ATCase structure and allosteric inhibition
ATCase is an enzyme essential to the creation of pyrimidine which make up DNA nucleotide bases (CTP is the final product). ATCase consists of two catalytic trimers (where substrate binds) and three regulatory dimers (where allosteric regulation takes place). The interaction of regulatory dimers and catalytic trimers is helped by Zinc atom. The presence of two catalytic trimers means that six substrate molecules can bind to one ATCase.
ATCase exhibits cooperatively, which means that as one substrate molecule binds the active sites become more and more likely to bind more substrate molecules. ATCase, therefore, can be in the R- relaxed(as the substrate binds, the structure changes so that affinity for the substrate increases) and T - tense state (without the substrate --> low catalytic activity).
Allosteric effectors of ATCase are : CTP and ATP. Allosteric effects bind to the regulatory and not catalytic sites of the enzyme. How do they effect the enzyme? CTP is an allosteric inhibitor that stabilizes the T state of the enzyme and makes it less likely to bind to the substrate. It makes sense since the more product we produce, the less we should want to make in order not to waste anymore energy (negative inhibition). ATP, on the other hand, is an allosteric activator of ATCase stabilizing the R state of the enzyme and increasing it's affinity for the substrate. High levels of ATP signify that there are more purine bases than pyrimidine. Thus, CTPase is activated to balance out this inequality.
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