How is nucleotide biosynthesis regulated? How is the enzyme ATCase regulated?

Nucleotide biosynthesis is regulated by feedback inhibition of its key enzymes.

ATCase enzyme is an allosteric enzyme and is also regulated by feedback inhibition.

Explanation:

Nucleotide biosynthesis is regulated by feedback inhibition mechanisms. For example, the enzyme glutamine-PRPP amidotransferase of purine biosynthesis is inhibited by the end products namely IMP, AMP, and GMP. Therefore, higher concentrations of AMP or GMP inhibits purine biosynthesis.

The enzyme aspartate transcarbamoylase (ATCase) catalyzes the formation of N-Carbamoylaspartate from aspartate during pyrimidine biosynthesis. It is inhibited by CTP (cytidine 5'-triphosphate), the end product of the sequence. The allosteric subunits of the enzyme ATCase bind the allosteric inhibitor, CTP.

When CTP is not bound to the regulatory subunits, the enzyme is maximally active. Accumulation of CTP in cells and its binding to the regulatory subunit transform the enzyme into inactive conformation.