Correct sequence of events in Krebs' cycle is

Options

A. Acetyl CoA `to` citrate `to` pyruvate a-ketoglutarate `to` succinate `to` malate `to` fumarate `to` OAA
B. Acetyl CoA `to` citric acid `to` a-keto-glutaric acid `to` succinic acid `to` fumaric acid `to` malic acid `to` OAA
C. Acetyl CoA`to` citric acid `to` malic acid `to alpha`-ketoglutaric acid `to` succinic acid `to` OAA
D. Pyruvic acid `to` Acetyl CoA `to` citrate `to` malate `to` fumarate

Answer

To determine the correct sequence of events in the Krebs cycle (also known as the citric acid cycle or TCA cycle), we can follow these steps:

### Step-by-Step Solution:

1. **Start with Acetyl Coenzyme A**: The cycle begins when Acetyl CoA (a 2-carbon molecule) combines with Oxaloacetate (a 4-carbon molecule) to form Citrate (a 6-carbon molecule).

2. **Formation of Citrate**: The reaction between Acetyl CoA and Oxaloacetate produces Citrate, which is the first step in the Krebs cycle.

3. **Conversion of Citrate to Isocitrate**: Citrate is then converted into Isocitrate through a rearrangement process.

4. **Formation of Alpha-Ketoglutarate**: Isocitrate is oxidized and decarboxylated to form Alpha-Ketoglutarate (a 5-carbon molecule). During this step, NAD+ is reduced to NADH and CO2 is released.

5. **Conversion to Succinyl CoA**: Alpha-Ketoglutarate undergoes another decarboxylation to form Succinyl CoA (a 4-carbon molecule). Again, NAD+ is reduced to NADH and CO2 is released.

6. **Formation of Succinate**: Succinyl CoA is converted into Succinate. This step involves substrate-level phosphorylation, where GDP is converted to GTP.

7. **Conversion of Succinate to Fumarate**: Succinate is then oxidized to Fumarate, with the reduction of FAD to FADH2.

8. **Formation of Malate**: Fumarate is hydrated to form Malate.

9. **Regeneration of Oxaloacetate**: Finally, Malate is oxidized to regenerate Oxaloacetate, reducing another NAD+ to NADH.

### Correct Sequence of Events:
- Acetyl CoA → Citrate → Isocitrate → Alpha-Ketoglutarate → Succinyl CoA → Succinate → Fumarate → Malate → Oxaloacetate

### Conclusion:
The correct sequence of events in the Krebs cycle is represented by the second option given in the question:
**Acetyl CoA → Citrate → Alpha-Ketoglutarate → Succinyl CoA → Succinate → Fumarate → Malate → Oxaloacetate.**