In which of the following steps of the TCA cycle does Substrate level phosphorylation occurs?
**Core Concept**
The TCA (tricarboxylic acid) cycle, also known as the citric acid cycle or Krebs cycle, is a key metabolic pathway that generates energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. Substrate-level phosphorylation is a type of energy coupling that directly generates ATP from the high-energy phosphate compound phosphoenolpyruvate (PEP) or other high-energy intermediates.
**Why the Correct Answer is Right**
Substrate-level phosphorylation in the TCA cycle occurs in the step catalyzed by Succinyl-CoA synthetase (also known as Succinic thiokinase). This enzyme catalyzes the conversion of succinyl-CoA to succinate, generating GTP (guanosine triphosphate) from GDP (guanosine diphosphate) and inorganic phosphate by directly transferring a phosphate group. This GTP molecule is then converted to ATP by the enzyme NADH dehydrogenase (Complex I) of the electron transport chain. This step represents a substrate-level phosphorylation event, where energy is directly coupled to the formation of ATP.
**Why Each Wrong Option is Incorrect**
* **Option A:** The step catalyzed by Citrate synthase is the first step of the TCA cycle and does not involve substrate-level phosphorylation.
* **Option B:** The step catalyzed by α-Ketoglutarate dehydrogenase is a critical step in the TCA cycle but does not involve substrate-level phosphorylation; instead, it involves the conversion of α-ketoglutarate to succinyl-CoA, releasing NADH and CO2.
* **Option D:** The step catalyzed by Malate dehydrogenase involves the conversion of malate to oxaloacetate, generating NADH as a byproduct but not involving substrate-level phosphorylation.
**Clinical Pearl / High-Yield Fact**
The TCA cycle is a crucial energy-producing pathway in the mitochondria, and understanding its steps, including substrate-level phosphorylation, is essential for appreciating how cells generate energy from the breakdown of nutrients.
**Correct Answer: C. Succinyl-CoA synthetase**