Q.81. Which one of the following is an example of substrate level phosphorylation?
(A) Glucose to Glucose 6-phosphate
(B) Fructose 6-phosphate to Fructose 1,6-bisphosphate
(C) 1,3-bisphosphoglycerate to 3-phosphoglycerate
(D) 2-phosphoglycerate to Phosphoenolpyruvate
Substrate level phosphorylation involves direct transfer of a phosphate group from a high-energy substrate to ADP, forming ATP without an electron transport chain. In this MCQ, option (C) is correct as it exemplifies this process in glycolysis.
Correct Answer
Option (C) 1,3-bisphosphoglycerate to 3-phosphoglycerate represents substrate level phosphorylation. The enzyme phosphoglycerate kinase catalyzes the direct transfer of the high-energy phosphate from 1,3-bisphosphoglycerate to ADP, producing ATP and 3-phosphoglycerate. This occurs in the payoff phase of glycolysis, yielding 2 ATP per glucose molecule.
Option Analysis
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(A) Glucose to Glucose 6-phosphate: This is the first step of glycolysis, primed by hexokinase using ATP hydrolysis, not generating ATP. No substrate level phosphorylation occurs here.
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(B) Fructose 6-phosphate to Fructose 1,6-bisphosphate: Phosphofructokinase-1 (PFK-1) adds a phosphate from ATP in this committed step, consuming energy rather than producing it.
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(C) 1,3-bisphosphoglycerate to 3-phosphoglycerate: Correct, as described above; the acyl phosphate donates directly to ADP.
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(D) 2-phosphoglycerate to Phosphoenolpyruvate: Enolase removes water to form the high-energy enol phosphate, but no phosphate transfer to ADP happens here. The next step (PEP to pyruvate) is the other substrate level phosphorylation site.
Glycolysis Context
Glycolysis produces net 2 ATP via two substrate level phosphorylations: one at this step and another from phosphoenolpyruvate to pyruvate. These are anaerobic and occur in the cytosol, vital for quick energy in cells.
