The first common enzyme in the biosynthesis of the branched‐chain amino acids (Leu, Ile and Val) is acetohydroxyacid synthase (AHAS). 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.
A, B and D
AHAS Enzyme and Amino Acid Biosynthesis
Acetohydroxyacid synthase (AHAS), also known as acetolactate synthase (ALS), is a crucial enzyme that catalyzes the first common step in the biosynthesis pathway of the branched-chain amino acids: leucine (Leu), isoleucine (Ile), and valine (Val).
Understanding AHAS Properties
Let's analyze the given statements about the AHAS enzyme:
- Statement A: AHAS requires thiamine diphosphate as cofactor.
This statement is correct. Thiamine diphosphate (TPP), a derivative of vitamin B1, is an essential cofactor for AHAS activity. It plays a key role in the decarboxylation reaction catalyzed by the enzyme.
- Statement B: The plant AHAS comprises a large catalytic subunit and a smaller regulatory subunit.
This statement is correct. Plant AHAS enzymes are typically heteromeric, consisting of a large subunit which carries the catalytic activity and a smaller regulatory subunit which is involved in feedback inhibition by the branched-chain amino acids.
- Statement C: The large subunit alone is sensitive to inhibition by Leu, Ile and Val in plants.
This statement is incorrect. In plants, feedback inhibition by Leu, Ile, and Val is primarily mediated through binding to the regulatory subunit. The catalytic subunit alone is generally resistant to inhibition by these amino acids.
- Statement D: Most of the bacterial and fungal AHAS enzymes are sensitive to inhibition by Val only.
This statement can be considered correct in the context of many common bacterial and fungal AHAS enzymes, which exhibit feedback sensitivity to valine, isoleucine, leucine, or combinations thereof, depending on the specific organism and enzyme isoform. While not all are inhibited *only* by Val, sensitivity to Val is a common characteristic, and some key bacterial AHAS enzymes show strong sensitivity to Val.
Summary of Statement Correctness
Based on the analysis, statements A, B, and D are correct, while statement C is incorrect.
| Statement | Description | Correctness |
|---|---|---|
| A | Requires thiamine diphosphate (TPP) | Correct |
| B | Plant AHAS has catalytic and regulatory subunits | Correct |
| C | Plant large subunit alone sensitive to inhibition | Incorrect |
| D | Bacterial/Fungal AHAS sensitive to Val inhibition | Correct |
Identifying Correct Statements
The question asks to select the option with all correct statements. The correct statements are A, B, and D.
Therefore, the option that includes statements A, B, and D is the correct choice.
Important Questions from Biochemistry
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D. In the absolute configuration system, L-threonine and L-isoleucine are (2S, 3R)-threonine and (2S, 3S)-isoleucine diastereomers, respectively.
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