Fig. 1: The mitochondrial respiratory chain.
The protonmotive complexes I, III and IV are depicted in the inner membrane, together with additional components of the chain, complex II, cytochrome c and ubiquinone (Q), the hydrogen donors NADH and succinate and the acceptor, O2. The combination of N,N,N’,N’-tetramethyl-phenylene-diamine (TMPD) and ascorbate (not shown for clarity; see the text) donates electrons directly to cytochrome c and thus engages complex IV only. Black arrows depict the hydrogen and electron transfers in the chain, eventually reducing O2 to water. Red arrows symbolise the associated protonmotive events where Zn is the effective protonmotive H+/e− stoichiometry, 2 for complex I and complex IV, 1 for complex III (see “Methods”) and n is the name of the complex. Typical average redox potential differences (ΔE) across each complex are given. The net free energy change (ΔG) for any combination of complexes is given as the arithmetic sum of the relevant redox potential difference and the protonmotive force (ΔP) multiplied by the H+/e− stoichiometry (Z), where ΔE has a negative sign and is larger than ΔP for the case where redox chemistry drives proton translocation.
