In the TCA cycle ,`FADH_2` is formed during

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 determine during which step of the TCA cycle (Krebs cycle) FADH2 is formed, we can follow these steps:

### Step 1: Understand the TCA Cycle
The TCA cycle, also known as the Krebs cycle or citric acid cycle, is 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.

**Hint:** Familiarize yourself with the overall purpose and products of the TCA cycle.

### Step 2: Identify the Key Steps
In the TCA cycle, several key reactions occur, including the conversion of citric acid to various intermediates. The cycle begins with the combination of acetyl-CoA and oxaloacetic acid to form citric acid.

**Hint:** Recall the sequence of reactions in the TCA cycle and the intermediates formed.

### Step 3: Focus on the Formation of FADH2
FADH2 is specifically formed during the conversion of succinate to fumarate. This step is crucial as it involves the reduction of FAD to FADH2.

**Hint:** Pay attention to the specific reactions where FAD is involved, as it is directly related to the formation of FADH2.

### Step 4: Analyze the Options
1. **Succinyl CoA to Succinate** - This step produces NADH, not FADH2.
2. **Citrate to Cis-Aconitate** - This step does not produce FADH2.
3. **Succinate to Fumarate** - This is the correct step where FADH2 is produced.
4. **Fumarate to Malate** - This step does not produce FADH2.

**Hint:** Eliminate options that do not involve the formation of FADH2 based on your understanding of the reactions.

### Conclusion
The correct answer is that FADH2 is formed during the conversion of **succinate to fumarate** in the TCA cycle.

**Final Answer:** FADH2 is formed during the conversion of succinate to fumarate.