N-acetylglutamate synthase (NAGS; EC 2.3.1.1) is a nuclear-encoded, mitochondrial-matrix enzyme that catalyzes the formation of N-acetyl-L-glutamate (NAG) from L-glutamate and acetyl-CoA. In mammals, NAG is the obligate allosteric activator of carbamoyl phosphate synthetase 1 (CPS1), the first and rate-limiting enzyme of the urea cycle; NAGS therefore functions as the regulatory gate of hepatic ureagenesis and ammonia detoxification. Although NAG is not consumed stoichiometrically in the urea cycle, the cycle cannot proceed without it. The enzyme is a homotetramer whose subunits comprise an N-terminal amino-acid-kinase (AAK) domain that binds the allosteric activator L-arginine (which enhances mammalian NAGS activity) and a C-terminal GCN5-related N-acetyltransferase (NAT) domain that is catalytically competent on its own. NAGS is expressed mainly in liver, kidney and small intestine and is imported into the mitochondrion via a cleaved N-terminal transit peptide. Biallelic loss-of-function variants cause N-acetylglutamate synthase deficiency, an autosomal recessive urea cycle disorder presenting with hyperammonemia (without orotic aciduria) that, uniquely among urea cycle disorders, is specifically treatable with the NAG analogue carglumic acid.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006526 L-arginine biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) annotation to arginine biosynthesis. NAGS catalyzes the acetylation of L-glutamate to N-acetyl-L-glutamate, the committed first step of the acetylated ornithine route to L-arginine (UniProt PATHWAY UPA00068: "N(2)-acetyl-L-ornithine from L-glutamate: step 1/4"). This is the conserved ancestral function of the family across bacteria, plants and fungi. In mammals the NAG product is primarily used to activate CPS1 in the urea cycle rather than to make net arginine, but the enzymatic role at the head of the acetylornithine pathway is real and correctly captured. Reason: Well-supported family-conserved involvement in the acetylglutamate branch that leads to arginine biosynthesis; consistent with UniProt PATHWAY annotation. Supporting Evidence: PMID:23894642 In microorganisms and plants, NAG is further converted to NAG phosphate by NAG kinase (NAGK, EC 2.7.2.8) to continue the L-arginine biosynthetic pathway |
| GO:0005759 mitochondrial matrix | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) annotation placing NAGS activity in the mitochondrial matrix. Human NAGS was purified from human liver mitochondria and carries a cleaved N-terminal mitochondrial transit peptide; the enzyme acts in the matrix where its substrates and CPS1 reside. Reason: Core subcellular location, supported experimentally by purification from human liver mitochondria and by the presence of a mitochondrial targeting sequence. Supporting Evidence: PMID:7126172 was isolated from human liver mitochondria by precipitation with (NH4)2SO4 |
| GO:0004042 L-glutamate N-acetyltransferase activity, acting on acetyl-CoA as donor | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) annotation of the core catalytic activity (EC 2.3.1.1): acetylation of L-glutamate by acetyl-CoA to yield N-acetyl-L-glutamate and CoA (RHEA:24292). This is the defining molecular function of NAGS and is directly supported by human enzymology and structure. Reason: This is the primary, experimentally established molecular function of the gene product. Supporting Evidence: PMID:12459178 N-acetylglutamate synthase (NAGS, E.C. 2.3.1.1) is a mitochondrial enzyme catalyzing the formation of N-acetylglutamate (NAG) |
| GO:0006536 glutamate metabolic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) annotation to glutamate metabolism. NAGS consumes L-glutamate as a substrate, so the annotation is not incorrect, but "glutamate metabolic process" is a broad parent term that does not convey the specific biological role of NAGS (making the CPS1 activator NAG / feeding the acetylornithine pathway). Reason: True but generic; L-glutamate is the substrate, yet the specific processes (arginine biosynthesis / urea cycle regulation) are more informative and are captured by other annotations. Retain as a peripheral, non-core process term. Supporting Evidence: PMID:23894642 catalyzes the conversion of AcCoA and L-glutamate to CoA and N-acetyl-L-glutamate (NAG) |
| GO:0004042 L-glutamate N-acetyltransferase activity, acting on acetyl-CoA as donor | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (IEA) annotation of the core catalytic activity from combined automated methods (ARBA, RHEA:24292, InterPro, ortholog transfer). Consistent with the experimentally established EC 2.3.1.1 activity. Reason: Correct and specific molecular-function assignment; agrees with experimental annotations. |
| GO:0005759 mitochondrial matrix | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic (IEA) annotation from the UniProt Swiss-Prot subcellular-location mapping (SL-0170, mitochondrion matrix). Matches the experimentally supported matrix localization. Reason: Correct location, corroborated by purification of the enzyme from liver mitochondria and by the IBA/TAS matrix annotations. |
| GO:0006526 L-arginine biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (IEA) annotation to arginine biosynthesis from InterPro/UniPathway (UPA00068). Same rationale as the IBA arginine-biosynthesis annotation: NAGS performs the committed acetylglutamate-forming step of the acetylornithine pathway. Reason: Consistent with the conserved pathway role and with the IBA annotation of the same term. |
| GO:0016747 acyltransferase activity, transferring groups other than amino-acyl groups | IEA GO_REF:0000002 | MODIFY | Summary: Electronic (IEA) InterPro2GO annotation derived from the generic GNAT/N-acetyltransferase domain signature (IPR000182). This is a broad parent term ("acyltransferase activity, transferring groups other than amino-acyl groups") that only states NAGS is an acyltransferase. The specific, experimentally established activity of this enzyme, GO:0004042 (L-glutamate N-acetyltransferase activity, acting on acetyl-CoA as donor), is already annotated, so the generic parent is an over-annotation. Reason: The generic acyltransferase term should be refined to the known specific activity GO:0004042. NAGS is not a general acyltransferase acting on multiple acceptors; its physiological (and only demonstrated) acetyl-acceptor is L-glutamate. Replace with the specific EC 2.3.1.1 term. Proposed replacements: L-glutamate N-acetyltransferase activity, acting on acetyl-CoA as donor Supporting Evidence: PMID:23894642 The NAT domain has a typical GCN5-related NAT fold and a site that catalyzes NAG synthesis |
| GO:0090461 intracellular glutamate homeostasis | IDA PMID:21757002 The nuclear receptor FXR regulates hepatic transport and met... | KEEP AS NON CORE | Summary: IDA annotation (BHF-UCL) from a study showing that the nuclear receptor FXR regulates hepatic glutamine/glutamate metabolism and directly induces NAGS expression via an FXRE in the NAGS promoter. In this context NAGS contributes to handling of the intracellular glutamate pool by consuming glutamate to form NAG. The cached publication is abstract-only (full_text_available: false); the experimental annotation reflects the curator's reading of the full text and should not be removed. Reason: NAGS uses glutamate as a substrate and its expression is coupled to hepatic glutamate/glutamine metabolism, so a role in intracellular glutamate homeostasis is defensible but peripheral. It is not the core evolved function (production of the CPS1 activator NAG), so retain as non-core rather than remove; defer to the experimental curator. Supporting Evidence: PMID:21757002 Glutamine is taken up by periportal hepatocytes and is the major source of ammonia for urea synthesis and glutamate for N-acetylglutamate (NAG) synthesis, which is catalyzed by the N-acetylglutamate synthase (NAGS). |
| GO:0000050 urea cycle | IDA PMID:21757002 The nuclear receptor FXR regulates hepatic transport and met... | ACCEPT | Summary: IDA annotation (BHF-UCL) to the urea cycle. NAGS produces N-acetyl-L-glutamate, the obligate allosteric activator of CPS1, the first enzyme of the urea cycle; loss of NAGS causes hyperammonemia because CPS1 is inactive without NAG. NAGS is thus the regulatory gate of ureagenesis. The cached reference is abstract-only, but the urea-cycle role of NAGS is independently and strongly established by multiple sources. Reason: Core biological role. Although NAG is not stoichiometrically consumed in the cycle, NAGS is indispensable for urea cycle function; the annotation correctly captures this involvement. Supporting Evidence: PMID:21757002 major source of ammonia for urea synthesis and glutamate for N-acetylglutamate (NAG) synthesis, which is catalyzed by the N-acetylglutamate synthase (NAGS) PMID:12459178 Patients with NAGS deficiency develop hyperammonemia because CPSI is inactive without NAG. |
| GO:0004042 L-glutamate N-acetyltransferase activity, acting on acetyl-CoA as donor | TAS PMID:21757002 The nuclear receptor FXR regulates hepatic transport and met... | ACCEPT | Summary: TAS annotation of the core catalytic activity (EC 2.3.1.1). Redundant with the multiple experimental IDA/EXP and IBA/IEA annotations of the same term; correct. Reason: Correct assignment of the specific molecular function; duplicate of the experimentally supported activity. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9955697 | ACCEPT | Summary: TAS annotation (Reactome) placing NAGS in the mitochondrial matrix, consistent with all other localization evidence. Reason: Correct core location; agrees with the IBA/IEA/TAS matrix annotations. |
| GO:0004042 L-glutamate N-acetyltransferase activity, acting on acetyl-CoA as donor | EXP PMID:12459178 Cloning and expression of the human N-acetylglutamate syntha... | ACCEPT | Summary: Experimental (EXP) annotation. Recombinant human NAGS (conserved domain) complemented an NAGS-deficient E. coli strain and displayed arginine-responsive NAGS catalytic activity, directly demonstrating the EC 2.3.1.1 activity. Reason: Direct experimental demonstration of the core catalytic activity in the human enzyme. Supporting Evidence: PMID:12459178 the recombinant protein has arginine-responsive NAGS catalytic activity |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: High-throughput (HTP) annotation from a quantitative high-confidence human mitochondrial proteome study, identifying NAGS as a mitochondrial protein. Consistent with the more precise matrix localization; "mitochondrion" is a correct but less specific parent term. Reason: Correct localization supported by proteomics; the specific compartment (matrix) is captured by other annotations. |
| GO:0004042 L-glutamate N-acetyltransferase activity, acting on acetyl-CoA as donor | IDA PMID:23894642 Crystal structure of the N-acetyltransferase domain of human... | ACCEPT | Summary: IDA annotation from the crystal structure and enzymology of the human NAGS N-acetyltransferase (NAT) domain in complex with N-acetyl-L-glutamate. The study directly measured NAGS catalytic activity (with substrate-dependence, mutagenesis of active-site residues) confirming EC 2.3.1.1. Reason: Direct experimental (structural + kinetic) demonstration of the core catalytic activity. Supporting Evidence: PMID:23894642 NAGS deficiency results in elevated levels of plasma ammonia which is neurotoxic. We report herein the first crystal structure of human NAGS, that of the catalytic N-acetyltransferase (hNAT) domain with N-acetyl-L-glutamate bound at 2.1 Å resolution. |
| GO:0004042 L-glutamate N-acetyltransferase activity, acting on acetyl-CoA as donor | IDA PMID:7126172 Purification and properties of acetyl-CoA:L-glutamate N-acet... | ACCEPT | Summary: IDA annotation from the classic purification and kinetic characterization of acetyl-CoA:L-glutamate N-acetyltransferase (EC 2.3.1.1) from human liver, establishing the substrate/product kinetics of the enzyme. Reason: Direct experimental demonstration of the core catalytic activity in native human enzyme. Supporting Evidence: PMID:7126172 Acetyl-CoA:L-glutamate N-acetyltransferase (amino acid acetyltransferase, EC 2.3.1.1) was isolated from human liver mitochondria |
| GO:0005739 mitochondrion | IDA PMID:7126172 Purification and properties of acetyl-CoA:L-glutamate N-acet... | ACCEPT | Summary: IDA annotation of mitochondrial localization, based on purification of the enzyme from human liver mitochondria. Correct; the finer matrix localization is captured by other annotations. Reason: Correct experimentally supported localization; consistent with the matrix annotations. Supporting Evidence: PMID:7126172 was isolated from human liver mitochondria |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-70542 | ACCEPT | Summary: TAS annotation (Reactome) for the reaction "glutamate + acetyl CoA => N-acetyl glutamate + CoA" localizing NAGS to the mitochondrial matrix. Consistent with all other localization evidence. Reason: Correct core location; duplicate of the matrix annotations. |
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