Aspartate transcarbamylase control

Carbamyl-L-aspartate is the key precursor in the biosynthesis of pyrimidines. The enzyme aspartate transcarbamylase is inhibited by several pyrimidine nucleotides, notably cytidine triphosphate, and is activated by ATP, a purine nucleotide. Thus the enzyme is under feedback regulation, and controls the relative concentration of pyrimidine and purine nucleotides. 

In this experiment we will examine some of the properties of the aspartate transcarbamylase of Escherichia coli, which is typical of many enzymes subject to feedback inhibition and which has been studied extensively. Aspartate transcarbamylase (ATCase) catalyzes the first reaction unique to the biosynthesis of pyrimidine nucleotides. ATCase is subject to specific inhibition by quite low concentrations of one of its end products, cytidine 5 -triphosphate (CTP). This relationship and two other regulatory interactions important to the control of pyrimidine biosynthesis are summarized in Figure 9-1. 

Mathieu, M. (1987) Utilization of aspartate transcarbamylase activity in the study of neuroendocrinal control of gametogenesis in Mytilus edulis. J. Exp. Biol., 241, 247-252. 

Another form of spatial organization of metabolism that is often seen in eukaryotes but is less common in bacteria involves enzyme aggregates or multifunctional enzymes. An example is seen in S. cerevisiae where the first two reactions in pyrimidine nucleotide biosynthesis, the synthesis of carbamyl phosphate and the carbamylation of aspartate, are catalyzed by a single bifunctional protein (31). Both reactions are subject to feedback inhibition by UTP, in contrast to the situation inB. subtilis where aspartate transcarbamylase activity is not controlled. It is possible that an evolutionary advantage of the fusion of the genes... 

The intramitochondrial location of the arginine-specific carbamyl phosphate synthetase in N. crassa has the additional advantage of assuring separate pools of carbamyl phosphate for arginine and pyrimidine biosynthesis (41). Since this precludes the utilization of carbamyl phosphate produced in the course of arginine biosynthesis by aspartate transcarbamylase and of the pyrimidine-specific carbamyl phosphate by ornithine transcarbamylase, the control of these reac-... 

The pathotype responsible for tomato stem canker produces 3 related toxins (Fig. 3). Aspartate and certain products of aspartate metabolism (e.g. orotic acid) protect tomato plants against the toxins, and it is thought that toxicity may be due to inhibition of aspartate transcarbamylase. Susceptibility to the disease is controlled by a single genetic locus with 2 alleles. 

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. 

Sometimes the action of an enzyme is controlled by a substance other than the substrate or products. These enzymes are called allosteric enzymes. The site on the enzyme molecule that reacts with the allosteric control molecule is different from the active reaction site of the enzyme. The separate allosteric control sites and active sites have been observed by x-ray diffraction in the enzyme aspartate transcarbamylase, an enzyme of molecular mass 310,000 d which consists of six subunits. 

The synthesis of the enzyme in E. coli is controlled by a feedback mechanism involAring pyrimidine nucleotides 476-478). Aspartate transcarbamylase has been crystallized from E. coli 478a). 

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