Question

In all the following reactions, reduction of NAD⁺ to NADH occurs, except

• A. isocitric acid $\to \alpha$ -ketoglutaric acid
• B. malic acid $\to$ oxaloacetic acid
• C. pyruvic acid $\to$ acetyl coenzyme
• D. succinic acid $\to$ fumaric acid

Answer 1

Answer: (D)

During the step of Krebs' cycle, where succinic acid undergoes oxidation or dehydrogenation to form Fumaric acid, FAD is reduced to FADH₂ and enzyme involved in this step is succinic acid dehydrogenase.
Conversion of isocitric acid to $\alpha$ -ketoglutaric acid, malic acid to oxaloacetic acid and pyruvic acid to acetyl Co-A, all involve the reduction of NAD to NADH + H⁺.