Fig. 2.

Normal mitochondrial respiratory chain and nutrient metabolism. Reducing agents (NADH or FADH₂) are generated from glycolysis and Krebs cycle of glucose metabolism and β-oxidation of fatty acids. While NADH or FADH₂ are oxidized to NAD⁺ or FAD, the electrons are carried to complex I (NADH-ubiquinone reductase), complex II (succinated ubiquinone reductase), complex III (ubiquinone-cytochrome c reductase), complex IV (cytochrome oxidase), and finally to O₂, which produces H₂O. Oxidation of NADH or FADH₂ generates protons that are pumped to intermembrane space through complex I, III, and IV. The pumped protons increase electrochemical gradient across the membrane. This proton gradient is the driving force for F0F1-ATPase (ATP synthase) to produce ATP