Feedback inhibition is a key regulatory mechanism. Aspartate transcarbamoylase (ATCase) in bacteria is inhibited by CTP, the end product of pyrimidine biosynthesis, and is activated by ATP. ATCase is an example of which type of enzyme regulation?

• A Allosteric regulation — the inhibitor/activator binds a site distant from the active site and causes conformational changes ✓
• B Covalent modification by phosphorylation
• C Competitive product inhibition at the active site by CTP
• D Zymogen activation by proteolytic cleavage

Explanation

ATCase is the paradigm allosteric enzyme. CTP (the end product of pyrimidine synthesis) binds regulatory subunits (r subunits) at a site distant from the catalytic subunits (c subunits), inducing the T (taut, less active) conformation. ATP (purine, signals adequate energy) binds the same regulatory site and promotes the R (relaxed, active) conformation, balancing purine and pyrimidine pools. ATCase shows sigmoidal (cooperative) kinetics with substrate aspartate. This contrasts with competitive product inhibition, where the product binds the active site. ATCase is not regulated by phosphorylation or zymogen activation in mammals (mammalian CAD trifunctional protein is regulated differently).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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