4.17 : Allosteric Proteins-ATCase

Binding sites linkages can regulate a protein's function. For example, enzyme activity is often regulated through a feedback mechanism where the end product of the biochemical process serves as an inhibitor.

Aspartate transcarbamoylase (ATCase) is a cytosolic enzyme that catalyzes the condensation of L-aspartate and carbamoyl phosphate to N-carbamoyl-L-aspartate. This reaction is the first step in pyrimidine biosynthesis. UTP and CTP, the end products of the pyrimidine synthesis pathway, inhibit the activity of ATCase, the enzyme that catalyzes the first essential step of this pathway. Binding of UTP and CTP to the enzyme negatively regulates the linked catalytic site when the concentration of pyrimidines is high, relative to the concentration of purines in the cell. This phenomenon is known as feedback inhibition and is essential in maintaining the right amounts of metabolites in an organism.

ATCase is part of the CAD multi-enzyme complex, part of the pyrimidine biosynthesis pathway, along with carbamoyl phosphate synthetase II and dihydroorotase. Pyrimidines are essential for DNA synthesis during cell division, therefore, inhibition of ATCase activity slowing down tumor growth in cancer.