The mammalian mitochondrial genome and its protein-coding gene repertoire involved in the oxidative phosphorylation pathway. (A) Schematic representation of genes within mammalian mitochondrial genome (~7,000 bp). Genes on the outer circle are transcribed from the light-strand. Location of the tRNAs (red boxes) conform to the canonical placental mammalian arrangement. Abbreviations: HSP, putative heavy-strand promoter; OHR, origin of heavy-strand replication; OLR, origin of light-strand replication. (B) Simplified view of the mitochondrial oxidative phosphorylation machinery. Complexes I (NADH dehydrogenase) and II (succinate dehydrogenase) receive electrons from either NADH or FADH2. Electrons are then carried between complexes by the carrier molecules coenzyme Q/ubiquinone (UQ) and cytochrome c (CYC). The potential energy of these electron transfer events is used to pump protons against the gradient, from the mitochondrial matrix into the intermembrane space [Complexes I and III (cytochrome bc1) and IV (cytochrome c oxidase)]. ATP synthesis by Complex V (ATP synthase) is driven by the proton gradient, and occurs in the mitochondrial matrix. MM: mitochondria matrix; IM: intermembrane space.