Which statement best defines an allosteric enzyme in the context of allosteric regulation?

• A. An enzyme that catalyzes a reaction only when two different substrates bind simultaneously to the same active site with equal affinity.
• B. An enzyme that is active only after irreversible proteolytic cleavage of a zymogen into a mature form.
• C. An enzyme whose activity is modulated by an effector binding at a site other than the active site, causing a conformational change that alters catalysis.
• D. An enzyme that binds its substrate covalently at the active site as an obligatory step in catalysis.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the concept of allosteric regulation, which involves the modulation of an enzyme's activity through the binding of molecules (effectors) at sites other than the enzyme's active site.
2. Step 2: Recognize that an allosteric enzyme has a specific site called the allosteric site, distinct from the active site, where effectors bind and induce conformational changes.
3. Step 3: Note that these conformational changes can either increase or decrease the enzyme's catalytic activity, thus regulating the enzyme function in response to cellular signals.
4. Step 4: Compare the given statements to this definition: the correct statement should describe an enzyme whose activity is modulated by effector binding at a site other than the active site, causing a conformational change.
5. Step 5: Conclude that the best definition of an allosteric enzyme is the one describing modulation by effectors binding at a non-active site, leading to altered catalysis.