Which enzyme catalyzes the conversion of succinyl-CoA to succinate in the TCA cycle and requires which cofactor?

Explanation

## Succinyl-CoA to Succinate Conversion ### Enzyme Identity **Key Point:** Succinyl-CoA synthetase (also called succinate thiokinase) catalyzes the only substrate-level phosphorylation step in the TCA cycle. ### Cofactor Requirement **High-Yield:** This enzyme requires **GTP (or ATP in some tissues)** as the high-energy phosphate donor. The reaction is: $$\text{Succinyl-CoA} + \text{GDP} + \text{Pi} \rightarrow \text{Succinate} + \text{GTP} + \text{CoA}$$ ### Clinical Significance **Clinical Pearl:** This is the only step in the TCA cycle that directly generates a high-energy phosphate bond (GTP/ATP). In tissues with high ATP demand (muscle, brain), this GTP is rapidly converted to ATP by nucleoside diphosphate kinase. ### Mnemonic **Mnemonic:** **S**uccinyl-CoA **S**ynthetase = **S**ubstrate-level phosphorylation — the only direct energy capture in the cycle.