Q.28 The example of substrate level phosphorylation in glycolysis is
(A)Conversion of Glucose to Glucose-6-phosphate
(B) Conversion of Glyceraldehyde-3-phosphate to 1,3-Bisphosphoglycerate
(C) Conversion of 1,3-Bisphosphoglycerase to 3-Phosphoglycerate
(D)Conversion of Dihydroxyacetone phosphate to Glyceraldehyde-3-phosphate
Correct Answer: (C) Conversion of 1,3-Bisphosphoglycerate to 3-Phosphoglycerate
Substrate level phosphorylation in glycolysis occurs twice per glucose molecule: once in this step (catalyzed by phosphoglycerate kinase) and once later (when phosphoenolpyruvate converts to pyruvate). Here, the high-energy phosphate from 1,3-bisphosphoglycerate (1,3-BPG) transfers directly to ADP, producing ATP and 3-phosphoglycerate (3-PG). No electron transport chain is involved—it’s a direct enzymatic handoff.
This step yields 2 ATP net from glycolysis (after early investments), powering cells anaerobically.
Why Not the Other Options? Full Explanations
(A) Conversion of Glucose to Glucose-6-Phosphate
This is the first glycolysis step, catalyzed by hexokinase or glucokinase. ATP donates a phosphate to glucose, forming glucose-6-phosphate (G6P).
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This is phosphorylation using ATP (an investment phase), not substrate level phosphorylation.
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No ATP is produced here; it’s consumed.
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Incorrect for the query.
(B) Conversion of Glyceraldehyde-3-Phosphate to 1,3-Bisphosphoglycerate
Glyceraldehyde-3-phosphate dehydrogenase oxidizes G3P using NAD⁺ and inorganic phosphate (Pi), adding it to form 1,3-BPG.
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This creates a high-energy acyl phosphate but does not produce ATP. The phosphate comes from Pi, not a substrate transfer to ADP.
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ATP generation happens in the next step.
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Incorrect—it’s preparatory for substrate level phosphorylation.
(D) Conversion of Dihydroxyacetone Phosphate to Glyceraldehyde-3-Phosphate
Triose phosphate isomerase interconverts dihydroxyacetone phosphate (DHAP) and G3P via enediol intermediate—no phosphates or ATP involved.
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Pure isomerization; no phosphorylation occurs.
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Balances triose sugars from fructose-1,6-bisphosphate split but skips energy steps.
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Incorrect entirely.
Quick Summary Table: Glycolysis Steps and Phosphorylation
| Option | Step | Enzyme | ATP Produced? | Type of Phosphorylation |
|---|---|---|---|---|
| (A) | Glucose → G6P | Hexokinase | No (consumed) | ATP-dependent |
| (B) | G3P → 1,3-BPG | GAPDH | No | Oxidation + Pi addition |
| (C) | 1,3-BPG → 3-PG | Phosphoglycerate kinase | Yes | Substrate level |
| (D) | DHAP → G3P | Triose phosphate isomerase | No | None (isomerization) |
Why Master Substrate Level Phosphorylation in Glycolysis?
Grasping this boosts scores in molecular biology exams and clarifies anaerobic ATP production in muscles or yeast. It contrasts with aerobic yields (up to 36-38 ATP total).
For visuals, check Khan Academy or NCERT diagrams. Practice more MCQs on glycolysis payoff phase!
