Back in 1962, Gerhart and Pardee developed a model for the regulation of the act

Question

Back in 1962, Gerhart and Pardee developed a model for the regulation of the activity of the ATCase enzyme by CTP and ATP, using the pathway given in Figure 16.1. Describe that model, using information presented here as well as what you have learned about allosteric enzymes. Be sure to include a sentence explaining the physiological significance of your model.

CASE 16 Allosteric Regulation of ATCase Table 16.1: Names and Abbreviations of Nucleic Acid Bases, Nucleotides and Nucleosides. (Based on Voet and Voet, 1995. Nucleotide Base formula Nucleoside Nucleotide Base I X H X ribose Nucleotide X ribose X deoxyribose OH triphosphate X ribose triphosphate phosphate AMP dATP ATP Adenine Adenosine dGTP Guanine Guanosine GMP GTP dCTP Cytosine Cytidine CMP CTP UTP NH Uracil Uridine UMP

Explanation / Answer

ATCase enzyme has distinct regulatory and catalytic sites. The catalytic subunit is involved in catalyzing the committed step in the synthesis of pyrimidine nucleotides such as cytidine triphosphate (CTP). When, the concentrations of CTP are low, the substrate binds to the catalytic site and ATCase-driven synthesis of CTP increases until sufficient amount of CTP is produced. Structurally CTP is different from its substrate; therefore, it cannot bind at the catalytic site. The regulatory subunit of ATCase has no catalytic activity, but has binding site for CTP. Such sites are called as allosteric or regulatory sites. When, concentrations of CTP increases, excess CTP binds to the regulatory site of ATCase and inhibits biosynthesis of CTP.

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