Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).

Assertion (A) : Iodoacetate selectively inhibits the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase.
Reason (R) : Iodoacetate is a chelating agent that chelates the magnesium ion and makes it unavailable for the catalytic action of glyceraldehyde-3-phosphate dehydrogenase.

In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Always remember: Iodoacetate inhibits GAPDH by alkylating active-site Cysteine thiol (\(\text{-SH}\)) groups, not by chelating metal ions.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Enzyme inhibitors can block biochemical pathways by binding to active sites or altering essential functional groups.

Step 2: Detailed Explanation:
Assertion (A) is correct because iodoacetate is a well-known, potent, and selective irreversible inhibitor of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), an essential enzyme in the glycolytic pathway.
Reason (R) is incorrect because iodoacetate does not act as a chelating agent.
Instead, it is an alkylating agent that covalently reacts with the highly reactive sulfhydryl (\(\text{-SH}\)) group of the essential cysteine residue (Cys-149) in the active site of GAPDH.
This covalent modification prevents the enzyme from binding to its substrate.
Other glycolytic inhibitors, like fluoride, work by chelating magnesium (\(\text{Mg}^{2+}\)) ions to inhibit the enzyme enolase, but this mechanism does not apply to iodoacetate.

Step 3: Final Answer:
Thus, Assertion (A) is true but Reason (R) is false.