The enzyme that catalyzes a reaction of substrate level phosphorylation in cytric acid cycle is
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The cleavage of Succinyl-CoA is the only step in the entire citric acid cycle that directly produces a high-energy nucleoside triphosphate (GTP or ATP) without requiring the mitochondrial respiratory chain.
Step 1: Understanding the Concept:
Substrate-level phosphorylation is a metabolic reaction that results in the formation of ATP or GTP by the direct transfer of a phosphoryl group to ADP or GDP from another phosphorylated compound, without the direct involvement of the electron transport chain and oxidative phosphorylation. Detailed Explanation:
In the citric acid cycle (Krebs cycle), there is only one step where substrate-level phosphorylation occurs:
- This reaction is the conversion of Succinyl-CoA to Succinate.
- The enzyme catalyzing this reaction is Succinyl-CoA synthetase (also known as succinyl thiokinase).
- The high-energy thioester bond of succinyl-CoA is cleaved, and the released energy is conserved by driving the phosphorylation of GDP (or ADP) to form GTP (or ATP).
- In animal tissues, two isoforms of succinyl-CoA synthetase exist: one is specific for GDP and the other is specific for ADP.
- The other listed enzymes catalyze different types of reactions:
1. Succinate dehydrogenase oxidizes succinate to fumarate, generating $\text{FADH}_2$.
2. Fumarase hydrates fumarate to malate.
3. Malate dehydrogenase oxidizes malate to oxaloacetate, generating $\text{NADH} + \text{H}^+$. Step 2: Final Answer:
Succinyl-CoA synthetase is the enzyme responsible for substrate-level phosphorylation in the citric acid cycle.
