Question

Some of the exceptional properties of regulatory/allosteric enzymes are:
(A) Their Kinetics do not obey the Michaelis-Menten equation.
(B) They are mostly monomeric in nature.
(C) They have more than one substrate binding sites and the substrate binding to different sites is mutually independent, exclusive and noncooperative.
(D) Binding of effector molecules to them may lead to their activation or inhibition.

• A. (A), (B) and (D) only.
• B. (A), (C) and (D) only.
• C. (A), (B), (C) and (D).
• D. (A) and (D) only.

Hint:

Allosteric enzymes are always multimeric proteins showing cooperative binding (which gives them sigmoidal curves instead of hyperbolic Michaelis-Menten curves). They are regulated via conformation shifts by activators and inhibitors.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Allosteric or regulatory enzymes are specialized proteins that regulate metabolic pathways.
Unlike simple Michaelis-Menten enzymes, allosteric enzymes exhibit conformational changes upon binding of substrates and regulatory effector molecules.

Step 2: Detailed Explanation:
Let us analyze each of the stated properties of allosteric enzymes:
- (A) Their Kinetics do not obey the Michaelis-Menten equation: This is correct.
Allosteric enzymes do not produce the classic hyperbolic curve in a velocity-versus-substrate concentration plot.
Instead, they exhibit sigmoidal (S-shaped) curves due to cooperative binding.
- (B) They are mostly monomeric in nature: This is incorrect.
Almost all regulatory allosteric enzymes are oligomeric proteins containing multiple subunits (polypeptide chains) that interact with one another.
- (C) Substrate binding to different sites is mutually independent, exclusive, and noncooperative: This is incorrect.
The sigmoidal kinetic profile of allosteric enzymes is a direct consequence of positive cooperativity, where substrate binding at one active site alters the conformation of adjacent subunits, increasing their affinity for subsequent substrate molecules.
- (D) Binding of effector molecules to them may lead to their activation or inhibition: This is correct.
Allosteric effectors bind reversibly to regulatory sites distinct from the active site, inducing conformational changes that either stabilize the active state (allosteric activators) or the inactive state (allosteric inhibitors).
Thus, only statements (A) and (D) are correct.

Step 3: Final Answer:
The exceptional properties of allosteric enzymes are (A) and (D) only.