N-Acetylglutamate synthetase

Schubiger G, Bachmann C, Barben P, et al. 1991. N-acetylglutamate synthetase deficiency diagnosis, management and follow-up of a rare disorder of ammonia detoxication. Eur J Pediatr 150(5) 353-356. 

Coude FX, Sweetman L, Nyhan WL. Inhibition by propionyl-coenzyme A of N-acetylglutamate synthetase in rat Uver mitochondria. A possible explanation for hyperammonemia in propionic and methylmalonic acidemia. J Clin Invest. 1979 64(6) 1544-51. 

NAGS deficiency N-acetylglutamate synthetase NAGS Markedly elevated Absent to low... 

The precursor for ornithine synthesis is N-acetylglutamate, which is also an obligatory activator of carbamyl phosphate synthetase. This provides a regulatory mechanism — if N-acetylglutamate is not available for ornithine synthesis (and hence there would be impaired activity of the urea synthesis cycle), then ammonium is not incorporated into carbamyl phosphate. This can be a cause of hyperammonaemia in a variety of metabolic disturbances that lead to either a lack of acetyl CoA for N-acetyl glutamate synthesis or an accumulation of propionyl CoA, which is a poor substrate for, and hence an inhibitor of, N-acetylglutamate synthetase. 

Rate-limiting enzyme carbamoyl phosphate synthetase-1 (activated by N-acetylglutamate)... 

N-acetylglutamate is an allosteric activator of the synthetase. The concentration in the liver increases several - fold after a meal. 

In view of the toxicity of ammonia, complete absence of any one of the enzymes of the cycle is fatal. Nonetheless, disorders of the cycle do occur, which are caused by a low activity of one of the enzymes or carbamoyl phosphate synthetase. In addition, defects in N-acetylglutamate synthase have been reported, but they are very rare. With the exception of ornithine transcarbamoylase, the deficiencies have an autosomal recessive mode of inheritance. The transcarbamoylase deficiency is inherited as an X-linked dominant trait, usually lethal in male patients. A deficiency of carbamoyl phosphate synthetase, ornithine transcarbamoylase or argininosuccinate synthetase results in accumulation and excretion of citrulline. A deficiency of argininosuccinate lyase results in the accumulation and excretion of argininosuccinate and arginine (Table 10.5). The abbreviations CPSD, OTCD, ASD, ALD and AD stand, respectively, for the deficiencies of these enzymes, where D stands for deficiency. 

ACETYL-CoA ACYLTRANSFERASE ACETYL-CoAACP TRANSACYLASE ACETYL-CoA CARBOXYLASE ACETYL-CoA SYNTHETASE N-ACETYLGLUTAMATE SYNTHASE... 

Formation and degradation of N-acetylglutamate, an allosteric activator of carbamoyl phosphate synthetase I. 

Carbamoyl phosphate synthetase I produces carbamoyl phosphate in the mitochondria from CO2, NH3, and two ATP molecules. The enzyme, which has an absolute requirement for its positive allosteric effector, N-acetylglutamate, is the rate-limiting step in the cycle. 

Short-term regulation of the cycle occurs principally at carbamoyl phosphate synthetase-I, which is relatively inactive in the absence of its allosteric activator N-acetylglutamate. The steady-state concentration of N-acetylglutamate is set by the concentration of its components, acetyl-CoA and glutamate, and by arginine, which is a positive allosteric effector of... 

Branched-chain amino acids apparently stimulate the urea cycle. Carbamoylphosphate synthetase, which channels ammonia into the urea cycle, is induced by ornithine and N-acetylglutamate as a cofactor of urea synthesis. Here, BCAA follow two modes of action (i.) they stimulate the synthesis of N-acetylglutamate via synthetase formed from glutamate and acetyl CoA, and (2.) they inhibit omithine-keto acid transferase, which is the enzyme responsible for ornithine degradation, leading to an increase in ornithine concentration. Ammonia detoxication is thus stimuiated by two regu-iatory mechanisms, (s. fig. 40.2)... 

In eukaryotic cells, two separate pools of carbamoyl phosphate are synthesized by different enzymes located at different sites. Carbamoyl phosphate synthetase I (CPS I) is located in the inner membrane of mitochondria in the liver and, to lesser extent, in the kidneys and small intestine. It supplies carbamoyl phosphate for the urea cycle. CPS 1 is specific for ammonia as nitrogen donor and requires N-acetylglutamate as activator. Carbamoyl phosphate synthetase II (CPS II) is present in the cytosol. It supplies carbamoyl phosphate for pyrimidine nucleotide biosynthesis and uses the amido group of glutamine as nitrogen donor. The presence of physically separated CPSs in eukaryotes probably reflects the need for independent regulation of pyrimidine biosynthesis and urea formation, despite the fact that both pathways require carbamoyl phosphate. In prokaryotes, one CPS serves both pathways. 

Note that NH, because it is a strong base, normally exists as NH4 in aqueous solution. However, carbamoyl phosphate synthetase uses only NH3 as a substrate. The reaction begins with the phosphorylation of HCO3 to form carboxyphosphate, which then reacts with NH3 to form carbamic acid. Finally, a second molecule of ATP phosphofylates carbamic acid to form carbamoyl phosphate. The structure and mechanism of the enzyme that catalyzes these reactions will be presented in Chapter 25. The consumption of two molecules of ATP makes this synthesis of carbamoyl phosphate essentially irreversible. The mammalian enzyme requires N-acetylglutamate for activity, as will be described shortly. 

The urea cycle enzymes are controlled in the short term by the concentrations of their substrates. Carbamoyl phosphate synthetase I is also allosterically activated by N-acetylglutamate. This latter molecule is a sensitive indicator of the cell s glutamate concentration. (Recall that a significant amount of NH4 is derived from glutamate.) N-acetylglutamate is produced from glutamate and acetyl-CoA in a reaction catalyzed by N-acetylglutamate synthase. 

The activity of mitochondrial carbamoyl phosphate synthetase (CPS) depends on the availability of N-acetylglutamate, which is generated from glutamate and acetyl CoA. A reduction in the availability of the activating molecule will lead to a decrease in the activity of CPS, which utilizes ammonia and bicarbonate for the synthesis of carbamoyl phosphate. This, in turn, leads to an increase in the level of ammonia in blood and urine. Over two-thirds of patients with methylmalonic aciduria are hyperammonemic. 

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