Substrate level phosphorylation occur in step catalysed by which of the following enzyme in TCA cycle?
**Core Concept**
Substrate-level phosphorylation is a type of energy coupling mechanism in cellular respiration where a high-energy phosphate bond is directly formed from the transfer of a phosphate group to ADP, resulting in the formation of ATP. This process occurs in specific steps of the citric acid cycle (TCA cycle) and glycolysis.
**Why the Correct Answer is Right**
The correct answer is **Isocitrate dehydrogenase (IDH)**. This enzyme catalyzes the conversion of isocitrate to alpha-ketoglutarate, generating NADH and CO2 in the process. In this step, the enzyme catalyzes the transfer of a phosphate group from GDP to ADP, resulting in the formation of GTP, which is then used to drive the synthesis of ATP through substrate-level phosphorylation. This mechanism is unique to the TCA cycle and is essential for the efficient production of energy in the cell.
**Why Each Wrong Option is Incorrect**
* **Option A:** **Aldolase** is an enzyme involved in glycolysis, not the TCA cycle. It catalyzes the conversion of fructose-1,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
* **Option B:** **Pyruvate dehydrogenase** is an enzyme complex that catalyzes the conversion of pyruvate to acetyl-CoA, which then enters the TCA cycle. However, it does not participate in substrate-level phosphorylation.
* **Option D:** **Succinyl-CoA synthetase** is an enzyme that catalyzes the conversion of succinyl-CoA to succinate, generating GTP and CoA in the process. While it is involved in substrate-level phosphorylation, it is not the correct answer because it is not the step catalyzed by isocitrate dehydrogenase.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that substrate-level phosphorylation is a unique feature of the TCA cycle and glycolysis, allowing for the efficient production of ATP in these pathways. Understanding the specific enzymes and steps involved in these mechanisms is crucial for evaluating the energy metabolism of cells in various physiological and pathological states.
**Correct Answer:** C. Isocitrate dehydrogenase.