Questions 1. Gerhart and Pardee measured ATCase activity in the presence of a va
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Questions 1. Gerhart and Pardee measured ATCase activity in the presence of a variety of purine and pyrimidine derivatives. Their results are presented in Table 16.2. What compound(s) were the most effective inhibitors? activators? Explain the significance of the metabolites that served as inhibitors or activators in the context of the biosynthetic pathway presented in Figure 16.1
Background Aspartate transcarbamoylase (ATCase) catalyzes an early step in the synthesis of the pyrimidine nucleotides UTP and CTP. The enzyme catalyzes the condensation of carbamoyl phosphate and aspartate to form carbamoyl aspartate. The reaction pathway is shown in Figure 16.1. The enzyme has been fairly well characterized. It is known to consist of six regulatory subunits and six catalytic subunits. In this case, we examine the properties of ATCase isolated from E. coli to illustrate some of the important regulatory properties of multi-subunit enzymes. As an early enzyme in a multi-step pathway, the ase reaction is a logical one to regulate the synthesis of pyrimidine nucleotides. Both purine nucleotides and pyrimidine nucleotides are needed in roughly equal amounts as substrates for DNA synthesis in rapidly dividing cells. The regulation of the ATCase enzyme ensures a proper balance of purine and pyrimidine pools in E. coli. The goal in this case was to identify the cellular metabolites that serve as activators and inhibitors of ATCase. HCO3 Glutamine ATP ATCase Carbamoyl phosphate Aspartate N-carbamoylaspartate UMP UTP Figure 16.1: Pyrimidine synthetic pathway. CTPExplanation / Answer
1. CTP is an inhibitor and ATP is an activator.
CTP
ATP
(i) The high concentration of ATP signals a high concentration of purine nucleotides in the cell. The increase in ATCase activity will balance the purine and pyrimidine pools.
(ii) A high concentration of ATP indicates that there is significant energy reserved in the cell to promote mRNA synthesis and DNA replication that leads to the synthesis of pyrimidines.
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