Q72. Which of the following catalyze(s) substrate–level phosphorylation?
(A) ATP synthase
(B) Succinate thiokinase
(C) Phosphofructokinase
(D) Pyruvate kinase
Succinate thiokinase and pyruvate kinase catalyze substrate-level phosphorylation.
Question Breakdown
Substrate-level phosphorylation involves direct transfer of a phosphate group from a high-energy substrate to ADP (or GDP), forming ATP (or GTP) without involving an electron transport chain. This occurs in glycolysis and the TCA cycle. Options (B) and (D) are correct.
Option Analysis
(A) ATP synthase
ATP synthase produces ATP via oxidative phosphorylation, using a proton gradient across the mitochondrial membrane to drive ADP phosphorylation. No direct substrate phosphate transfer happens here.
(B) Succinate thiokinase
Also called succinyl-CoA synthetase, this enzyme converts succinyl-CoA + Pi + ADP/GDP to succinate + CoA + ATP/GTP in the TCA cycle. The high-energy thioester bond in succinyl-CoA provides the phosphate for direct transfer.
(C) Phosphofructokinase
This glycolytic enzyme phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate using ATP as the phosphate donor. It consumes ATP rather than producing it through substrate-level means.
(D) Pyruvate kinase
In glycolysis, pyruvate kinase transfers a phosphate from phosphoenolpyruvate (PEP) to ADP, yielding pyruvate and ATP. The high-energy enol phosphate in PEP enables direct phosphorylation.
Substrate-level phosphorylation plays a key role in cellular energy production during glycolysis and the TCA cycle. Enzymes like succinate thiokinase and pyruvate kinase directly transfer phosphate groups to ADP, generating ATP without membrane-bound complexes.
Key Enzymes Involved
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Succinate thiokinase (Succinyl-CoA synthetase): Catalyzes succinyl-CoA to succinate, producing GTP (equivalent to ATP). Essential in TCA cycle for one ATP per glucose.
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Pyruvate kinase: Converts phosphoenolpyruvate to pyruvate in glycolysis, yielding 2 ATP per glucose. Critical payoff phase enzyme.
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Other examples include phosphoglycerate kinase in glycolysis.
Comparison Table
| Enzyme | Pathway | Substrate → Product | Nucleotide Produced |
|---|---|---|---|
| Succinate thiokinase | TCA cycle | Succinyl-CoA → Succinate | GTP/ATP |
| Pyruvate kinase | Glycolysis | PEP → Pyruvate | ATP |
| ATP synthase | Oxidative | Proton gradient | ATP |
| Phosphofructokinase | Glycolysis | F6P → F1,6BP (uses ATP) | None |
Importance for CSIR NET
These reactions highlight anaerobic ATP yield (4 ATP gross in glycolysis, net 2). Succinate thiokinase links TCA to energy conservation, vital for exam questions on bioenergetics.
