GCSH is the H-protein of the mitochondrial glycine cleavage system (GCS). It is a small, non-catalytic lipoyl-carrier protein: a lipoyl group is covalently attached to a specific lysine (N6-lipoyllysine, Lys107) and acts as a swinging arm that shuttles the reaction intermediate between the other GCS components. During glycine cleavage the H-protein accepts the aminomethyl moiety produced by the P-protein (glycine dehydrogenase/decarboxylating, GLDC), presents the aminomethyl-dihydrolipoyl intermediate to the T-protein (aminomethyltransferase, AMT) which releases ammonia and transfers the methylene group to tetrahydrofolate, and its resulting dihydrolipoyl form is reoxidised by the L-protein (dihydrolipoyl dehydrogenase, DLD). Because it is a substrate-carrier rather than an enzyme, its most informative functional attributes are its covalently bound lipoyl cofactor, its participation in glycine catabolism/decarboxylation, and its membership in the glycine cleavage complex. GCSH is localised to the mitochondrion/mitochondrial matrix and its lipoyl cofactor is generated in place by octanoyl transfer (LIPT2) and sulfur insertion (LIAS); it additionally serves as a lipoyl donor for other mitochondrial 2-oxo-acid dehydrogenase E2 subunits (DLAT, DLST) via LIPT1, a moonlighting role in cellular bioenergetics. Biallelic loss-of-function variants in GCSH are a rare cause of nonketotic hyperglycinemia (glycine encephalopathy), presenting as a combined disorder of glycine metabolism and protein lipoylation (multiple mitochondrial dysfunctions syndrome 7).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) annotation placing GCSH in the mitochondrion. GCSH carries an N-terminal mitochondrial transit peptide and the entire glycine cleavage system operates in the mitochondrial matrix; localization is experimentally confirmed for the human protein. Reason: Correct compartment, well supported across the GcvH family and by experimental evidence for the human protein. The more precise mitochondrial matrix location is captured by other annotations. Supporting Evidence: PMID:36190515 Representative merged images (40X objective lens) showing colocalization of mitochondrial marker cytochrome C (red) and GCS H-protein (green) in knockdown COS7 cells co-transfected with pCMV-GCSH WT or variants. |
| GO:0005960 glycine cleavage complex | IBA GO_REF:0000033 | ACCEPT | Summary: GCSH is the H-protein subunit of the glycine cleavage complex. This is the defining, core cellular-component annotation for the gene and is strongly supported phylogenetically and experimentally. Reason: Core annotation. GCSH is one of the four canonical GCS proteins (P/H/T/L) and functions as the lipoyl-carrier subunit of the complex. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt SUBUNIT: Interacts with GLDC (By similarity). The glycine cleavage PMID:36190515 Lipoate is also a critical cofactor of the glycine cleavage enzyme system (GCS), where the H-protein itself is the lipoate carrier of this enzyme complex. |
| GO:0019464 glycine decarboxylation via glycine cleavage system | IBA GO_REF:0000033 | ACCEPT | Summary: GCSH participates in glycine decarboxylation via the glycine cleavage system. Its lipoyl arm accepts the aminomethyl group from the P-protein and presents it to the T-protein, making it essential to the cleavage reaction. Core biological-process annotation. Reason: Core function, precisely describing the GCS pathway in which GCSH acts. Strongly supported phylogenetically and by the functional literature. Supporting Evidence: PMID:36190515 Glycine is decarboxylated to carbon dioxide and the resulting aminomethyl-group transferred to lipoyl-H-protein as aminomethyl-lipoate. The T-protein releases ammonia while transferring the methyl-group to tetrahydrofolate. The L-protein reoxidizes the reduced lipoyl-group. |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation from the UniProt subcellular-location mapping (SL-0173 Mitochondrion). Correct but less specific than the mitochondrial matrix annotations. Reason: Consistent with experimental localization; a broadly correct IEA. Retained as a supporting, non-core localization annotation. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005960 glycine cleavage complex | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO electronic annotation (GCV_H family IPR002930/IPR017453) placing GCSH in the glycine cleavage complex. Duplicates the well-supported IBA/TAS complex-membership annotation. Reason: Correct family-to-complex mapping, consistent with the experimentally established role of GCSH as the H-protein subunit. Supporting Evidence: PMID:36190515 Lipoate is also a critical cofactor of the glycine cleavage enzyme system (GCS), where the H-protein itself is the lipoate carrier of this enzyme complex. |
| GO:0019464 glycine decarboxylation via glycine cleavage system | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO electronic annotation to the GCS decarboxylation process. Duplicates the core IBA/TAS process annotation. Reason: Correct family-based mapping to the pathway in which GCSH acts. Supporting Evidence: PMID:36190515 Glycine is decarboxylated to carbon dioxide and the resulting aminomethyl-group transferred to lipoyl-H-protein as aminomethyl-lipoate. The T-protein releases ammonia while transferring the methyl-group to tetrahydrofolate. The L-protein reoxidizes the reduced lipoyl-group. |
| GO:0005515 protein binding | IPI PMID:16189514 Towards a proteome-scale map of the human protein-protein in... | MARK AS OVER ANNOTATED | Summary: Bare protein binding IPI from a large-scale yeast two-hybrid interactome screen. The interactor is not a glycine-cleavage-system or lipoylation-pathway partner and this term conveys no specific molecular function for GCSH. Reason: protein binding (GO:0005515) is uninformative; the supporting high-throughput interaction does not correspond to GCSH's known biology (a substrate-carrier acting via its covalent lipoyl arm). Per curation policy this experimental IPI is not removed but flagged as an over-annotation. The informative molecular function for GCSH is its lipoyl-carrier role, captured by the proposed lipoic acid binding term. Supporting Evidence: PMID:16189514 Here we describe an initial version of a proteome-scale map of |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: Bare protein binding IPI from the HuRI binary interactome map. The recorded interactors (e.g. NMI, MAGEA6/11, MED11, MIS18A, RHBDD2) are not GCS or lipoylation-pathway partners and the term is uninformative for GCSH function. Reason: protein binding (GO:0005515) tells us nothing about GCSH's actual function. The high-throughput binary hits do not reflect its role as the lipoyl-carrier H-protein of the GCS. Retained (experimental IPI) but flagged as over-annotation; the informative MF is the proposed lipoic acid binding term. Supporting Evidence: PMID:32296183 reference map of the human binary protein interactome |
| GO:0006091 generation of precursor metabolites and energy | TAS PMID:36190515 Pathogenic variants in GCSH encoding the moonlighting H-prot... | KEEP AS NON CORE | Summary: Reflects GCSH's moonlighting role in lipoate biosynthesis and transfer to bioenergetic 2-oxo-acid dehydrogenases (PDH via DLAT, 2-KGDH via DLST), which is upstream of energy metabolism. Correct but a broad, non-core process for this gene. Reason: GCSH indirectly supports energy metabolism by providing the lipoyl cofactor required for PDH and 2-KGDH activity, but the term is very general and describes a downstream consequence rather than GCSH's direct molecular role. Kept as a non-core annotation. Supporting Evidence: PMID:36190515 The H-protein is pivotal in the biosynthesis and transfer of the cofactor lipoate to several critical cellular energetics enzymes |
| GO:0019464 glycine decarboxylation via glycine cleavage system | TAS PMID:1671321 The glycine cleavage system: structure of a cDNA encoding hu... | ACCEPT | Summary: Author-traceable annotation (original human H-protein cDNA paper) to the glycine cleavage decarboxylation process. Duplicates and corroborates the core IBA annotation. Reason: Core process, supported by the foundational literature establishing GCSH as the H-protein of the glycine cleavage system. Supporting Evidence: PMID:1671321 which encodes the entire structure of a file:human/GCSH/GCSH-uniprot.txt FUNCTION: The glycine cleavage system catalyzes the degradation of |
| GO:0051604 protein maturation | TAS PMID:36190515 Pathogenic variants in GCSH encoding the moonlighting H-prot... | KEEP AS NON CORE | Summary: Captures GCSH's moonlighting role as a lipoyl donor for maturation (lipoylation) of the E2 subunits DLAT and DLST. Legitimate but a secondary, non-core process relative to glycine cleavage. Reason: GCSH provides the lipoyl group transferred by LIPT1 to mature bioenergetic E2 enzymes, so the annotation is accurate; however this moonlighting maturation role is distinct from GCSH's core carrier function in the GCS. Kept as non-core. A more specific term such as protein lipoylation (GO:0009249) would better describe this role. Supporting Evidence: PMID:36190515 The H-protein is pivotal in the biosynthesis and transfer of the cofactor lipoate to several critical cellular energetics enzymes file:human/GCSH/GCSH-uniprot.txt in the lipoylation of enzymes involved in cellular energetics such as |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-5693967 | ACCEPT | Summary: Reactome-traceable localization to the mitochondrial matrix, where the P-protein step of glycine cleavage produces the aminomethyl-lipoyl-GCSH intermediate. Precise, core localization. Reason: The glycine cleavage system and GCSH act in the mitochondrial matrix; this is the most specific and correct localization annotation. Supporting Evidence: PMID:36190515 Representative merged images (40X objective lens) showing colocalization of mitochondrial marker cytochrome C (red) and GCS H-protein (green) in knockdown COS7 cells co-transfected with pCMV-GCSH WT or variants. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-5693977 | ACCEPT | Summary: Reactome-traceable matrix localization (T-protein step transferring the GCSH-bound aminomethyl group to tetrahydrofolate). Precise, core localization. Reason: Correct and specific localization for GCSH within the GCS reaction cycle in the matrix. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: High-throughput proteomics annotation placing GCSH in the high-confidence human mitochondrial proteome (MitoCoP). Correct compartment; less specific than the matrix annotations. Reason: Orthogonal high-throughput support for mitochondrial localization. Supporting Evidence: PMID:34800366 defined a human mitochondrial high-confidence proteome (MitoCoP) of 1,134 protein-coding genes |
| GO:0005739 mitochondrion | EXP PMID:36190515 Pathogenic variants in GCSH encoding the moonlighting H-prot... | ACCEPT | Summary: Direct experimental evidence: GCS H-protein colocalizes with the mitochondrial marker cytochrome C by immunofluorescence in COS7 cells. Confirms mitochondrial localization. Reason: Experimental localization for the human protein; strongly supports the mitochondrion annotation. The matrix sub-compartment is captured by other annotations. Supporting Evidence: PMID:36190515 Representative merged images (40X objective lens) showing colocalization of mitochondrial marker cytochrome C (red) and GCS H-protein (green) in knockdown COS7 cells co-transfected with pCMV-GCSH WT or variants. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-5694018 | ACCEPT | Summary: Reactome-traceable matrix localization (L-protein reoxidation of dihydrolipoyl-GCSH). Precise, core localization. Reason: Correct and specific localization for GCSH within the matrix GCS reaction cycle. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-6792572 | ACCEPT | Summary: Reactome-traceable matrix localization associated with the lipoyl-transfer reaction from lipoyl-GCSH to DBT/DLST (moonlighting lipoylation role). Correct compartment. Reason: GCSH and the lipoyl-transfer machinery act in the mitochondrial matrix; correct and specific localization. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-6793591 | ACCEPT | Summary: Reactome-traceable matrix localization associated with LIAS synthesis of lipoyl-GCSH. Correct compartment. Reason: Lipoyl cofactor synthesis on GCSH occurs in the mitochondrial matrix; correct and specific localization. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9858752 | ACCEPT | Summary: Reactome-traceable matrix localization associated with the lipoyl-transfer reaction from lipoyl-GCSH to DLAT (moonlighting lipoylation role). Correct compartment. Reason: GCSH-mediated lipoyl transfer to DLAT occurs in the mitochondrial matrix; correct and specific localization. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-6793590 | ACCEPT | Summary: Reactome-traceable matrix localization associated with the LIPT2 octanoyl-transfer reaction generating octanoyl-GCSH. Correct compartment. Reason: Lipoyl-cofactor biogenesis on GCSH occurs in the mitochondrial matrix; correct and specific localization. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005739 mitochondrion | TAS PMID:3348809 Cloning of cDNA encoding human H-protein, a constituent of t... | ACCEPT | Summary: Author-traceable annotation from the human H-protein cloning paper. Correct compartment; less specific than the matrix annotations. Reason: Consistent with all other localization evidence; foundational reference for GCSH identity. Supporting Evidence: PMID:3348809 a constituent of the glycine cleavage |
| GO:0005960 glycine cleavage complex | TAS PMID:3348809 Cloning of cDNA encoding human H-protein, a constituent of t... | ACCEPT | Summary: Author-traceable annotation establishing GCSH (H-protein) as a constituent of the glycine cleavage system. Core complex-membership annotation. Reason: Core annotation, directly supported by the cloning paper that identifies GCSH as an H-protein constituent of the glycine cleavage system. Supporting Evidence: PMID:3348809 a constituent of the glycine cleavage |
| GO:0031405 lipoic acid binding | IDA PMID:1671321 The glycine cleavage system: structure of a cDNA encoding hu... | NEW | Summary: GCSH binds one lipoic acid (lipoate) cofactor covalently on Lys107 (N6-lipoyllysine), via its lipoyl-binding domain. This covalently bound lipoyl group is the swinging arm that carries the reaction intermediate through the glycine cleavage cycle; lipoic acid binding is the informative molecular function of this otherwise non-catalytic carrier protein. Reason: GOA lists only bare protein binding as a molecular function. The lipoyl-carrier role is GCSH's defining molecular activity and is directly supported by the covalent (R)-lipoate cofactor (PMID:1671321), the N6-lipoyllysine modification at position 107, and the lipoyl-binding (biotinyl/lipoyl attachment) domain in UniProt. GO:0031405 lipoic acid binding is the appropriate, informative term. Proposed as a new molecular-function annotation. Supporting Evidence: file:human/GCSH/GCSH-uniprot.txt Binds 1 lipoyl cofactor covalently. file:human/GCSH/GCSH-uniprot.txt N6-lipoyllysine PMID:36190515 Lipoate is also a critical cofactor of the glycine cleavage enzyme system (GCS), where the H-protein itself is the lipoate carrier of this enzyme complex. |
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Download this section (compressed HTML)Q: Is there a dedicated GO molecular-function term for the lipoyl-swinging-arm/substrate-carrier role of GCSH-type H-proteins, distinct from the generic lipoic acid binding, that would better capture its aminomethyl-carrier activity within the glycine cleavage complex?
Q: How should GCSH's moonlighting role as a lipoyl donor for DLAT/DLST (protein lipoylation) be represented relative to its core carrier role in the GCS, e.g. via protein lipoylation (GO:0009249)?
Experiment: Structural and kinetic characterization of the swinging lipoyl arm (e.g. the role of Ser67 in protecting the aminomethyl group) to define GCSH's carrier mechanism in the human GCS.
Hypothesis: The swinging lipoyl arm of GCSH protects and transfers the aminomethyl intermediate during glycine cleavage, a carrier mechanism separable from its lipoyl-donor role.
Type: structural biology / enzyme kinetics
Experiment: Quantify the relative contribution of GCSH to glycine cleavage versus E2-enzyme lipoylation in human cells (e.g. using separation-of-function GCSH variants) to clarify the two activities.
Hypothesis: GCSH's contributions to glycine cleavage and to E2-enzyme lipoylation are genetically separable moonlighting activities.
Type: cell biology / biochemistry
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