10. Pyrurate dehydrogenase is subject to feedback inhibition by its products in glycolysis. Some of the chemical compounds which might be involved in the process, are listed below:
A. NADH                   B. FADH
C. Acetyl-CoA           D. Acetaldehyde
Which one of the following combinations of above chemical compounds is involved in feedback inhibition of pyruvate dehydrogenase?
(1) A and B              (2) B and C
(3) C and D              (4) A and C

Introduction

Pyruvate dehydrogenase (PDH) is a critical enzyme complex linking glycolysis and the citric acid cycle by converting pyruvate to acetyl-CoA. Its activity is finely regulated through feedback inhibition by its products to maintain cellular energy homeostasis.

Explanation of Each Compound

(A) NADH

• Correct. NADH, produced by PDH activity, signals high cellular energy status and inhibits PDH to prevent excess acetyl-CoA production.

(B) FADH

• Incorrect. FADH itself is not a standard feedback inhibitor. FADH₂ is produced in some reactions but is less directly involved in PDH inhibition.

(C) Acetyl-CoA

• Correct. As the immediate product of PDH, acetyl-CoA accumulation inhibits PDH to regulate substrate flow into the citric acid cycle.

(D) Acetaldehyde

• Incorrect. Acetaldehyde is not related to PDH regulation; it is more involved in fermentation pathways.

Why A and C Are Correct

NADH and acetyl-CoA provide negative feedback to PDH, thereby controlling the metabolic flux from glycolysis to aerobic respiration efficiently. This regulation prevents unnecessary energy expenditure and balances metabolic needs.

Understanding these regulatory mechanisms is essential for biochemistry students and researchers studying cellular metabolism and metabolic diseases.