In Krebs cycle guanosine triphosphate is formed during the conversion of

Options

A. Isocitrate to oxalosuccinate
B. Oxalosuccinate to `alpha`-ketoglutarate
C. Succinyl CoA to succinate
D. Fumarate to malate

Answer

**Step-by-Step Solution:**

1. **Understanding the Krebs Cycle:**
The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a series of biochemical reactions used by all aerobic organisms to generate energy. During this cycle, various substrates are converted into different products, and energy carriers like NADH and GTP are produced.

2. **Identifying the Options:**
The question asks during which conversion in the Krebs cycle guanosine triphosphate (GTP) is formed. The options provided are:
- A) Isocitrate to oxalosuccinate
- B) Oxalosuccinate to alpha-ketoglutarate
- C) Succinyl-CoA to succinate
- D) Fumarate to malate

3. **Analyzing Each Option:**
- **Option A: Isocitrate to oxalosuccinate**
- This reaction is catalyzed by isocitrate dehydrogenase and produces NADH, not GTP. Thus, this option is incorrect.

- **Option B: Oxalosuccinate to alpha-ketoglutarate**
- This reaction is also catalyzed by isocitrate dehydrogenase and releases carbon dioxide, but does not produce GTP. Therefore, this option is incorrect.

- **Option C: Succinyl-CoA to succinate**
- This reaction is catalyzed by succinate thiokinase (also known as succinyl-CoA synthetase). In this reaction, GTP is synthesized from GDP. This option is correct.

- **Option D: Fumarate to malate**
- This reaction is catalyzed by fumarase and does not produce GTP. It involves the addition of water to fumarate to form malate. Thus, this option is incorrect.

4. **Conclusion:**
The correct answer is **Option C: Succinyl-CoA to succinate**, where guanosine triphosphate (GTP) is formed.

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