In Krebs cycle, `FADH_(2)` is formed during conversion of

Options

A. Conversion of succinyl Co-A to succinate
B. conversion of citrate to cis-aconitate
C. conversion of succinate to fumarate
D. conversion of fumarate to malate

Answer

To solve the question regarding the formation of FADH2 in the Krebs cycle, we can analyze each option step by step.

### Step-by-Step Solution:

1. **Understand the Krebs Cycle**: The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a series of enzymatic reactions that produce energy through the oxidation of acetyl-CoA. During this cycle, various intermediates are formed, and electron carriers like NADH and FADH2 are generated.

2. **Evaluate Each Option**:
- **Option 1: Conversion of Succinyl CoA to Succinate**: This reaction produces GTP (or ATP) but does not produce FADH2. Therefore, this option is incorrect.

- **Option 2: Conversion of Citrate to Cis-Aconitate**: This step involves the isomerization of citrate to cis-aconitate and does not produce FADH2. Instead, it is associated with NADPH production. Thus, this option is also incorrect.

- **Option 3: Conversion of Succinate to Fumarate**: This reaction is catalyzed by the enzyme succinate dehydrogenase and is the specific step in the Krebs cycle where FADH2 is produced. Therefore, this option is correct.

- **Option 4: Conversion of Fumarate to Malate**: This reaction involves the hydration of fumarate to malate and does not produce FADH2. Hence, this option is incorrect.

3. **Conclusion**: After evaluating all the options, the correct answer is that FADH2 is formed during the conversion of succinate to fumarate in the Krebs cycle.

### Final Answer:
FADH2 is formed during the conversion of **succinate to fumarate** in the Krebs cycle.

---