N-acetylglutamate synthase

N-Acetylglutamate synthase
N-acetylglutamate synthase/kinase tetramer, Maricaulis maris
Identifiers
Symbol	NAGS
Entrez	162417
HUGO	17996
OMIM	608300
RefSeq	NM_153006
UniProt	Q8N159
Other data
EC number	2.3.1.1
Locus	Chr. 17 q21.31

N-acetylglutamate synthase (NAGS) is an enzyme that catalyses the production of N-Acetylglutamate (NAG) from glutamate and acetyl-CoA.

Put simply NAGS catalyzes the following reaction:

NAGS, a member of the N-acetyltransferase family of enzymes, is present in both prokaryotes and eukaryotes, although its role and structure differ widely depending on the species. NAG can be used in the production of ornithine and arginine, two important amino acids, or as an allosteric cofactor for carbamoyl phosphate synthase (CPS1). In mammals, NAGS is expressed primarily in the liver and small intestine, and is localized to the mitochondrial matrix.

Most prokaryotes (bacteria) and lower eukaryotes (fungi, green algae, plants, etc.) produce NAG through orinithine acetyltransferase (OAT), which is part of a ‘cyclic’ ornithine production pathway. NAGS is therefore used in a supportive role, replenishing NAG reserves as required. In some plants and bacteria, however, NAGS catalyzes the first step in a ‘linear’ arginine production pathway.

The protein sequences of NAGS between prokaryotes, lower eukaryotes and higher eukaryotes have shown a remarkable lack of similarity. Sequence identity between prokaryotic and eukaryotic NAGS is largely <30%, while sequence identity between lower and higher eukaryotes is ~20%.

Enzyme activity of NAGS is modulated by L-arginine, which acts as an inhibitor in plant and bacterial NAGS, but an effector in vertebrates. While the role of arginine as an inhibitor of NAG in ornithine and arginine synthesis is well understood, there is some controversy as to the role of NAG in the urea cycle. The currently accepted role of NAG in vertebrates is as an essential allosteric cofactor for CPS1, and therefore it acts as the primary controller of flux through the urea cycle. In this role, feedback regulation from arginine would act to signal NAGS that ammonia is plentiful within the cell, and needs to be removed, accelerating NAGS function. As it stands, the evolutionary journey of NAGS from essential synthetic enzyme to primary urea cycle controller is yet to be fully understood.