Which processes of Krebs' cycle are associated with both decarboxylations and dehydrogenation ?

Options

A. Succinate `rarr` Fumarate, Fumarate `rarr` Malate
B. Malate `rarr` Oxaloacetate, Succinate `rarr` Fumarate
C. `alpha`-Ketoglutaric acid `rarr` Succinate, Malate `rarr` Oxaloacetate
D. Isocitrate `rarr alpha`-Ketoglutaric acid, `alpha`-ketoglutaric acid `rarr` Succinate.

Answer

To answer the question regarding which processes of the Krebs cycle are associated with both decarboxylation and dehydrogenation, we need to analyze the options provided and understand the definitions of decarboxylation and dehydrogenation.

### Step-by-Step Solution:

1. **Understanding Decarboxylation and Dehydrogenation**:
- **Decarboxylation**: This is the process where a carbon atom is removed from a molecule, typically in the form of carbon dioxide (CO₂).
- **Dehydrogenation**: This involves the removal of hydrogen from a molecule, often resulting in the formation of NADH or FADH₂, which are electron carriers.

2. **Analyzing the Options**:
- **Option 1**: Succinate to fumarate, then fumarate to malate.
- **Succinate to fumarate**: This step involves dehydrogenation (FAD to FADH₂) but no decarboxylation occurs.
- **Fumarate to malate**: No decarboxylation or dehydrogenation occurs here.
- **Conclusion**: Incorrect option.

- **Option 2**: Malate to oxaloacetate, then succinate to fumarate.
- **Malate to oxaloacetate**: This step involves dehydrogenation (NAD⁺ to NADH) but no decarboxylation occurs.
- **Succinate to fumarate**: Only dehydrogenation occurs here.
- **Conclusion**: Incorrect option.

- **Option 3**: Alpha-ketoglutaric acid to succinate, then malate to oxaloacetate.
- **Alpha-ketoglutaric acid to succinate**: This step involves both decarboxylation (CO₂ is released) and dehydrogenation (NAD⁺ to NADH).
- **Malate to oxaloacetate**: Only dehydrogenation occurs here.
- **Conclusion**: This option has one step that meets the criteria, but not both steps.

- **Option 4**: Isocitrate to alpha-ketoglutaric acid, then alpha-ketoglutaric acid to succinate.
- **Isocitrate to alpha-ketoglutaric acid**: This step involves both decarboxylation (CO₂ is released) and dehydrogenation (NAD⁺ to NADH).
- **Alpha-ketoglutaric acid to succinate**: This step also involves both decarboxylation (CO₂ is released) and dehydrogenation (NAD⁺ to NADH).
- **Conclusion**: Correct option.

3. **Final Answer**: The correct answer is **Option 4**: Isocitrate to alpha-ketoglutaric acid and alpha-ketoglutaric acid to succinate are both associated with decarboxylation and dehydrogenation.