Biochemistry — Glycolysis energy yield: During glycolysis, which enzyme-catalyzed step represents a principal ATP-generating substrate-level phosphorylation event?

Introduction / Context:
Glycolysis converts glucose to pyruvate with net ATP gain via substrate-level phosphorylation. Two steps generate ATP directly: the phosphoglycerate kinase reaction and the pyruvate kinase reaction. Recognizing these steps is essential for understanding pathway energetics.

Given Data / Assumptions:

• Substrate-level phosphorylation generates ATP from a high-energy phosphate intermediate without an electron transport chain.
• Glycolysis includes oxidation, phosphorylation, and isomerization reactions.
• The question asks for a principal ATP-generating step.

Concept / Approach:
Phosphoglycerate kinase transfers phosphate from 1,3-bisphosphoglycerate to ADP to form ATP and 3-phosphoglycerate. This is the first ATP-forming step in the payoff phase. Pyruvate kinase later transfers phosphate from phosphoenolpyruvate to ADP to form ATP and pyruvate. Dehydrogenase steps yield NADH, not ATP, and phosphofructokinase consumes ATP.

Step-by-Step Solution:

Verification / Alternative check:
Stoichiometry: per glucose, two 1,3-BPG molecules each yield an ATP via phosphoglycerate kinase, confirming its role as a principal ATP contributor.

Why Other Options Are Wrong:

• Pyruvate kinase also makes ATP but is the second ATP-forming step; the question emphasizes recognizing a principal substrate-level phosphorylation, and phosphoglycerate kinase is a canonical answer.
• Glyceraldehyde-3-phosphate dehydrogenase forms NADH, not ATP.
• Phosphofructokinase consumes ATP and regulates flux.

Common Pitfalls:
Equating total ATP formation only with the terminal pyruvate kinase step; remember there are two ATP-yielding reactions in glycolysis.

Final Answer:
Phosphoglycerate kinase