oxidative phosphorylation

Oxidative phosphorylation couples electron transport with ATP synthesis. NADH and FADH2 generated from glycolysis, the citric acid cycle, and fatty acid oxidation donate electrons to Complexes I and II of the electron transport chain, respectively. Electrons flow through Complexes I→III→IV (with coenzyme Q and cytochrome c as mobile carriers), ultimately reducing O2 to H2O at Complex IV. This electron flow pumps H+ ions from the mitochondrial matrix into the intermembrane space at Complexes I, III, and IV, creating an electrochemical gradient (proton-motive force). Protons flow back into the matrix through ATP synthase (Complex V), driving the phosphorylation of ADP to ATP.

Key clinical correlations: - Uncouplers (e.g., 2,4-dinitrophenol, aspirin overdose, thermogenin/UCP1 in brown fat) dissipate the proton gradient without ATP production, causing increased O2 consumption, heat generation (hyperthermia), and decreased ATP yield. - ETC inhibitors: Rotenone (Complex I), Antimycin A (Complex III), Cyanide/CO/Azide (Complex IV) block electron flow, halting ATP synthesis and increasing lactic acid production. - ATP synthase inhibitors: Oligomycin blocks Complex V directly. - Mitochondrial diseases (e.g., MELAS, Leigh syndrome, Kearns-Sayre syndrome) result from mutations in mitochondrial DNA affecting ETC complexes, leading to lactic acidosis and multisystem dysfunction, especially in high-energy-demand tissues (muscle, brain). - Malignant hyperthermia and neuroleptic malignant syndrome involve uncontrolled heat production related to mitochondrial dysfunction/calcium release.

Understanding this pathway is essential for explaining energy failure in ischemia, toxin exposure, and mitochondrial myopathies, as well as pharmacologic principles (e.g., metformin’s inhibition of Complex I contributing to lactic acidosis risk).