Substrate phosphorylation in TCA occurs when

Options

A. Succinic acid changes to fumaric acid
B. Fumaric acid changes to malic acid
C. Succinyl CoA changes to succinic acid
D. Oxalosuccinic acid changes to `alpha`-ketoglutaric acid

Answer

### Step-by-Step Solution:

1. **Understanding Substrate Phosphorylation**:
- Substrate phosphorylation is a biochemical process where a phosphate group is transferred from a substrate to ADP (adenosine diphosphate) or GDP (guanosine diphosphate), resulting in the formation of ATP (adenosine triphosphate) or GTP (guanosine triphosphate).
- This process occurs during specific steps in metabolic pathways, including the TCA cycle.

2. **Identifying the TCA Cycle**:
- The TCA cycle, also known as the Krebs cycle or citric acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy.
- It involves several key steps, each catalyzed by specific enzymes.

3. **Analyzing the Given Options**:
- The question provides four options regarding the steps in the TCA cycle where substrate phosphorylation might occur:
1. Succinic acid to fumaric acid
2. Fumaric acid to malic acid
3. Succinyl-CoA to succinic acid
4. Oxalo succinic acid to alpha-ketoglutaric acid

4. **Identifying the Correct Step**:
- Among the options, we need to identify which step involves the transfer of a phosphate group from a substrate to ADP or GDP.
- The only step in the TCA cycle that involves substrate phosphorylation is the conversion of **succinyl-CoA to succinic acid**.
- During this conversion, a phosphate group is transferred to ADP, resulting in the formation of ATP. This is the only step in the cycle where ATP is generated through substrate-level phosphorylation.

5. **Conclusion**:
- Therefore, the correct answer to the question is **option 3: succinyl-CoA changes to succinic acid**.