Aspartate transcarbamylase formation

Animal and bacterial enzymes that utilize or synthesize carbamyl phosphate have activity with acetyl phosphate. Acyl phosphatase hydrolyzes both substrates, and maybe involved in the specific dynamic action of proteins. Ornithine and aspartic transcarbamylases also synthesize acetylornithine and acetyl aspartate. Finally, bacterial carbamate kinase and animal carbamyl phosphate synthetase utilize acetyl phosphate as well as carbamyl phosphate in the synthesis of adenosine triphosphate. The synthesis of acetyl phosphate and of formyl phosphate by carbamyl phosphate synthetases is described. The mechanism of carbon dioxide activation by animal carbamyl phosphate synthetase is reviewed on the basis of the findings concerning acetate and formate activation. 

Reactions 3 and 4 indicate that with aspartic acid, aspartic transcarbamylase, and carbamyl-P or acetyl-P, either carbamyl aspartate or acetyl aspartate can be formed. Carbamyl aspartate is the first intermediate in the formation of pyrimidines, and acetyl aspartate, of unknown function, is the amino acid derivative present in the largest concentration in brain of most species (43). 

A particularly intuitive application of this concept may be the experimental anticancer drug N-(phosphonoacetyl)-L-aspartate (PALA). The first step in the de novo biosynthesis of the pyrimidine nucleotide formation in the cell involves the condensation of carbamoyl phosphate with L-aspartic acid catalyzed by the enzyme aspartate transcarbamylase (Eq. 2.14).2 One can postulate a transition state, as shown in Eq. 2.14. 

Fig. is. Model for the mechanism of homotropic cooperativity in aspartate transcarbamylase. Shown schematically are the two extreme conformations in the T state and the R state. The binding of the substrates at one active site induces the domain closure in that catalytic chain and requires a quaternary conformational change which allows the 240s loops of the upper and lower catalytic chains to move to their final positions. The formation of the R state, in a concerted fashion, is further stabilized by a variety of new interactions as shown. [Reprinted with permission from Ref. (/).]... 

Aspartate transcarbamylase (ATCase) catalyzes the formation of carbamoyl aspartate with CP and aspartic acid as substrates. It is the first specific enzyme for the pyrimidine pathway, and it holds a special place in the historical development of end-product control at this level. The concept of feedback inhibition as an important regulatory mechanism evolved from the initial discovery by Yates and Pardee [90] that CTP is a potent inhibitor of ATCase. It has since developed into a prototype for a regulatory protein with classic allosteric properties. A thorough characterization of the enzyme and its properties has been made through the combined efforts of Gerhart, Pardee, Schachman, and Changeux [91-97]. A summary of these studies follows. 

While the studies of Boyland and Roller and Elion and co-workers, which were conducted in vivo, do suggest that urethane has a specificity for pyrimidine biosynthesis, Kaye could not demonstrate in vitro any significant inhibition by urethane of several enzymes involved in nucleic acid metabolism. Both urethane and its A -hydroxy metabolite bear a structural resemblance to carbamyl phosphate and carbamyl-L-aspartate. The enzyme aspartate transcarbamylase begins pyrimidine biosynthesis by catalyzing the formation of carbamyl-L-aspartate from carbamyl phosphate and l-aspartate. Giri and Bhide have reported that in vivo administration of urethane decreased aspartate transcarbamylase activity of lung tissue of adult male and (to a lesser extent) female mice no in vitro inhibition could be demonstrated. 

Carbamoyl phosphate (CP) serves as a substrate for two separate transcarbamylase enzymes. One of these, in a reaction with aspartic acid, yields carbamoyl aspartate, the first specific precursor in the UMP pathway the other, in a similar reaction with ornithine, has a similar role for the eventual synthesis of arginine. Thus, CP serves as a common precursor for both UMP and arginine, and special regulation of its formation must be obtained to assure a balanced supply of both end products. The problem is handled in a variety of ways by different organisms. 

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