Proton pumping redox coupled

The oxidation/reduction of redox cofactors in biological systems is often coupled to proton binding/release either at the cofactor itself or at local amino acid residues, which provides the basic mechanochem-ical part of a proton pump such as that foimd in cytochrome c oxidase (95). Despite a thermodynamic cycle that provides that coupling of protonation of amino acids to the reduction process will result in a 60 mV/pH decrease unit in the reduction potential per proton boimd between the pAa values in the Fe(III) and Fe(II) states, the essential pumping of protons in the respiratory complexes has yet to be localized within their three-dimensional structures. 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

Fig. 4. Proposed functions of the hydroxyfarnesylethyl group of heme A. (A) A possible electron transfer pathway formed by overlapping of rr-electron orbitals in the alkyl chain with that of the pyrrole. (B) A side-on coordination of the terminal double bond to Cub and a coordination of a deprotonated form of the double bond to Fe j. A proposed proton-pumping mechanism including the redox-coupled change in the two coordination states.

Rich (1995) has proposed that the proton pump in cytochrome c oxidase is driven mainly by electrostatic interactions (or repulsion) between protons in a proton trap and protons transferred from the matrix side to the O2 reduction site for neutralizing oxides (O ) are produced by O2 reduction. In this mechanism, a structural change for gating proton transfer from the matrix side to the proton trap is required for a complete cycle of redox-coupled proton pumping. However, no such structural change has been detected. 

Of the four redox centres, haem a is the most likely one to be coupled to proton translocation (Fig. 3.6). It exhibits heterogeneous oxidoreduction kinetics, membrane sidedness with respect to proton dependence, and looses the proton-dependence of oxidoreduction simultaneously with loss of proton pumping when subunit III is removed from the enzyme (see below, and discussion in Refs. 92, 99). 

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