Question

ATCase and pepsin are regulated by very different mechanisms, yet there is one feature or characteristic that they both share when they are activated. What is it?

Using ATCase as a model, describe allosteric regulation.

Answer 1

Aspartate transcarbamoylase (ATCase) is a cytosolic enzyme that catalyzes the condensation of L-aspartate and carbamoyl phosphate (CP) to produce N-carbamoyl-L-aspartate (CAA). This reaction is the first committed step of the pyrimidine biosynthetic pathway. Allosteric Regulation. Enzymes have an area called the active site, where they bind substrates, like the hamburger, and turn them into products or food for cells. When a molecule binds an allosteric site, it alters the enzyme's shape, or confirmation, which then changes how the enzyme functions. An allosteric inhibitor binds to the enzyme, inducing it to assume an inactive form. Allosteric inhibition is the negative control of enzyme activity, by the binding an inhibitory substance (effector molecule) to the enzyme. This binding doesn't happen at the active site but leads to a conformational change which - in the negative control - reduces their affinity for their substrates. In case of pepsin also proteolysis results in the irreversible conformational change.

ATCase (aspartate transcarbamoylase), a rate-limiting step in bacterial pyrimidine biosynthesis allosterism

-ATP = activator
-CTP = is a feedback inhibitor (is the molecule produced at the end of the pathway)