The first common enzyme in the biosynthesis of the branched-chain amino acids (Leu, Ile, and Val) is acetohydroxyacid synthase (AHAS). The following statements are made about the enzyme:
A. AHAS requires thiamine diphosphate as cofactor.
B. The plant AHAS comprises a large catalytic subunit and a smaller regulatory subunit.
C. The large subunit alone is sensitive to inhibition by Leu, Ile, and Val in plants.
D. Most of the bacterial and fungal AHAS enzymes are sensitive to inhibition by Val only.

Select the option with all correct statements.

Correct option is D

Explanation-

A. AHAS requires thiamine diphosphate as cofactor. 
Correct – AHAS (also known as ALS, acetolactate synthase) uses thiamine diphosphate (ThDP or TPP) as a coenzyme for its catalytic activity.

B. The plant AHAS comprises a large catalytic subunit and a smaller regulatory subunit. 
Correct – In plants, AHAS is a heteromeric enzyme, with a large catalytic subunit and a smaller regulatory subunit involved in feedback regulation.

C. The large subunit alone is sensitive to inhibition by Ile, Leu, and Val in plants. 
Incorrect – The large subunit alone is catalytically active but not fully feedback-inhibited by amino acids. The regulatory subunit is essential for sensitivity to Leu, Ile, and Val.

D. Most of the bacterial and fungal AHAS enzymes are sensitive to inhibition by Val only. 
Correct – In many bacterial and fungal systems, Val is the dominant feedback inhibitor of AHAS activity.

Correct Combination -  A, B, and D
Answer: Option d – A, B and D only