranscarbamylase– pages 3-4). Exon 3 encodes the carbamoyl phosphate (CP)-binding site, while exon 9 encodes the ornithine (ORN)-binding site (couchet2021ornithinetranscarbamylase– pages 3-4). The gene encodes a 354-residue precursor polypeptide with a molecular weight of approximately 39.9 kDa (couchet2021ornithinetranscarbamylase– pages 3-4, diazmunoz2010molecularandbiochemical pages 1-2). The X-linked inheritance of the OTC gene has significant clinical implications, as hemizygous males with loss-of-function mutations present with severe disease, while heterozygous females display variable phenotypes influenced by X-chromosome inactivation patterns (yilmaz2023genetictherapyapproaches pages 1-2, couchet2021ornithinetranscarbamylase– pages 7-8).

2. Protein Structure and Quaternary Assembly

The key biochemical properties of human OTC are summarized below:

Property	Human OTC summary
Gene symbol	OTC (ornithine transcarbamylase) (diazmunoz2010molecularandbiochemical pages 2-3, couchet2021ornithinetranscarbamylase– pages 3-4)
UniProt ID	P00480
EC number	EC 2.1.3.3 (couchet2021ornithinetranscarbamylase– pages 1-3, couchet2021ornithinetranscarbamylase– pages 7-8)
Gene location	Xp21.1 on the short arm of the X chromosome (arranz2007estimationofthe pages 1-2, couchet2021ornithinetranscarbamylase– pages 3-4)
Gene size	~73 kb (diazmunoz2010molecularandbiochemical pages 2-3, arranz2007estimationofthe pages 1-2, couchet2021ornithinetranscarbamylase– pages 3-4)
Number of exons	10 exons and 9 introns (diazmunoz2010molecularandbiochemical pages 2-3, arranz2007estimationofthe pages 1-2, couchet2021ornithinetranscarbamylase– pages 3-4)
Precursor molecular weight	~39.9 kDa precursor (couchet2021ornithinetranscarbamylase– pages 3-4, diazmunoz2010molecularandbiochemical pages 1-2)
Mature protein molecular weight	~36.1 kDa mature protein (couchet2021ornithinetranscarbamylase– pages 3-4)
Amino acids	354 aa precursor; 322 aa mature reported in older review and gene-structure sources, and 321 aa mature reported in a newer review after leader peptide cleavage (32 aa leader) (diazmunoz2010molecularandbiochemical pages 2-3, arranz2007estimationofthe pages 1-2, couchet2021ornithinetranscarbamylase– pages 3-4)
Quaternary structure	Obligate homotrimer; dish-like trimer with 3-fold symmetry; active sites formed at subunit interfaces (couchet2021ornithinetranscarbamylase– pages 4-6, couchet2021ornithinetranscarbamylase– pages 1-3, diazmunoz2010molecularandbiochemical pages 1-2)
Subcellular localization	Mitochondrial matrix, with partial/non-covalent association to the inner mitochondrial membrane and participation in multienzyme/channeling assemblies (couchet2021ornithinetranscarbamylase– pages 3-4, couchet2021ornithinetranscarbamylase– pages 4-6, couchet2021ornithinetranscarbamylase– pages 1-3)
Primary tissues of expression	Highest physiologic expression/activity in liver and small intestine/intestinal mucosa (couchet2021ornithinetranscarbamylase– pages 1-3, couchet2021ornithinetranscarbamylase– pages 4-6, erdal2025aminoacidmetabolism pages 10-12)
Km for carbamoyl phosphate	Reported 0.26 mM in a recent review; older review cites 26 μM (couchet2021ornithinetranscarbamylase– pages 6-7, diazmunoz2010molecularandbiochemical pages 1-2)
Km for ornithine	Reported 0.4 mM in a recent review; older review cites 40 μM (couchet2021ornithinetranscarbamylase– pages 6-7, diazmunoz2010molecularandbiochemical pages 1-2)
pH optimum	~pH 8 (diazmunoz2010molecularandbiochemical pages 2-3)
Reaction catalyzed	Carbamoyl phosphate + L-ornithine → L-citrulline + phosphate (+ H⁺); transfer of the carbamoyl group to the δ-amino group of ornithine (couchet2021ornithinetranscarbamylase– pages 6-7, couchet2021ornithinetranscarbamylase– pages 4-6, couchet2021ornithinetranscarbamylase– pages 1-3)
Key catalytic residues / motifs	Active site spans adjacent subunits; important residues/motifs include Asp231, His168, Arg141, Arg330, Cys303, Leu304; conserved STRTR motif in the carbamoyl-phosphate-binding region and HCLP motif in the ornithine-binding/catalytic region (shi2000crystalstructureof pages 4-6, allewell1999molecularrecognitionby pages 4-6, diazmunoz2010molecularandbiochemical pages 1-2)
Inhibitors	L-norvaline (reported Ki = 71 μM), L-α-aminobutyrate, γ-aminobutyrate, L-leucine, L-isoleucine, L-valine, L-lysine (alternative substrate/inhibitory competitor), inorganic phosphate (product inhibition), Zn(II), and reducing agents (couchet2021ornithinetranscarbamylase– pages 6-7, diazmunoz2010molecularandbiochemical pages 1-2)

Table: This table summarizes the core biochemical, structural, genetic, and localization properties of human ornithine transcarbamylase (OTC). It is useful as a quick reference for functional annotation and for distinguishing catalytic mechanism, cellular context, and clinically relevant biochemical features.

Domain Organization

Each OTC monomer has a bilobal structure composed of two distinct structural domains surrounding a deep cleft: an N-terminal domain that binds CP and a C-terminal domain that binds ORN (couchet2021ornithinetranscarbamylase– pages 3-4, couchet2021ornithinetranscarbamylase– pages 4-6). Both domains share an αβα-topology with a central 5-stranded parallel β-sheet flanked by α-helices, organized as a 3-layer (α/β) sandwich structure (diazmunoz2010molecularandbiochemical pages 1-2, couchet2021ornithinetranscarbamylase– pages 4-6). The two domains are connected by two long interdomain helices (h5 and h12) (couchet2021ornithinetranscarbamylase– pages 4-6). The N-terminal domain contains a conserved Ser-Thr-Arg-Thr-Arg (STRTR) motif for CP binding, while the C-terminal domain contains the conserved Leu-His-Cys-Leu-Pro (LHCLP) motif critical for catalysis (diazmunoz2010molecularandbiochemical pages 1-2). Additionally, human OTC contains an internal trefoil knot structure that contributes to protein stability (diazmunoz2010molecularandbiochemical pages 2-3).

Trimeric Assembly

The mature protein assembles into obligate homotrimers forming a dish-like structure with 3-fold rotational symmetry and approximately 100 Å diameter (couchet2021ornithinetranscarbamylase– pages 4-6). All three active sites open to the concave face of the dish-like structure, and importantly, active sites are formed at the interfaces between adjacent monomers, with each active site receiving contributions from residues of a neighboring subunit (diazmunoz2010molecularandbiochemical pages 1-2, couchet2021ornithinetranscarbamylase– pages 4-6). This trimeric assembly is obligatory for catalytic function (couchet2021ornithinetranscarbamylase– pages 4-6).

The crystal structure of human OTC complexed with CP and the ornithine analog L-norvaline has been resolved at 1.9 Å resolution, providing detailed insight into the active site architecture and catalytic mechanism (shi2000crystalstructureof pages 4-6).

3. Enzymatic Reaction and Catalytic Mechanism

Reaction Catalyzed

OTC (EC 2.1.3.3) is a carbamoyl transferase that catalyzes the transfer of a carbamoyl group from carbamoyl phosphate to the δ-amino group of L-ornithine, producing L-citrulline, inorganic phosphate, and a proton (couchet2021ornithinetranscarbamylase– pages 6-7, couchet2021ornithinetranscarbamylase– pages 4-6, couchet2021ornithinetranscarbamylase– pages 1-3). This reaction is highly thermodynamically favorable, with a ΔG° of approximately −63 kcal/mol (couchet2021ornithinetranscarbamylase– pages 6-7).

Kinetic Mechanism

The reaction follows an ordered bi-bi mechanism with Michaelis-Menten kinetics. CP binds first to the deeper pocket of the active site, forming a binary complex that induces a global closure of the active site cleft between the two domains (couchet2021ornithinetranscarbamylase– pages 6-7, couchet2021ornithinetranscarbamylase– pages 4-6). This conformational change then allows ORN binding, which triggers a further induced-fit mechanism moving the SMG loop to shield and fix the active site (couchet2021ornithinetranscarbamylase– pages 4-6). Product release is also ordered, with citrulline released before inorganic phosphate (couchet2021ornithinetranscarbamylase– pages 6-7). The Km values for the substrates have been reported as 0.26 mM for CP and 0.4 mM for ORN at pH 7.7 (couchet2021ornithinetranscarbamylase– pages 6-7), with optimal catalytic activity around pH 8 (diazmunoz2010molecularandbiochemical pages 2-3).

Catalytic Residues and Mechanism

The catalytic mechanism involves several key steps. The δ-amino group of ornithine performs a nucleophilic attack on the carbonyl carbon of CP (couchet2021ornithinetranscarbamylase– pages 4-6, allewell1999molecularrecognitionby pages 4-6). Cys-303 functions as a general base, abstracting a proton from the δ-amino group of ornithine; the basicity of Cys-303 is enhanced by a hydrogen bond to Asp-263 (shi2000crystalstructureof pages 4-6, allewell1999molecularrecognitionby pages 4-6). A tetrahedral intermediate is formed and stabilized by positively charged residues His-168, Arg-141, and Arg-330, which interact with the partially negatively charged carbonyl oxygen of CP (shi2000crystalstructureof pages 4-6). His-168 also participates in proton transfer to facilitate product release (shi2000crystalstructureof pages 4-6). The residues C303 and L304 stabilize the transition state (couchet2021ornithinetranscarbamylase– pages 4-6). Additional key substrate-binding residues include Asp-231, Met-236, Leu-125, Ser-235, and Asn-167, which form the ORN-binding pocket through electrostatic, hydrophobic, and hydrogen-bonding interactions (allewell1999molecularrecognitionby pages 4-6).

Substrate Specificity and Inhibition

While OTC is highly specific for L-ornithine, the enzyme can also accept other basic amino acids as alternative substrates. Notably, L-lysine can serve as an alternative substrate, producing homocitrulline (couchet2021ornithinetranscarbamylase– pages 6-7). OTC activity is competitively inhibited by several amino acids, including L-norvaline (Ki = 71 μM), L-α-aminobutyrate, γ-aminobutyrate, L-leucine, L-isoleucine, and L-valine (couchet2021ornithinetranscarbamylase– pages 6-7, diazmunoz2010molecularandbiochemical pages 1-2). Inorganic phosphate acts as a product inhibitor, and the enzyme is also inhibited by Zn(II) and reducing agents (diazmunoz2010molecularandbiochemical pages 1-2).

4. Subcellular Localization and Mitochondrial Import

Mitochondrial Targeting and Import

OTC is encoded by a nuclear gene and synthesized as a 39.9 kDa precursor protein (pre-OTC) on free ribosomes in the cytoplasm (mori1982ornithinetranscarbamylasein pages 1-3, mori1982ornithinetranscarbamylasein pages 3-4). The precursor contains a canonical N-terminal signal peptide of 32 amino acids (approximately 3,400–4,000 daltons) that serves as a mitochondrial targeting sequence (couchet2021ornithinetranscarbamylase– pages 3-4, mori1982ornithinetranscarbamylasein pages 1-3). After synthesis, the precursor is released into the cytosol and then transported across both the outer and inner mitochondrial membranes into the matrix in an energy-dependent process requiring membrane potential, mitochondrial integrity, potassium and magnesium ions, cytosolic proteins, and specific mitochondrial matrix proteases (mori1982ornithinetranscarbamylasein pages 1-3, mori1982ornithinetranscarbamylasein pages 9-12). The half-life of the precursor in the cytosol is approximately 12 minutes (mori1982ornithinetranscarbamylasein pages 1-3). Processing of the precursor to the mature 36.1 kDa form occurs during, rather than after, transport into mitochondria, mediated by processing proteases located mainly in the matrix and partly in the intermembrane space (mori1982ornithinetranscarbamylasein pages 9-12).

Localization Within Mitochondria

The mature OTC protein resides in the mitochondrial matrix, where it assembles into homotrimers and attaches to the inner mitochondrial membrane through non-covalent interactions with anionic phospholipids, particularly cardiolipin (couchet2021ornithinetranscarbamylase– pages 3-4, couchet2021ornithinetranscarbamylase– pages 4-6, couchet2021ornithinetranscarbamylase– pages 1-3). In liver tissue, OTC represents approximately 3–4% of total mitochondrial proteins, and the half-life of active OTC in rat liver ranges from 6–9 days (couchet2021ornithinetranscarbamylase– pages 3-4). Importantly, OTC co-localizes with carbamoyl phosphate synthetase 1 (CPS1) at the inner membrane, forming part of a multi-enzyme channeling assembly with other mitochondrial urea cycle enzymes (NAGS, CPS1, and OTC) that facilitates metabolite flux (couchet2021ornithinetranscarbamylase– pages 4-6).

Tissue Distribution

OTC is predominantly expressed in two organs: the liver, where it functions as an integral part of the urea cycle for ammonia detoxification, and the small intestine, where it synthesizes citrulline for systemic export (couchet2021ornithinetranscarbamylase– pages 1-3, couchet2021ornithinetranscarbamylase– pages 4-6). Hepatic OTC activity is substantially higher than intestinal activity (4,110 μmol·g⁻¹·h⁻¹ in liver vs. 100 μmol·g⁻¹·h⁻¹ in intestinal mucosa) (couchet2021ornithinetranscarbamylase– pages 6-7). Under certain pathological conditions, such as hepatotoxicity or active liver regeneration, OTC can be released from mitochondria into the bloodstream, where it may function as a signaling molecule (diazmunoz2010molecularandbiochemical pages 1-2).

5. Role in the Urea Cycle and Metabolic Pathways

The Urea Cycle

OTC catalyzes the second step of the hepatic urea cycle, directly downstream of CPS1, which catalyzes the rate-limiting first step by producing CP from ammonia, bicarbonate, and ATP (couchet2021ornithinetranscarbamylase– pages 4-6, erdal2025aminoacidmetabolism pages 10-12). The citrulline produced by OTC is then exported from the mitochondrial matrix to the cytosol, where it is condensed with aspartate by argininosuccinate synthetase (ASS1), cleaved by argininosuccinate lyase (ASL) to produce arginine and fumarate, and finally hydrolyzed by arginase to yield urea and regenerate ornithine, completing the cycle (couchet2021ornithinetranscarbamylase– pages 1-3). Ornithine is imported back into the mitochondrial matrix via the ornithine carriers ORC1/ORNT1 and ORC2/ORNT2 located in the inner mitochondrial membrane and is channeled directly to OTC bound at this location (couchet2021ornithinetranscarbamylase– pages 4-6).

Intestinal Citrulline Production

In the intestine, OTC plays a distinct role from its hepatic function. Intestinal OTC synthesizes citrulline that is exported systemically into the plasma rather than being channeled into a local urea cycle (couchet2021ornithinetranscarbamylase– pages 1-3, couchet2021ornithinetranscarbamylase– pages 8-9). This intestinal citrulline contributes to whole-body arginine biosynthesis through the renal arginine synthesis pathway and plays a major role in amino acid homeostasis, particularly of L-glutamine and L-arginine (couchet2021ornithinetranscarbamylase– pages 1-3, couchet2021ornithinetranscarbamylase– pages 8-9).

Regulation of OTC Expression and Activity

OTC expression is regulated at multiple levels. Transcriptionally, the OTC gene promoter is activated by hepatocyte nuclear factor 4 (HNF-4) and the mitochondrial metabolism regulator PGC1α, while COUP-TF2 provides inhibitory regulation (couchet2021ornithinetranscarbamylase– pages 1-3). The promoter also contains cAMP response elements and glucocorticoid receptor binding sequences, as well as a urea cycle element (diazmunoz2010molecularandbiochemical pages 2-3). OTC expression responds to nutritional state: high-protein diets increase OTC mRNA and protein expression through mechanisms potentially involving AMPK signaling (couchet2021ornithinetranscarbamylase– pages 3-4). The tumor suppressor p53 represses translation of both CPS1 and OTC (couchet2021ornithinetranscarbamylase– pages 3-4).

Post-translationally, OTC activity is regulated through reversible acetylation: acetylation at K88 reduces enzymatic activity in response to nutrient signals, while Sirt3-mediated deacetylation increases OTC activity during fasting states, thereby linking OTC function to the metabolic demands of the organism (couchet2021ornithinetranscarbamylase– pages 3-4). Additionally, the enzyme shows developmental regulation, with rapid accumulation in the first three weeks of postnatal life in rats, reaching adult levels by weaning (couchet2021ornithinetranscarbamylase– pages 6-7).

6. Clinical Significance: OTC Deficiency

Prevalence and Inheritance

OTC deficiency (OTCD) is an X-linked disorder and the most common urea cycle disorder, accounting for approximately 50% of all urea cycle hereditary disorders (couchet2021ornithinetranscarbamylase– pages 7-8). Prevalence estimates range from 1 in 14,000 to 1 in 80,000 live births (yilmaz2023genetictherapyapproaches pages 1-2, couchet2021ornithinetranscarbamylase– pages 7-8).

Clinical Manifestations

Hemizygous males with complete OTC deficiency typically present with severe neonatal-onset hyperammonemia, characterized by hypotonia, somnolence, and potentially hyperammonemic coma or death (couchet2021ornithinetranscarbamylase– pages 7-8, yilmaz2023genetictherapyapproaches pages 2-4). Patients with partial OTC deficiency present later with milder symptoms. Heterozygous females display highly variable phenotypes due to random X-chromosome inactivation, ranging from asymptomatic states to severe metabolic and neuropsychiatric manifestations (zarantebahamon2025chronicandvariable pages 5-6). Clinical deterioration is often triggered by catabolic stressors such as febrile illness, infection, or pregnancy (zarantebahamon2025chronicandvariable pages 5-6). Corticosteroid administration can exacerbate hyperammonemia not only through protein catabolism but also by suppressing urea-cycle-related gene expressions (couchet2021ornithinetranscarbamylase– pages 3-4).

Current Treatments and Gene Therapy

Current standard care includes protein-restricted diets, ammonia scavenger drugs (sodium benzoate, sodium phenylbutyrate), and arginine/citrulline supplementation, though these measures do not prevent all hyperammonemic episodes (yilmaz2023genetictherapyapproaches pages 1-2, couchet2021ornithinetranscarbamylase– pages 7-8, yilmaz2023genetictherapyapproaches pages 2-4). Liver transplantation remains the only currently curative option but is limited by donor shortage, surgical challenges in young infants, and lifelong immunosuppression requirements (yilmaz2023genetictherapyapproaches pages 1-2, yilmaz2023genetictherapyapproaches pages 2-4).

As a well-characterized monogenic disorder, OTCD is a prime candidate for gene therapy. Multiple modalities are under active development, including adeno-associated virus (AAV)-based gene addition therapy, mRNA therapy encapsulated in lipid nanoparticles, and CRISPR/Cas9 genome editing (yilmaz2023genetictherapyapproaches pages 1-2, yilmaz2023genetictherapyapproaches pages 7-9, yilmaz2023genetictherapyapproaches pages 9-10). Phase 3 clinical trials with AAV8 vectors have shown positive outcomes, with patients achieving metabolic stability and discontinuing ammonia-scavenger medications and protein-restricted diets (yilmaz2023genetictherapyapproaches pages 7-9). Restoring even 10% of normal OTC enzyme activity can significantly improve the clinical phenotype (yilmaz2023genetictherapyapproaches pages 2-4). Pharmacological chaperone approaches have also been explored, with recent high-throughput screening identifying compounds that stabilize OTC protein and enhance enzymatic activity and ureagenesis in patient-derived hepatocytes (couchet2021ornithinetranscarbamylase– pages 7-8).

7. Emerging Roles in Cancer and Chronic Disease

Beyond its classical role in nitrogen metabolism, OTC has emerging significance in cancer and chronic disease. Many tumor types—including colorectal carcinoma, hepatocellular carcinoma, glioblastoma, and pediatric sarcomas—show deficient OTC expression (couchet2021ornithinetranscarbamylase– pages 8-9). This metabolic reprogramming redirects nitrogen away from the urea cycle toward anabolic pathways that favor tumor growth, simultaneously increasing tumor dependency on exogenous arginine and making arginine deprivation therapy a potential therapeutic approach (couchet2021ornithinetranscarbamylase– pages 8-9). In hepatocellular carcinoma, lower OTC expression correlates with larger tumor size and advanced grade, while OTC silencing increases cell proliferation (couchet2021ornithinetranscarbamylase– pages 9-10). Downregulated OTC expression has also been observed in non-alcoholic steatohepatitis, contributing to hyperammonemia and increased liver fibrosis risk (couchet2021ornithinetranscarbamylase– pages 9-10).

8. Summary

Human ornithine transcarbamylase (OTC; P00480) is a mitochondrial matrix enzyme that catalyzes the transfer of a carbamoyl group from carbamoyl phosphate to L-ornithine, producing L-citrulline and inorganic phosphate. This reaction constitutes the second step of the hepatic urea cycle and is essential for ammonia detoxification. OTC is synthesized as a cytoplasmic precursor with an N-terminal mitochondrial targeting peptide, imported into mitochondria, and processed to its mature form, which assembles into obligate homotrimers at the inner mitochondrial membrane. The enzyme is predominantly expressed in liver and intestine, with distinct metabolic roles in each tissue: hepatic ureagenesis and intestinal citrulline export for systemic arginine biosynthesis. OTC activity is regulated transcriptionally by HNF-4 and PGC1α, and post-translationally through reversible acetylation. Deficiency of OTC causes the most common urea cycle disorder, an X-linked condition with significant morbidity and mortality that is now a leading candidate for gene therapy approaches. Emerging evidence also implicates OTC dysregulation in cancer metabolism and chronic liver disease.

References

(diazmunoz2010molecularandbiochemical pages 2-3): Mauricio Diaz-Munoz and Rolando Hernandez-Munoz. Molecular and biochemical features of the mitochondrial enzyme ornithine transcarbamylase: a possible new role as a signaling factor. Jul 2010. URL: https://doi.org/10.2174/092986710791331031, doi:10.2174/092986710791331031. This article has 17 citations and is from a peer-reviewed journal.
(arranz2007estimationofthe pages 1-2): J. A. Arranz, Encarnació Riudor, C. Marco-Marín, and Vicente Rubio. Estimation of the total number of disease-causing mutations in ornithine transcarbamylase (otc) deficiency. value of the otc structure in predicting a mutation pathogenic potential. Journal of Inherited Metabolic Disease, 30:217-226, Mar 2007. URL: https://doi.org/10.1007/s10545-007-0429-x, doi:10.1007/s10545-007-0429-x. This article has 53 citations and is from a peer-reviewed journal.
(couchet2021ornithinetranscarbamylase– pages 3-4): Morgane Couchet, Charlotte Breuillard, Christelle Corne, John Rendu, Béatrice Morio, Uwe Schlattner, and Christophe Moinard. Ornithine transcarbamylase – from structure to metabolism: an update. Frontiers in Physiology, Oct 2021. URL: https://doi.org/10.3389/fphys.2021.748249, doi:10.3389/fphys.2021.748249. This article has 82 citations.
(diazmunoz2010molecularandbiochemical pages 1-2): Mauricio Diaz-Munoz and Rolando Hernandez-Munoz. Molecular and biochemical features of the mitochondrial enzyme ornithine transcarbamylase: a possible new role as a signaling factor. Jul 2010. URL: https://doi.org/10.2174/092986710791331031, doi:10.2174/092986710791331031. This article has 17 citations and is from a peer-reviewed journal.
(yilmaz2023genetictherapyapproaches pages 1-2): Berna Seker Yilmaz and Paul Gissen. Genetic therapy approaches for ornithine transcarbamylase deficiency. Biomedicines, 11:2227, Aug 2023. URL: https://doi.org/10.3390/biomedicines11082227, doi:10.3390/biomedicines11082227. This article has 36 citations.
(couchet2021ornithinetranscarbamylase– pages 7-8): Morgane Couchet, Charlotte Breuillard, Christelle Corne, John Rendu, Béatrice Morio, Uwe Schlattner, and Christophe Moinard. Ornithine transcarbamylase – from structure to metabolism: an update. Frontiers in Physiology, Oct 2021. URL: https://doi.org/10.3389/fphys.2021.748249, doi:10.3389/fphys.2021.748249. This article has 82 citations.
(couchet2021ornithinetranscarbamylase– pages 1-3): Morgane Couchet, Charlotte Breuillard, Christelle Corne, John Rendu, Béatrice Morio, Uwe Schlattner, and Christophe Moinard. Ornithine transcarbamylase – from structure to metabolism: an update. Frontiers in Physiology, Oct 2021. URL: https://doi.org/10.3389/fphys.2021.748249, doi:10.3389/fphys.2021.748249. This article has 82 citations.
(couchet2021ornithinetranscarbamylase– pages 4-6): Morgane Couchet, Charlotte Breuillard, Christelle Corne, John Rendu, Béatrice Morio, Uwe Schlattner, and Christophe Moinard. Ornithine transcarbamylase – from structure to metabolism: an update. Frontiers in Physiology, Oct 2021. URL: https://doi.org/10.3389/fphys.2021.748249, doi:10.3389/fphys.2021.748249. This article has 82 citations.
(erdal2025aminoacidmetabolism pages 10-12): Ranya Erdal, Kıvanç Birsoy, and Gokhan Unlu. Amino acid metabolism in liver mitochondria: from homeostasis to disease. Metabolites, 15:446, Jul 2025. URL: https://doi.org/10.3390/metabo15070446, doi:10.3390/metabo15070446. This article has 7 citations.
(couchet2021ornithinetranscarbamylase– pages 6-7): Morgane Couchet, Charlotte Breuillard, Christelle Corne, John Rendu, Béatrice Morio, Uwe Schlattner, and Christophe Moinard. Ornithine transcarbamylase – from structure to metabolism: an update. Frontiers in Physiology, Oct 2021. URL: https://doi.org/10.3389/fphys.2021.748249, doi:10.3389/fphys.2021.748249. This article has 82 citations.
(shi2000crystalstructureof pages 4-6): Dashuang Shi, Hiroki Morizono, Mika Aoyagi, Mendel Tuchman, and Norma M. Allewell. Crystal structure of human ornithine transcarbamylase complexed with carbamoyl phosphate and l‐norvaline at 1.9 å resolution. Proteins: Structure, 39:271-277, Jun 2000. URL: https://doi.org/10.1002/(sici)1097-0134(20000601)39:4<271::aid-prot10>3.0.co;2-e, doi:10.1002/(sici)1097-0134(20000601)39:4<271::aid-prot10>3.0.co;2-e. This article has 53 citations.
(allewell1999molecularrecognitionby pages 4-6): Norma M. Allewell, Dashuang Shi, Hiroki Morizono, and Mendel Tuchman. Molecular recognition by ornithine and aspartate transcarbamylases. Aug 1999. URL: https://doi.org/10.1021/ar950262j, doi:10.1021/ar950262j. This article has 25 citations and is from a domain leading peer-reviewed journal.
(mori1982ornithinetranscarbamylasein pages 1-3): Masataka Mori, Satoshi Miura, Tetsuo Morita, Masaki Takiguchi, and Masamiti Tatibana. Ornithine transcarbamylase in liver mitochondria. Molecular and Cellular Biochemistry, 49:97-111, Nov 1982. URL: https://doi.org/10.1007/bf00242488, doi:10.1007/bf00242488. This article has 62 citations and is from a peer-reviewed journal.
(mori1982ornithinetranscarbamylasein pages 3-4): Masataka Mori, Satoshi Miura, Tetsuo Morita, Masaki Takiguchi, and Masamiti Tatibana. Ornithine transcarbamylase in liver mitochondria. Molecular and Cellular Biochemistry, 49:97-111, Nov 1982. URL: https://doi.org/10.1007/bf00242488, doi:10.1007/bf00242488. This article has 62 citations and is from a peer-reviewed journal.
(mori1982ornithinetranscarbamylasein pages 9-12): Masataka Mori, Satoshi Miura, Tetsuo Morita, Masaki Takiguchi, and Masamiti Tatibana. Ornithine transcarbamylase in liver mitochondria. Molecular and Cellular Biochemistry, 49:97-111, Nov 1982. URL: https://doi.org/10.1007/bf00242488, doi:10.1007/bf00242488. This article has 62 citations and is from a peer-reviewed journal.
(couchet2021ornithinetranscarbamylase– pages 8-9): Morgane Couchet, Charlotte Breuillard, Christelle Corne, John Rendu, Béatrice Morio, Uwe Schlattner, and Christophe Moinard. Ornithine transcarbamylase – from structure to metabolism: an update. Frontiers in Physiology, Oct 2021. URL: https://doi.org/10.3389/fphys.2021.748249, doi:10.3389/fphys.2021.748249. This article has 82 citations.
(yilmaz2023genetictherapyapproaches pages 2-4): Berna Seker Yilmaz and Paul Gissen. Genetic therapy approaches for ornithine transcarbamylase deficiency. Biomedicines, 11:2227, Aug 2023. URL: https://doi.org/10.3390/biomedicines11082227, doi:10.3390/biomedicines11082227. This article has 36 citations.
(zarantebahamon2025chronicandvariable pages 5-6): Ana María Zarante-Bahamón, Jenniffer A. Romero-Morales, and Jorge Luis Ramón-Gómez. Chronic and variable manifestations of ornithine transcarbamylase deficiency in heterozygous carriers: a case series of three colombian patients. Journal of Inborn Errors of Metabolism and Screening, Jan 2025. URL: https://doi.org/10.1590/2326-4594-jiems-2025-0002, doi:10.1590/2326-4594-jiems-2025-0002. This article has 0 citations.
(yilmaz2023genetictherapyapproaches pages 7-9): Berna Seker Yilmaz and Paul Gissen. Genetic therapy approaches for ornithine transcarbamylase deficiency. Biomedicines, 11:2227, Aug 2023. URL: https://doi.org/10.3390/biomedicines11082227, doi:10.3390/biomedicines11082227. This article has 36 citations.
(yilmaz2023genetictherapyapproaches pages 9-10): Berna Seker Yilmaz and Paul Gissen. Genetic therapy approaches for ornithine transcarbamylase deficiency. Biomedicines, 11:2227, Aug 2023. URL: https://doi.org/10.3390/biomedicines11082227, doi:10.3390/biomedicines11082227. This article has 36 citations.
(couchet2021ornithinetranscarbamylase– pages 9-10): Morgane Couchet, Charlotte Breuillard, Christelle Corne, John Rendu, Béatrice Morio, Uwe Schlattner, and Christophe Moinard. Ornithine transcarbamylase – from structure to metabolism: an update. Frontiers in Physiology, Oct 2021. URL: https://doi.org/10.3389/fphys.2021.748249, doi:10.3389/fphys.2021.748249. This article has 82 citations.

Artifacts

Edison artifact artifact-00

Citations

diazmunoz2010molecularandbiochemical pages 2-3
diazmunoz2010molecularandbiochemical pages 1-2
shi2000crystalstructureof pages 4-6
allewell1999molecularrecognitionby pages 4-6
mori1982ornithinetranscarbamylasein pages 1-3
mori1982ornithinetranscarbamylasein pages 9-12
zarantebahamon2025chronicandvariable pages 5-6
yilmaz2023genetictherapyapproaches pages 7-9
yilmaz2023genetictherapyapproaches pages 2-4
arranz2007estimationofthe pages 1-2
yilmaz2023genetictherapyapproaches pages 1-2
erdal2025aminoacidmetabolism pages 10-12
mori1982ornithinetranscarbamylasein pages 3-4
yilmaz2023genetictherapyapproaches pages 9-10
https://doi.org/10.2174/092986710791331031,
https://doi.org/10.1007/s10545-007-0429-x,
https://doi.org/10.3389/fphys.2021.748249,
https://doi.org/10.3390/biomedicines11082227,
https://doi.org/10.3390/metabo15070446,
https://doi.org/10.1002/(sici
https://doi.org/10.1021/ar950262j,
https://doi.org/10.1007/bf00242488,
https://doi.org/10.1590/2326-4594-jiems-2025-0002,

📚 Additional Documentation

Notes

(OTC-notes.md)

OTC (Ornithine transcarbamylase / ornithine carbamoyltransferase, P00480) — review notes

Core biology (verified)

X-linked, nuclear-encoded, mitochondrial-matrix urea-cycle enzyme; EC 2.1.3.3.
Catalyzes: carbamoyl phosphate + L-ornithine -> L-citrulline + phosphate + H(+) (Rhea:RHEA:19513).
UniProt: "Catalyzes the second step of the urea cycle, the condensation of carbamoyl phosphate with L-ornithine to form L-citrulline (PubMed:2556444, PubMed:6372096, PubMed:8112735)." Some sources number this the third step; either way it is the OTC-catalyzed condensation.
Pathway (UniProt): "Nitrogen metabolism; urea cycle; L-citrulline from L-ornithine and carbamoyl phosphate: step 1/1."
Subunit (UniProt): "Homotrimer." (PubMed:10813810, PubMed:9852088)
Localization (UniProt): "Mitochondrion matrix" (PubMed:3895227).
Family (UniProt): "aspartate/ornithine carbamoyltransferase superfamily. OTCase family."
Tissue: "Mainly expressed in liver and intestinal mucosa."
PTM/regulation: acetylation at Lys-88 negatively regulates activity in response to nutrient signals (PubMed:19318352).

Import / precursor processing

Nuclear-encoded precursor with 32-aa N-terminal leader (4 arginines, no acidic residues); leader cleaved after import into the matrix.
PMID:6372096
PMID:6372096
Basic arginine residues in the leader are required for import and proteolytic cleavage.
PMID:3895227
PMID:3895227

Enzyme activity / disease functional evidence

Missense mutations abolish/reduce OTC activity; expression in Cos1 cells confirms causality.
PMID:2556444
PMID:2556444
PMID:8112735
PMID:8112735

OTC deficiency (OTCD; MONDO:0010703)

Most common urea cycle disorder; deficiency -> impaired citrulline synthesis -> hyperammonemia.
dismech: "OTCD typically leads to mitochondrial enzyme dysfunction, preventing the synthesis of citrulline from carbamoyl phosphate and ornithine"
dismech: "carbamoylphosphate synthetase 1 and ornithine transcarbamylase are present in the mitochondrial matrix" (confirms matrix)

Localization proteomics

PMID:34800366 MitoCoP high-confidence mitochondrial proteome (HTP) — supports mitochondrial localization.

Annotation judgments

Strong experimental core kept ACCEPT: GO:0004585 (MF), GO:0000050 (urea cycle), GO:0019240 (L-citrulline biosynthetic), GO:0005759 (matrix), GO:0097272 (ammonium homeostasis).
GO:0006526 (L-arginine biosynthetic process) — IBA; urea cycle is the biosynthetic route to arginine in some contexts; KEEP_AS_NON_CORE (downstream pathway role, not OTC's direct product).
GO:0006593 (L-ornithine catabolic process) — IDA; ornithine is the consumed substrate; KEEP_AS_NON_CORE (accurate but a substrate-centric framing of the same MF).
GO:0042802 (identical protein binding) — IEA Ensembl; reflects homotrimer; MODIFY toward homotrimerization/structural constituent role; keep non-core.
Ensembl Compara (GO_REF:0000107) transfers from rat P00481 that are over-annotations for human OTC: liver development, midgut development, response to xenobiotic/zinc/insulin/nutrient/biotin, phospholipid binding, monoatomic anion homeostasis, phosphate ion binding, mitochondrial inner membrane (wrong compartment; OTC is matrix). -> MARK_AS_OVER_ANNOTATED / REMOVE (inner membrane REMOVE: wrong compartment).
Cytosol TAS (Reactome R-HSA-9956513/9956502/9959881) reflects precursor/mutant-trafficking + gene-expression modeling, not the mature enzyme localization -> REMOVE/MARK_AS_OVER_ANNOTATED as a steady-state location claim.
GO:0016597 (amino acid binding), GO:0042301 (phosphate ion binding) reflect substrate/product binding within catalysis; keep non-core (subsumed by MF).

Deep research

falcon deep-research file was NOT present within the ~12 min poll window; review grounded in UniProt, GOA, dismech disorder file, and cached publications.

📄 View Raw YAML

id: P00480
gene_symbol: OTC
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  OTC (ornithine transcarbamylase / ornithine carbamoyltransferase, EC 2.1.3.3)
  is a nuclear-encoded, mitochondrial-matrix enzyme of the urea cycle. It
  catalyzes the condensation of carbamoyl phosphate with L-ornithine to form
  L-citrulline plus inorganic phosphate, the committed step that links carbamoyl
  phosphate (produced by CPS1) to citrulline en route to hepatic ureagenesis and
  ammonia detoxification. The protein is synthesized in the cytosol as a larger
  precursor with a cleavable, arginine-rich N-terminal mitochondrial targeting
  presequence, imported into the mitochondrial matrix, and proteolytically
  processed to the mature subunit, which assembles into a catalytically active
  homotrimer. OTC is expressed mainly in liver and intestinal mucosa and belongs
  to the aspartate/ornithine carbamoyltransferase superfamily (OTCase family).
  The X-linked gene is the site of loss-of-function variants causing ornithine
  carbamoyltransferase deficiency (OTCD), the most common urea cycle disorder,
  in which impaired citrulline synthesis reduces urea-cycle flux and produces
  hyperammonemia.
existing_annotations:
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      Phylogenetic (IBA) assertion that OTC is active in mitochondria. Correct
      but less specific than the well-supported mitochondrial matrix
      localization; retained as accurate parent-level localization.
    action: ACCEPT
    reason: >-
      OTC is a mitochondrial-matrix enzyme; the mitochondrion is the correct
      compartment at this level. The more specific matrix term is captured
      separately (GO:0005759).
- term:
    id: GO:0006526
    label: L-arginine biosynthetic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      IBA-transferred role in L-arginine biosynthesis. OTC produces citrulline,
      which is a precursor for argininosuccinate and thence arginine; the urea
      cycle is the biosynthetic route to arginine. This is a downstream pathway
      role rather than OTC's direct biosynthetic product.
    action: KEEP_AS_NON_CORE
    reason: >-
      OTC's direct product is L-citrulline, not L-arginine; arginine biosynthesis
      is a downstream consequence of the urea cycle in which OTC participates.
      Biologically defensible at the pathway level but not the core function of
      the enzyme.
- term:
    id: GO:0019240
    label: L-citrulline biosynthetic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      IBA assertion that OTC is involved in L-citrulline biosynthesis. This is
      exactly the direct product of the OTC reaction and is strongly supported
      experimentally and by the UniProt pathway annotation.
    action: ACCEPT
    reason: >-
      L-citrulline is the direct product of the reaction OTC catalyzes; UniProt
      records "Nitrogen metabolism; urea cycle; L-citrulline from L-ornithine
      and carbamoyl phosphate: step 1/1." Core biological process.
    supported_by:
    - reference_id: PMID:2556444
      supporting_text: >-
        An arginine to glutamine mutation in residue 109 of human ornithine
        transcarbamylase completely abolishes enzymatic activity in Cos1 cells.
- term:
    id: GO:0004585
    label: ornithine carbamoyltransferase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      Phylogenetic (IBA) assignment of the diagnostic molecular function.
      Concordant with experimental IDA/EXP annotations and UniProt catalytic
      activity. Core function.
    action: ACCEPT
    reason: >-
      This is the defining enzymatic activity of OTC (EC 2.1.3.3), supported by
      multiple experimental annotations and UniProt.
- term:
    id: GO:0004585
    label: ornithine carbamoyltransferase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated (IEA, multi-method) assignment of the core ornithine
      carbamoyltransferase activity, backed by EC 2.1.3.3 / RHEA:19513 and ARBA.
      Consistent with the experimental core.
    action: ACCEPT
    reason: >-
      Correct core molecular function; redundant with the experimental and IBA
      annotations of the same term.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: located_in
  review:
    summary: >-
      ARBA machine-learning electronic annotation to mitochondrion. Correct
      compartment at the organelle level.
    action: ACCEPT
    reason: >-
      OTC is a mitochondrial protein; the more specific matrix location is
      captured by GO:0005759.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: >-
      UniProt SubCell mapping (SL-0170) to mitochondrial matrix, matching the
      experimentally established localization of the mature enzyme.
    action: ACCEPT
    reason: >-
      Mitochondrial matrix is the correct, experimentally supported location of
      mature OTC.
    supported_by:
    - reference_id: PMID:6372096
      supporting_text: >-
        We have deduced the complete primary structure of the precursor of a
        human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC)
- term:
    id: GO:0006520
    label: amino acid metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      InterPro2GO electronic annotation to the broad amino acid metabolic
      process. Correct but very general; the specific urea-cycle and
      citrulline/arginine biosynthesis terms convey the actual role.
    action: KEEP_AS_NON_CORE
    reason: >-
      True at a high level (OTC metabolizes ornithine and produces citrulline)
      but too general to be a core annotation; more specific terms are present.
- term:
    id: GO:0016597
    label: amino acid binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Electronic annotation for amino acid binding, reflecting OTC's binding of
      the amino acid substrate L-ornithine within catalysis. Subsumed by the
      catalytic molecular function.
    action: KEEP_AS_NON_CORE
    reason: >-
      Ornithine binding is part of the catalytic mechanism captured by
      GO:0004585; substrate binding is not an independent core function.
- term:
    id: GO:0016743
    label: carboxyl- or carbamoyltransferase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro2GO annotation to the parent carbamoyltransferase activity class.
      Correct but less specific than the exact GO:0004585.
    action: KEEP_AS_NON_CORE
    reason: >-
      This is a parent of the precise ornithine carbamoyltransferase activity
      term already annotated; retained as an accurate but general classification.
- term:
    id: GO:0000050
    label: urea cycle
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: >-
      Automated annotation placing OTC in the urea cycle, consistent with the
      experimental IDA annotation and UniProt pathway. Core process.
    action: ACCEPT
    reason: >-
      OTC catalyzes the citrulline-forming step of the urea cycle; central core
      process.
- term:
    id: GO:0001889
    label: liver development
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer (from rat OTC, P00481) of a
      liver-development role. OTC is highly expressed in liver but there is no
      evidence it functions in liver morphogenesis/development; this is an
      over-annotation conflating tissue of expression with a developmental role.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Tissue-restricted metabolic expression (liver, intestinal mucosa) is not
      evidence of participation in organ development. Transferred by Ensembl
      Compara from a paralog/ortholog experiment; not appropriate as a human OTC
      function.
- term:
    id: GO:0005543
    label: phospholipid binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: enables
  review:
    summary: >-
      Ensembl Compara ortholog-transfer annotation of phospholipid binding. OTC
      is a soluble matrix enzyme with no established phospholipid-binding
      function; not supported by structure or biochemistry.
    action: REMOVE
    reason: >-
      No evidence that human OTC binds phospholipids; a soluble matrix
      carbamoyltransferase. Electronic ortholog transfer without biological
      support.
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: located_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer localizing OTC to the mitochondrial
      inner membrane. This is the wrong compartment: mature OTC is a soluble
      matrix enzyme, not an inner-membrane protein.
    action: REMOVE
    reason: >-
      UniProt and experimental data place mature OTC in the mitochondrial
      matrix, not the inner membrane. The inner-membrane assignment is an
      incorrect electronic transfer.
    supported_by:
    - reference_id: PMID:6372096
      supporting_text: >-
        We have deduced the complete primary structure of the precursor of a
        human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC)
- term:
    id: GO:0007494
    label: midgut development
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer of a midgut-development role. OTC is
      expressed in intestinal mucosa but is not known to drive midgut
      development; over-annotation conflating expression site with development.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Expression in intestinal mucosa does not equate to a role in midgut
      morphogenesis; transferred electronically without biological support for
      human OTC.
- term:
    id: GO:0009410
    label: response to xenobiotic stimulus
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer (rat) of a "response to xenobiotic"
      process. Not a described function of human OTC; likely reflects a
      rat-specific expression-response experiment.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      No evidence for human OTC participating in a xenobiotic response as a core
      or peripheral function; electronic transfer from a rodent stimulus study.
- term:
    id: GO:0010043
    label: response to zinc ion
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer of a zinc-response process. Not an
      established human OTC function; over-annotation from rodent data.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      No biological support that human OTC functions in a response to zinc ion;
      transferred electronically.
- term:
    id: GO:0019240
    label: L-citrulline biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: >-
      Automated (multi-method) annotation of L-citrulline biosynthesis, matching
      the direct product of OTC and the experimental annotations of the same
      term. Core process.
    action: ACCEPT
    reason: >-
      Redundant with the experimental IDA and IBA annotations; correctly
      captures OTC's direct biosynthetic product.
- term:
    id: GO:0031667
    label: response to nutrient levels
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer of a "response to nutrient levels"
      process. While OTC activity is modulated by Lys-88 acetylation in response
      to nutrient signals (UniProt PTM), this ortholog-transferred process
      annotation is a peripheral, non-core physiological response rather than a
      demonstrated human OTC function at this term.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Electronic ortholog transfer; any nutrient-responsive regulation of OTC is
      an upstream modulation of its activity, not a core process the enzyme
      executes. Not appropriate as a core annotation.
- term:
    id: GO:0032868
    label: response to insulin
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer of an insulin-response process. Not a
      described function of human OTC; over-annotation from rodent data.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      No biological support that human OTC participates in a response to insulin
      as a gene function; electronic transfer.
- term:
    id: GO:0042301
    label: phosphate ion binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: enables
  review:
    summary: >-
      Ensembl Compara ortholog-transfer of phosphate ion binding. Inorganic
      phosphate is a product of the OTC reaction, so transient phosphate
      interaction occurs within catalysis, but as an independent molecular
      function this is a peripheral, over-annotated activity.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Phosphate handling is part of the catalytic mechanism captured by
      GO:0004585; standalone "phosphate ion binding" transferred by Ensembl
      Compara is not a distinct core function.
- term:
    id: GO:0042802
    label: identical protein binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: enables
  review:
    summary: >-
      Ensembl Compara ortholog-transfer of identical protein binding. This
      reflects the biologically real fact that active OTC is a homotrimer;
      "identical protein binding" is an uninformative surrogate for the
      self-assembly / homotrimerization role.
    action: MODIFY
    reason: >-
      UniProt records the subunit structure as "Homotrimer"; the annotation
      should point to the structural/self-association role rather than generic
      identical protein binding. Non-core relative to catalysis.
    proposed_replacement_terms:
    - id: GO:0032403
      label: protein-containing complex binding
- term:
    id: GO:0055081
    label: monoatomic anion homeostasis
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer of a monoatomic anion homeostasis
      process. Not an established human OTC function; over-annotation from
      rodent data.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      No biological support that human OTC maintains monoatomic anion
      homeostasis; electronic ortholog transfer.
- term:
    id: GO:0070781
    label: response to biotin
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl Compara ortholog-transfer of a biotin-response process. Not a
      described function of human OTC; over-annotation from rodent data.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      No biological support that human OTC participates in a response to biotin;
      electronic transfer.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: >-
      Curated immunofluorescence (HPA) localization of OTC to the mitochondrion.
      Consistent with the established mitochondrial-matrix localization.
    action: ACCEPT
    reason: >-
      Direct imaging evidence for mitochondrial localization; correct compartment.
- term:
    id: GO:0004585
    label: ornithine carbamoyltransferase activity
  evidence_type: EXP
  original_reference_id: PMID:6372096
  qualifier: enables
  review:
    summary: >-
      Experimental demonstration of OTC enzymatic activity from expression of
      cloned human OTC cDNA. Core molecular function.
    action: ACCEPT
    reason: >-
      Expression of the cloned cDNA yielded catalytically active OTC, directly
      demonstrating the ornithine carbamoyltransferase activity.
    supported_by:
    - reference_id: PMID:6372096
      supporting_text: >-
        enzymatic activity was also detected.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: EXP
  original_reference_id: PMID:3895227
  qualifier: located_in
  review:
    summary: >-
      Experimental study of OTC precursor import establishing OTC as a
      mitochondrial-matrix enzyme. Core localization.
    action: ACCEPT
    reason: >-
      OTC is imported into and functions in the mitochondrial matrix; the
      arginine-rich leader peptide directs matrix import and cleavage.
    supported_by:
    - reference_id: PMID:3895227
      supporting_text: >-
        the human mitochondrial matrix enzyme, ornithine transcarbamoylase
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9956527
  qualifier: located_in
  review:
    summary: >-
      Reactome (TAS) localization of OTC to the mitochondrial matrix within a
      reaction where OTC variants fail to synthesize L-citrulline. Correct
      compartment.
    action: ACCEPT
    reason: >-
      Matrix localization is well established; the Reactome model correctly
      places OTC in the matrix.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9956513
  qualifier: located_in
  review:
    summary: >-
      Reactome (TAS) cytosol location arising from a model of OTC leader-sequence
      variants that are NOT targeted to mitochondria. This is a
      mistrafficking/precursor context, not the localization of the mature
      functional enzyme.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The cytosolic location reflects mislocalized targeting-signal variants
      (Reactome "Leader sequence variants of OTC aren't targeted to the
      mitochondria"), not the steady-state matrix localization of wild-type OTC.
      Misleading as a general localization claim.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9956502
  qualifier: located_in
  review:
    summary: >-
      Reactome (TAS) cytosol location from a model of OTC precursor targeting to
      the mitochondrial matrix. The cytosol appears only as the transient
      location of the newly synthesized precursor before import, not the mature
      enzyme's compartment.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Reflects the pre-import precursor stage of the import pathway, not the
      functional localization of mature OTC (mitochondrial matrix).
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9959881
  qualifier: located_in
  review:
    summary: >-
      Reactome (TAS) cytosol location from an "OTC gene expression" reaction.
      Reflects the biosynthesis/expression step, not the localization of the
      functional enzyme.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Derived from a gene-expression modeling reaction rather than the mature
      enzyme's compartment; not the functional localization of OTC.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  qualifier: located_in
  review:
    summary: >-
      High-throughput mitochondrial proteomics (MitoCoP) detecting OTC in the
      high-confidence mitochondrial proteome. Supports mitochondrial
      localization.
    action: ACCEPT
    reason: >-
      Proteomic evidence corroborating mitochondrial localization; consistent
      with the matrix assignment.
    supported_by:
    - reference_id: PMID:34800366
      supporting_text: >-
        mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP)
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8986181
  qualifier: located_in
  review:
    summary: >-
      Reactome (TAS) matrix localization within the presequence-proteolysis
      (PITRM1) reaction. Correct compartment for OTC.
    action: ACCEPT
    reason: >-
      Matrix localization is well supported; the Reactome presequence-processing
      model correctly places OTC in the matrix.
- term:
    id: GO:0000050
    label: urea cycle
  evidence_type: IDA
  original_reference_id: PMID:2556444
  qualifier: involved_in
  review:
    summary: >-
      Direct experimental annotation placing OTC in the urea cycle, from
      functional analysis of OTC and its deficiency in ammonia detoxification.
      Core process.
    action: ACCEPT
    reason: >-
      OTC catalyzes an essential step of the urea cycle; loss of activity
      impairs ureagenesis and causes hyperammonemia.
    supported_by:
    - reference_id: PMID:2556444
      supporting_text: >-
        Ornithine transcarbamylase (OTC) is an important enzyme in the
        detoxification of ammonia to urea, and its deficiency is the most common
        inborn error of ureagenesis in humans.
- term:
    id: GO:0004585
    label: ornithine carbamoyltransferase activity
  evidence_type: IDA
  original_reference_id: PMID:2556444
  qualifier: enables
  review:
    summary: >-
      Direct assay of OTC enzymatic activity (wild-type versus Arg109Gln mutant)
      confirming the ornithine carbamoyltransferase activity. Core molecular
      function.
    action: ACCEPT
    reason: >-
      Specific activity of expressed wild-type OTC was measured directly and
      contrasted with an inactive mutant, demonstrating the catalytic function.
    supported_by:
    - reference_id: PMID:2556444
      supporting_text: >-
        the specific activity of mutant OTC was 100-fold lower than that of wild
        type
- term:
    id: GO:0004585
    label: ornithine carbamoyltransferase activity
  evidence_type: IDA
  original_reference_id: PMID:8112735
  qualifier: enables
  review:
    summary: >-
      Direct measurement of OTC activity for wild-type and mutant cDNAs
      expressed in Cos1 cells, confirming ornithine carbamoyltransferase
      activity. Core molecular function.
    action: ACCEPT
    reason: >-
      Enzyme activities of expressed OTC variants were quantified directly
      (0-8.9% of normal for mutants), demonstrating the catalytic function.
    supported_by:
    - reference_id: PMID:8112735
      supporting_text: >-
        Predicted OTC activities of mutant OTC cDNAs ranged from 0% to 8.9% of
        the normal level
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: PMID:3472484
  qualifier: located_in
  review:
    summary: >-
      Curated (IDA) mitochondrial localization from work on targeting of
      nuclear-encoded proteins to the mitochondrial matrix. Correct compartment.
    action: ACCEPT
    reason: >-
      OTC is a well-established nuclear-encoded protein targeted to
      mitochondria; the more specific matrix term is also annotated.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: PMID:6372096
  qualifier: located_in
  review:
    summary: >-
      Curated (IDA) mitochondrial localization from characterization of the OTC
      precursor as a nuclear-encoded mitochondrial-matrix enzyme. Correct
      compartment.
    action: ACCEPT
    reason: >-
      OTC was characterized as a human mitochondrial matrix enzyme whose
      precursor is imported into mitochondria.
    supported_by:
    - reference_id: PMID:6372096
      supporting_text: >-
        We have deduced the complete primary structure of the precursor of a
        human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC)
- term:
    id: GO:0006593
    label: L-ornithine catabolic process
  evidence_type: IDA
  original_reference_id: PMID:2556444
  qualifier: involved_in
  review:
    summary: >-
      Direct annotation of OTC's involvement in L-ornithine consumption.
      L-ornithine is the amino acid substrate consumed in the OTC reaction; this
      is a substrate-centric framing of the same catalytic function.
    action: KEEP_AS_NON_CORE
    reason: >-
      Accurate (ornithine is consumed by OTC), but this is the substrate side of
      the citrulline-biosynthetic reaction rather than a distinct core process;
      OTC's core role is better captured by the citrulline-biosynthetic / urea
      cycle terms.
- term:
    id: GO:0006593
    label: L-ornithine catabolic process
  evidence_type: IDA
  original_reference_id: PMID:8112735
  qualifier: involved_in
  review:
    summary: >-
      Direct annotation of OTC's role in L-ornithine consumption, from
      expression analysis of OTC variants. Substrate-centric framing of the OTC
      reaction.
    action: KEEP_AS_NON_CORE
    reason: >-
      Ornithine is the consumed substrate of OTC; retained as an accurate but
      non-core substrate-side description of the same catalytic activity.
- term:
    id: GO:0019240
    label: L-citrulline biosynthetic process
  evidence_type: IDA
  original_reference_id: PMID:2556444
  qualifier: involved_in
  review:
    summary: >-
      Direct experimental annotation of OTC in L-citrulline biosynthesis (the
      product of the OTC reaction), from functional analysis of wild-type versus
      inactive mutant OTC. Core process.
    action: ACCEPT
    reason: >-
      L-citrulline is the direct product of OTC; enzyme-activity assays of
      wild-type and mutant OTC directly demonstrate this biosynthetic role.
    supported_by:
    - reference_id: PMID:2556444
      supporting_text: >-
        the specific activity of mutant OTC was 100-fold lower than that of wild
        type
- term:
    id: GO:0019240
    label: L-citrulline biosynthetic process
  evidence_type: IDA
  original_reference_id: PMID:8112735
  qualifier: involved_in
  review:
    summary: >-
      Direct experimental annotation of OTC in L-citrulline biosynthesis from
      expression/activity analysis of OTC variants in Cos1 cells. Core process.
    action: ACCEPT
    reason: >-
      Measured OTC activities of variants directly demonstrate the enzyme's
      L-citrulline-biosynthetic role.
    supported_by:
    - reference_id: PMID:8112735
      supporting_text: >-
        Predicted OTC activities of mutant OTC cDNAs ranged from 0% to 8.9% of
        the normal level
- term:
    id: GO:0097272
    label: ammonium homeostasis
  evidence_type: IMP
  original_reference_id: PMID:2556444
  qualifier: involved_in
  review:
    summary: >-
      IMP annotation linking OTC function to ammonium homeostasis: loss of OTC
      activity impairs ammonia detoxification (hyperammonemia). Physiologically
      central and well supported.
    action: ACCEPT
    reason: >-
      OTC deficiency is the most common inborn error of ureagenesis and causes
      hyperammonemia, directly implicating OTC in ammonium homeostasis via the
      urea cycle.
    supported_by:
    - reference_id: PMID:2556444
      supporting_text: >-
        Ornithine transcarbamylase (OTC) is an important enzyme in the
        detoxification of ammonia to urea, and its deficiency is the most common
        inborn error of ureagenesis in humans.
- term:
    id: GO:0097272
    label: ammonium homeostasis
  evidence_type: IMP
  original_reference_id: PMID:2575934
  qualifier: involved_in
  review:
    summary: >-
      IMP annotation of OTC in ammonium homeostasis, from OTC gene/mutation
      analysis in OTC deficiency. Consistent with the urea-cycle role in ammonia
      detoxification.
    action: ACCEPT
    reason: >-
      OTC deficiency (studied via gene structure and DNA diagnosis) manifests as
      a urea-cycle failure with hyperammonemia, supporting OTC's role in
      ammonium homeostasis.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-70560
  qualifier: located_in
  review:
    summary: >-
      Reactome (TAS) matrix localization within the core catalytic reaction
      (carbamoyl phosphate + ornithine => citrulline + orthophosphate). Correct
      compartment for the functional enzyme.
    action: ACCEPT
    reason: >-
      The catalytic reaction occurs in the matrix; Reactome correctly localizes
      OTC there.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9956502
  qualifier: located_in
  review:
    summary: >-
      Reactome (TAS) matrix localization from the OTC precursor
      targeting/processing reaction (post-import, matrix-processed form). Correct
      compartment for the mature enzyme.
    action: ACCEPT
    reason: >-
      The processed OTC resides in the matrix; this Reactome annotation captures
      the correct mature-enzyme compartment.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: NAS
  original_reference_id: PMID:6372096
  qualifier: located_in
  review:
    summary: >-
      Non-traceable author statement (NAS) that OTC is a mitochondrial-matrix
      enzyme, consistent with the experimental EXP/IDA matrix annotations. Correct
      compartment.
    action: ACCEPT
    reason: >-
      Concordant with the well-supported matrix localization of OTC.
    supported_by:
    - reference_id: PMID:6372096
      supporting_text: >-
        We have deduced the complete primary structure of the precursor of a
        human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC)
core_functions:
- description: >-
    Ornithine carbamoyltransferase activity in the mitochondrial matrix,
    catalyzing the condensation of carbamoyl phosphate with L-ornithine to form
    L-citrulline and inorganic phosphate (EC 2.1.3.3), the citrulline-forming
    step of the urea cycle.
  molecular_function:
    id: GO:0004585
    label: ornithine carbamoyltransferase activity
  directly_involved_in:
  - id: GO:0000050
    label: urea cycle
  - id: GO:0019240
    label: citrulline biosynthetic process
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  supported_by:
  - reference_id: PMID:2556444
    supporting_text: >-
      Ornithine transcarbamylase (OTC) is an important enzyme in the
      detoxification of ammonia to urea, and its deficiency is the most common
      inborn error of ureagenesis in humans.
  - reference_id: PMID:8112735
    supporting_text: >-
      Predicted OTC activities of mutant OTC cDNAs ranged from 0% to 8.9% of the
      normal level
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:2556444
  title: An arginine to glutamine mutation in residue 109 of human ornithine transcarbamylase
    completely abolishes enzymatic activity in Cos1 cells.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Abstract-only cache; directly establishes OTC's role in ammonia
      detoxification/ureagenesis and demonstrates loss of catalytic activity in a
      disease mutant. Supports the core MF and urea-cycle/citrulline annotations.
- id: PMID:2575934
  title: Structure of the ornithine transcarbamylase (OTC) gene and DNA diagnosis
    of OTC deficiency.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Abstract-only; OTC gene structure and DNA diagnosis of OTC deficiency.
      Supports the ammonium-homeostasis (deficiency) annotation via genotype.
- id: PMID:3472484
  title: 'Targeting of nuclear-encoded proteins to the mitochondrial matrix: implications
    for human genetic defects.'
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Title-only cache (no abstract text). Review of nuclear-encoded protein
      targeting to the mitochondrial matrix; supports mitochondrial localization
      annotation. No verbatim quote used.
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics
    in cellular context.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      MitoCoP high-confidence mitochondrial proteome; supports the HTP
      mitochondrial localization annotation.
- id: PMID:3895227
  title: Arginine in the leader peptide is required for both import and proteolytic
    cleavage of a mitochondrial precursor.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Abstract-only; establishes OTC as a mitochondrial-matrix enzyme whose
      arginine-rich leader peptide directs import and cleavage. Supports matrix
      localization.
- id: PMID:6372096
  title: Structure and expression of a complementary DNA for the nuclear coded precursor
    of human mitochondrial ornithine transcarbamylase.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Abstract-only; deduces the OTC precursor primary structure, defines it as a
      human mitochondrial-matrix enzyme, and detects enzymatic activity on
      expression. Supports MF and matrix localization.
- id: PMID:8112735
  title: Expression of four mutant human ornithine transcarbamylase genes in cultured
    Cos 1 cells relates to clinical phenotypes.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Abstract-only; quantifies OTC enzyme activities of wild-type and mutant
      cDNAs. Supports the core MF and citrulline-biosynthetic annotations.
- id: Reactome:R-HSA-70560
  title: carbamoyl phosphate + ornithine => citrulline + orthophosphate
  findings: []
- id: Reactome:R-HSA-8986181
  title: PITRM1 proteolyzes mitochondrial targeting peptides (presequences)
  findings: []
- id: Reactome:R-HSA-9956502
  title: OTC precursor is targeted to the mitochondrial matrix and processed
  findings: []
- id: Reactome:R-HSA-9956513
  title: Leader sequence variants of OTC aren't targeted to the mitochondria
  findings: []
- id: Reactome:R-HSA-9956527
  title: OTC variants don't synthesize L-citrulline
  findings: []
- id: Reactome:R-HSA-9959881
  title: OTC gene expression
  findings: []

OTC

Gene Description

Existing Annotations Review

Core Functions

Ornithine carbamoyltransferase activity in the mitochondrial matrix, catalyzing the condensation of carbamoyl phosphate with L-ornithine to form L-citrulline and inorganic phosphate (EC 2.1.3.3), the citrulline-forming step of the urea cycle.

References

Deep Research

Falcon

Comprehensive Research Report: Human Ornithine Transcarbamylase (OTC)

1. Gene Identity and Genomic Organization

2. Protein Structure and Quaternary Assembly

Domain Organization

Trimeric Assembly

3. Enzymatic Reaction and Catalytic Mechanism

Reaction Catalyzed

Kinetic Mechanism

Catalytic Residues and Mechanism

Substrate Specificity and Inhibition

4. Subcellular Localization and Mitochondrial Import

Mitochondrial Targeting and Import

Localization Within Mitochondria

Tissue Distribution

5. Role in the Urea Cycle and Metabolic Pathways

The Urea Cycle

Intestinal Citrulline Production

Regulation of OTC Expression and Activity

6. Clinical Significance: OTC Deficiency

Prevalence and Inheritance

Clinical Manifestations

Current Treatments and Gene Therapy

7. Emerging Roles in Cancer and Chronic Disease

8. Summary

Artifacts

Citations

📚 Additional Documentation

Notes

OTC (Ornithine transcarbamylase / ornithine carbamoyltransferase, P00480) — review notes

Core biology (verified)

Import / precursor processing

Enzyme activity / disease functional evidence

OTC deficiency (OTCD; MONDO:0010703)

Localization proteomics

Annotation judgments

Deep research

📄 View Raw YAML