Question

Why does rotenone inhibit oxidative phosphorylation when the substrate is pyruvate but not when it's succinate?

Answer 1

Ans. #1. Rotenone is an inhibitor of Complex I (NADH:Q oxidoreductase) of ETC. It blocks/inhibits oxidation of NADH complex I.

Pyruvate, when used as sole substrate, produces NADH and FADH₂ through ETC. Since NADH can’t be oxidized at complex I (because its inhibited by rotenone), there is no production of electrons from NADH-

            NADH -----------> NAD⁺ + 2e + 2H⁺

Further, deprivation of electrons supply to complex III and IV shuts down does not pumps electrons from mitochondrial matric (MM) to intermembrane space (IMS). No proton pump at complexes I, III and IV result no electrochemical gradient across the inner membrane.

Since there is no establishment of proton (electrochemical gradient) in presence of rotenone because of inhibition of complex I (assuming complex II is absent), no oxidative phosphorylation (ATP synthesis by ATP synthase) occurs.

#2. Succinate is oxidized at complex II (FADH2:Q oxidoreductase) of ETC. Rotenone does not inhibit complex II.

Oxidation of FADH2 at complex II yields electrons (FADH₂ ------> FAD + 2e + 2H⁺). When electrons are passed from complex II to III and then IV, there is establishment of proton gradient across the inner membrane as usual. The potential energy stored in proton gradient across the inner membrane is further harvested as ATP by ATP synthase- the process being called oxidative phosphorylation.

Conclusion: Rotenone inhibits oxidative phosphorylation when pyruvate is used as substrate because its product NADH can’t be oxidized at rotenone-inhibited complex I.

Use of succinate as substrate leaves oxidative phosphorylation unaffected because it does not inhibit complex II of ETC where succinate is oxidized.