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Biosynthesis and Degradation of Nucleotides:- Pyrimidine Nucleotide Biosynthesis Is Regulated by Feedback Inhibition

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  p868-869

2026-07-08

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Biosynthesis and Degradation of Nucleotides:- Pyrimidine Nucleotide Biosynthesis Is Regulated by Feedback Inhibition

Regulation of the rate of pyrimidine nucleotide synthesis in bacteria occurs in large part through aspartate transcarbamoylase (ATCase), which catalyzes the first reaction in the sequence and is inhibited by CTP, the end product of the sequence . The bacterial ATCase molecule consists of six catalytic subunits and six regulatory subunits . The catalytic subunits bind the substrate molecules, and the allosteric subunits bind the allosteric inhibitor, CTP. The entire ATCase molecule, as well as its subunits, exists in two conformations, active and inactive. When CTP is not bound to the regulatory subunits, the enzyme is maximally active. As CTP accumulates and binds to the regulatory subunits, they undergo a change in conformation. This change is transmitted to the catalytic sub units, which then also shift to an inactive conformation. ATP prevents the changes induced by CTP. Figure 1 shows the effects of the allosteric regulators on the activity of ATCase.

FIGURE 1 Allosteric regulation of aspartate transcarbamoylase by CTP and ATP. Addition of 0.8 mM CTP, the allosteric inhibitor of ATCase, increases the K_0.5 for aspartate (lower curve) and the rate of conversion of aspartate to N-carbamoylaspartate. ATP at 0.6 mM fully reverses this effect (middle curve).