Figure 2.

Diagrammatic representation of the mammalian ETC. Mitochondrial respiratory chain consists of four membrane-bound enzyme complexes (I–IV) and three of these are proton pumps. In addition, the ETC also contains two intermediary substrates (coenzyme Q and cytochrome c). The NADH, H⁺ and FADH₂ produced by the intermediate metabolism are oxidized further by the mitochondrial respiratory chain to establish an electrochemical gradient of protons, which is finally used by the F1F0-ATP synthase (complex V) to produce ATP, which is the only form of energy used by the cell. Electrons can also be delivered to the respiratory chain at the level of coenzyme Q by the ETF system or by the G3PDH system. ETC, electron transport chain; ETF, electron-transferring flavoprotein; G3PDH, glycerol 3 phosphate dehydrogenase; mtDNA, mitochondrial DNA; nDNA, nuclear DNA.