Compound M and the Muscle Cells: ATP Without Oxygen Failure
A research team grows human skeletal-muscle cells in sealed culture flasks filled with medium containing 25 mM glucose and equilibrated with room air. Each flask has a dissolved-oxygen sensor and a pH electrode. In control flasks, oxygen consumption is steady at 18 nmol per minute per 10^6 cells and ATP content is 42 nmol per 10^6 cells. The team then adds Compound M, which binds tightly to complex III of the electron transport chain and stops electron transfer. Sixty minutes later, oxygen consumption has fallen 85%, ATP content is 6 nmol per 10^6 cells, lactate in the medium has risen sixfold, and pH has dropped from 7.4 to 6.9. The medium still contains 22 mM glucose, and oxygen is still present at 60% of its original concentration. A second set of flasks receives Compound M plus an inhibitor of lactate dehydrogenase, the enzyme that converts pyruvate to lactate.
1. Explain why ATP fell so sharply even though glucose and oxygen were both still available, and estimate the ATP yield per glucose molecule in the Compound M flasks.
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Compound M blocks electron transfer at complex III, so NADH and FADH2 cannot be reoxidized through the chain. The proton gradient across the inner mitochondrial membrane collapses, and ATP synthase has no proton-motive force to drive it. Oxidative phosphorylation therefore produces almost no ATP even though O2 is present, because O2 can act as the terminal electron acceptor only if electrons reach complex IV.
The cells must fall back on glycolysis, which yields a net 2 ATP and 2 NADH per glucose. That NADH cannot be used by the blocked chain, so pyruvate is reduced to lactate to regenerate NAD+, giving roughly 2 ATP per glucose instead of about 32 from complete oxidation. That is about 6% of the normal yield, close to the observed 6/42 = 14% of control ATP, with the remainder supplied by glycolysis and existing ATP pools.
2. Predict what will happen in the flasks that receive Compound M plus the lactate dehydrogenase inhibitor, and justify your prediction using the role of NAD+.
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Glycolysis requires a continuous supply of NAD+. Lactate dehydrogenase is the reaction that reoxidizes NADH to NAD+ when the electron transport chain cannot. If lactate dehydrogenase is inhibited, NADH accumulates and glycolysis stalls at the glyceraldehyde-3-phosphate dehydrogenase step, so even the 2 ATP per glucose disappears.
ATP should therefore fall further and faster than with Compound M alone. Lactate should stay low instead of rising, and pyruvate should accumulate. Losing the last ATP source will impair ion pumps, so the cells should lose viability sooner. The result shows that fermentation is not a minor side pathway here; it is the only way to keep glycolysis running when respiration is blocked.
3. Explain why oxygen concentration in the flask does not fall to zero, and what that reveals about the limiting factor for ATP production.
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The block is at complex III, not at oxygen delivery. Because the cells' oxygen consumption has already dropped 85%, the remaining 15% of use cannot deplete a sealed flask within an hour, so oxygen stays at about 60% of its starting level.
This tells us oxygen is not the limiting factor. The limiting factor is electron transfer through the chain, and downstream of that, NAD+ regeneration. If hypoxia were the problem, adding oxygen would raise ATP production; here it would not, because the chain is chemically blocked. Low ATP with normal oxygen is a useful diagnostic signature of a respiratory-chain lesion rather than oxygen limitation.