Metabolic reactions in the mitochondria

The respiratory chain is composed of 4 multi-subunit complexes (I, II, III, and IV) linked by the mobile electron carriers coenzyme Q10 and cytochrome c. The reduced forms of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH₂) are formed from the citric acid cycle and the beta-oxidation of fatty acids in the mitochondrial matrix. The respiratory chain transfers electrons from NADH (via complex I) and from reduced flavoproteins (via complex II and electron transfer flavoprotein-coenzyme Q oxidoreductase [ETF-Qo]) to coenzyme Q10, then complex III, cytochrome c and finally complex IV. At the same time, complexes I, III, and IV pump electrons across the inner mitochondrial membrane from the matrix to the intermembrane space. The influx of these electrons (protons) back into the mitochondrial matrix releases energy that is used in the phosphorylation of ADP (adenosine diphosphate) to ATP (adenosine triphosphate) by complex V (ATP synthetase), which is also embedded in the inner mitochondrial membrane.