Carbamoyl phosphate synthetases

Martinez-Ramon, A., Knecht, E., Rubio, V., and Grisolia, S. (1990) Levels of carbamoyl phosphate synthetase I in livers of young and old rats assessed by activity and immunoassays and by electron microscopic immunogold procedures. J. Histochem. Cytochem. 38, 371-376. 

From ammonia through carbamoyl phosphate synthetase To urea... 

A. Pi/ erard, Control of the activity of Escherichia coli carbamoyl phosphate synthetase by antagonistic allosteric effectors. Science 154, 1572 1573 (1966). 

Urea, which contains two nitrogens, is synthesized in the liver from aspartate and carbamoyl phosphate, which in turn is produced from ammonium ion and carbon dioxide by mitochondrial carbamoyl phosphate synthetase. The urea cycle and the carbamoyl phosphate synthetase reaction are shown in Figure 1-17-2. 

Carbamoyl phosphate synthetase and ornithine transcarbamoylase are mitochondrial enzymes. 

The two conditions can be distinguished by an increase in orotic add and uracil, which occurs in ornithine transcarbamoylase deficiency, but not in the defldency of carbamoyl phosphate synthetase. Orotic acid and uracil are intermediates in pyrimidine synthrais (see Chapter 18). This pathway is stimulated by the accumulation of carbamoyl phosphate, the substrate for ornithine transcarbamoylase in the urea cycle and for aspartate transcarbamoylase in pyrimidine synthesis. 

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

Carbamoyl phosphate synthetase (no increase in orotic acid or uracil)... 

Pyrimidines are synthesized de novo in the cytoplasm from aspartate, COj, and glutamine as shown in Figure 1-18-2. Synthesis involves a cytoplasmic carbamoyl phosphate synthetase that differs from the mitochondrial enzyme with the same name used in the urea cycle. 

The carbamoyl phosphate synthetase (abbreviated to CPS-I) that is involved in the ornithine cycle differs from the enzyme that is involved in pyrimidine synthesis (carbamoyl phosphate synthetase-ll). The latter enzyme is cytosolic, requires glutamine for provision of nitrogen, rather than ammonia, and is regulated by different factors (Chapter 20). 

The of carbamoyl phosphate synthetase for ammonia is assumed to be above that of the ammonia concentration in the mitochondria, so that an increase in the latter should increase the activity of the enzyme. 

Condition Carbamoyl- phosphate synthetase Ornithine transcarbamoylase Argininosuccinate synthetase Argininosuccinate lyase Arginase... 

The concentration of ammonia in the liver is not saturating for carbamoyl phosphate synthetase, so that the greater the flux of ammonia into or within the liver, the higher the concentration of ammonia and the higher the activity of the synthetase. The effect of ammonia concentration is, therefore, a mass-action effect. 

Arginine stimulates carbamoyl-phosphate synthetase (an allosteric effect). 

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. 

CPS-I carbamoyl phosphate synthetase IRS insulin-receptor substrate... 

Carbamoyl phosphate synthetase formation in liver taken from tadpoles treated with thyroxine is enhanced by the addition of orotic acid, uracil or uridine (cytosine and adenosine had no effect). The synthesis of this enzyme is not affected by these pyrimidines in untreated animals. This indicates that there is a relative pyrimidine deficiency during thyroxine-induced metamorphosis [140]. 

Carbamoyl-phosphate synthetase (ammonia) [EC 6.3.4.16], also known as carbamoyl-phosphate synthetase I, catalyzes the reaction of two molecules of ATP with carbon dioxide, ammonia, and water to produce two molecules of ADP, orthophosphate, and carbamoyl phosphate. Carbamoyl-phosphate synthetase (glutamine-hydrolyzing) [EC 6.3.5.5], also known as carbamoyl-phosphate synthetase II, catalyzes the reaction of two molecules of ATP with carbon dioxide, glutamine, and water to produce two molecules of ADP, orthophosphate, glutamate, and carbamoyl phosphate. 

Enzymes utilizing four substrates in a single reaction. An example is carbamoyl phosphate synthetase which... 

PRPP H Orotate derived from glutamine. CPS-II, carbamoyl phosphate synthetase Asp,... 

D208A,D210A <13> (<13> 0.1% of wild type activity [18]) [18] E136A/E138A/E141A/K140A <13> (<13> not detrimental [18]) [18] Additional information <12> (<12> enzyme can replace in vivo carbamoyl phosphate synthetase of E. coli [17]) [17]... 

Pillai, R.P. Raushel, F.M. Villafranca, J.J. Stereochemistry of binding of thiophosphate analogs of ATP and ADP to carbamate kinase, glutamine synthetase, and carbamoyl-phosphate synthetase. Arch. Biochem. Biophys., 199, 7-15 (1980)... 

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