Reduction of NAD does not occur in which of the following?

Options

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

To solve the question regarding the reduction of NAD (nicotinamide adenine dinucleotide) and identify which reaction does not involve its reduction, we will analyze each option provided.

### Step-by-Step Solution:

1. **Understand the Question**: We need to find out in which of the given reactions NAD is not reduced. NAD is typically reduced to NADH in various metabolic pathways, particularly in cellular respiration.

2. **Analyze Each Option**:
- **Option 1: Isocitric acid to alpha-ketoglutaric acid**: This reaction occurs in the Krebs cycle and involves the enzyme isocitrate dehydrogenase. In this reaction, NAD is reduced to NADH.

- **Option 2: Malic acid to oxaloacetic acid**: This reaction is also part of the Krebs cycle and is catalyzed by malate dehydrogenase. Here, NAD is again reduced to NADH.

- **Option 3: Pyruvic acid to acetyl coenzyme**: This reaction occurs during the conversion of pyruvate to acetyl-CoA, catalyzed by the pyruvate dehydrogenase complex. In this process, NAD is reduced to NADH.

- **Option 4: Succinic acid to fumaric acid**: This reaction is part of the Krebs cycle and is catalyzed by succinate dehydrogenase. In this reaction, FAD (flavin adenine dinucleotide) is reduced to FADH2, not NAD.

3. **Identify the Correct Answer**: Based on the analysis, the reaction that does not involve the reduction of NAD is the conversion of succinic acid to fumaric acid. Therefore, the correct answer is:

**Succinic acid to fumaric acid.**