The oxidative decarboxylation occurs in the step

Options

A. Oxalosuccinate `to alpha`-ketoglutarate
B. Acetyl Co - A `to` Citrate
C. `alpha`-ketoglutrate `to` Succinyl co- A
D. Oxaloacetate `to` Citrate

Answer

To determine where oxidative decarboxylation occurs in the citric acid cycle (TCA cycle), we need to analyze the steps involved in the cycle and identify the reactions where carbon dioxide is released.

### Step-by-Step Solution:

1. **Understanding Oxidative Decarboxylation**:
- Oxidative decarboxylation is a biochemical reaction where a carboxyl group is removed from a molecule, resulting in the release of carbon dioxide (CO2). This process also involves the oxidation of the substrate.

2. **Identifying the Steps in the TCA Cycle**:
- The TCA cycle consists of several key steps:
- Acetyl-CoA combines with oxaloacetate to form citrate.
- Citrate is converted to isocitrate.
- Isocitrate is converted to alpha-ketoglutarate.
- Alpha-ketoglutarate is converted to succinyl-CoA.
- Succinyl-CoA is converted to succinate.
- Succinate is converted to fumarate.
- Fumarate is converted to malate.
- Malate is converted back to oxaloacetate.

3. **Analyzing Each Step for CO2 Release**:
- **Acetyl-CoA to Citrate**: No CO2 is released.
- **Citrate to Isocitrate**: No CO2 is released.
- **Isocitrate to Alpha-Ketoglutarate**: This is an oxidative decarboxylation step where CO2 is released.
- **Alpha-Ketoglutarate to Succinyl-CoA**: This is another oxidative decarboxylation step where CO2 is released.
- **Succinyl-CoA to Succinate**: No CO2 is released.
- **Succinate to Fumarate**: No CO2 is released.
- **Fumarate to Malate**: No CO2 is released.
- **Malate to Oxaloacetate**: No CO2 is released.

4. **Conclusion**:
- The oxidative decarboxylation occurs during the conversion of isocitrate to alpha-ketoglutarate and also during the conversion of alpha-ketoglutarate to succinyl-CoA. However, since the question asks for the specific step, the answer is:
- **Alpha-ketoglutarate to Succinyl-CoA** is the step where oxidative decarboxylation occurs.

### Final Answer:
The oxidative decarboxylation occurs in the step: **Alpha-ketoglutarate to Succinyl-CoA**.