The enzyme decarboxylase catalyses the following step

Options

A. conversion of citric acid to cis aconitic acid
B. fumaric and to malic acid
C. oxalosuccinic acid to `alpha-` ketoglutaric acid
D. malic acid to oxaloacetic acid

Explanation

To solve the question regarding the enzyme decarboxylase and its catalytic activity, we will analyze each option provided and determine which reaction involves the release of carbon dioxide, characteristic of decarboxylase activity.

### Step-by-Step Solution:

1. **Understanding Decarboxylase**:
- Decarboxylase is an enzyme that catalyzes the removal of a carboxyl group (-COOH) from a substrate, resulting in the release of carbon dioxide (CO2).

2. **Analyzing the Options**:
- **Option A**: Conversion of citric acid to cis-aconitic acid.
- This reaction is catalyzed by aconitase, not decarboxylase. No carbon dioxide is released in this step.
- **Conclusion**: Incorrect option.

- **Option B**: Fumaric acid to malic acid.
- This reaction is catalyzed by fumarase. There is no release of carbon dioxide in this conversion.
- **Conclusion**: Incorrect option.

- **Option C**: Oxalosuccinic acid to alpha-ketoglutaric acid.
- This reaction involves the conversion of oxalosuccinate to alpha-ketoglutarate and is catalyzed by isocitrate dehydrogenase. However, it also involves decarboxylation, meaning carbon dioxide is released during this process.
- **Conclusion**: Correct option.

- **Option D**: Malic acid to oxaloacetic acid.
- This reaction involves the conversion of malate to oxaloacetate. It is a dehydrogenation reaction and does not involve the release of carbon dioxide.
- **Conclusion**: Incorrect option.

3. **Final Conclusion**:
- The only reaction that involves the release of carbon dioxide and is catalyzed by decarboxylase activity is **Option C: Oxalosuccinic acid to alpha-ketoglutaric acid**.