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ATCase Allosteric Regulation
Aspartate transcarbamoylase or simply ATCase is the prototypical example of an enzyme that is regulated allosterically. ATCase catalyzes the formation of N-carbamoylaspartate from aspartate and carbamoyl phosphate. This is the first step in the biochemical synthesis of pyrimidine nitrogenous bases that are ultimately used to produce nucleoside triphosphates such as cytidine triphosphate (CTP). Early studies demonstrated that the rate of formation of N-carbamoylaspartate decreased as the concentration of the CTP increased inside the cell. This data suggested that CTP was an inhibitor to ATCase. Since CTP looks nothing like the original substrate molecules of ATCase, that implies that CTP does not bind to the active site of the enzyme but rather some other regulatory site. Therefore, that must mean that CTP is an allosteric inhibitor of ATCase and ATCase is in fact controlled allosterically by our cells. At low concentrations of CTP, not enough of it is present to inactivate the enzyme and so the enzyme will function efficiently to produce N-carbamoylaspartate at a higher rate. However at high concentrations of CTP, the CTP creates a negative feedback loop that causes the inhibition of ATCase and blocks the formation of the product. Like the majority of allosteric enzymes, ATCase behaves cooperatively. This must imply that its structure consists of multiple subunits.
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