In which one of the following steps of citric acid cycle, FAD is reduced to `FADH_(2)`

Options

A. Pyruvate `rarr` Acetyl CoA
B. Succninc acid `rarr` Malic acid
C. Malic acid `rarr` Oxaloacetic acid
D. Citric acid `rarr alpha`-Ketoglutaric acid

Answer

To determine in which step of the citric acid cycle (also known as the Krebs cycle) FAD is reduced to FADH2, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Citric Acid Cycle**: The citric acid cycle consists of a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.

2. **Identifying Key Reactions**: Within the citric acid cycle, various substrates are converted into products, and during these conversions, certain coenzymes are reduced or oxidized. FAD (flavin adenine dinucleotide) is one such coenzyme that can be reduced to FADH2.

3. **Finding the Relevant Reaction**: The reduction of FAD to FADH2 occurs specifically during the oxidation of succinate (succinic acid) to fumarate (fumaric acid). This reaction is catalyzed by the enzyme succinate dehydrogenase.

4. **Conclusion**: Therefore, the step in the citric acid cycle where FAD is reduced to FADH2 is when succinate is oxidized to fumarate.

5. **Final Answer**: The correct answer to the question is the conversion of succinic acid to fumaric acid.