Q.67 Select the enzyme involved in catalysis of substrate-level phosphorylation in citric acid cycle.
- Aconitase
- Succinate dehydrogenase
- Succinyl – CoA synthetase
- Fumarase
Succinyl-CoA synthetase catalyzes the sole substrate-level phosphorylation step in the citric acid cycle, converting succinyl-CoA to succinate while producing GTP (or ATP).
Correct Answer
Succinyl–CoA synthetase.
Option Analysis
Aconitase: Incorrect. This enzyme interconverts citrate and isocitrate via cis-aconitate in an isomerization reaction, with no phosphorylation involved.
Succinate dehydrogenase: Incorrect. It oxidizes succinate to fumarate, transferring electrons to ubiquinone for the electron transport chain (oxidative phosphorylation), not substrate-level.
Succinyl–CoA synthetase: Correct. It cleaves succinyl-CoA to succinate + CoA, using the energy to phosphorylate GDP/ADP to GTP/ATP via a phosphohistidine intermediate—true substrate-level phosphorylation.
Fumarase: Incorrect. This hydratase adds water across fumarate to form malate, a reversible non-redox, non-phosphorylating reaction.
Enzyme substrate-level phosphorylation citric acid cycle occurs once per turn at succinyl-CoA to succinate, generating GTP/ATP directly without electron transport.
Reaction Mechanism
Succinyl-CoA + Pi + GDP ⇌ Succinate + CoA + GTP. A histidine residue on the enzyme captures phosphate, then transfers it to GDP; this bypasses oxidative steps.
Cycle Context
Of 10 enzymes, only succinyl-CoA synthetase yields substrate-level ATP equivalent; others produce NADH/FADH2 for oxidative phosphorylation.
Enzyme Reaction Phosphorylation Type Aconitase Citrate ↔ Isocitrate None Succinate dehydrogenase Succinate → Fumarate Oxidative Succinyl-CoA synthetase Succinyl-CoA → Succinate Substrate-level Fumarase Fumarate → Malate None This step sustains ATP under hypoxia, linking to heme/ketone metabolism. For exams, distinguish it from respiratory chain phosphorylation.
