Energy transduction: proton transfer through the respiratory complexes

Abstract

A series of metalloprotein complexes embedded in a mitochondrial or bacterial membrane utilize electron transfer reactions to pump protons across the membrane and create an electrochemical potential (DeltamuH+). Current understanding of the principles of electron-driven proton transfer is discussed, mainly with respect to the wealth of knowledge available from studies of cytochrome c oxidase. Structural, experimental, and theoretical evidence supports the model of long-distance proton transfer via hydrogen-bonded water chains in proteins as well as the basic concept that proton uptake and release in a redox-driven pump are driven by charge changes at the membrane-embedded centers. Key elements in the pumping mechanism may include bound water, carboxylates, and the heme propionates, arginines, and associated water above the hemes. There is evidence for an important role of subunit III and proton backflow, but the number and nature of gating mechanisms remain elusive, as does the mechanism of physiological control of efficiency.

Figure 1

Complexes of the respiratory chain. These include Escherichia coli NADH dehydrogenase (145), succinate dehydrogenase 1NEN, bc₁ complex 1PP9, cytochrome c oxidase 1V54, and cytochrome c 1HRC.

Figure 2

CcO (R. sphaeroides numbering in structure 1M56) showing the D path (red) and K path (blue) with D-path waters (red spheres) and K-path waters (blue spheres). Heme a and a₃ (green) are stick structures with Ca and Mg metals (green spheres) and Cu metal (orange spheres).

Figure 3

Proposed oxygen reduction reactions at the active site of CcO during steady-state turnover. Abbreviations used are as follows: Y, Y288 (R. sphaeroides numbering); R, reduced; A, oxy; P, F, and O are as described in the text. Only substrate protons are indicated. The two phases of the catalytic cycle merge to a certain extent, but the metal reduction phase is essentially O to R, whereas the O₂ reduction phase is A to F.

Figure 4

The area surrounding heme a and a₃, including the nonredox Mg, is shown in the R. sphaeroides CcO (1M56) (16). The arginine pair interacts closely with the heme propionates. The nitrogen on W172 is close to the heme propionate and away from E286.

Figure 5

The absence of subunit III decreases the rate of proton uptake into the D pathway to the diffusion limit. Estimated bimolecular rate constants for the uptake of protons into the D pathway during steady-state turnover were calculated from the rates of steady-state activity of wild-type [WT, and without subunit III, WT III (−)] CcO. The red and gray lines represent diffusion-limited rate constants of 2–6 · 10¹⁰ M⁻¹s⁻¹, i.e., the range of rate constants expected if the diffusion of buffer to D132 through bulk solvent determines the rate of proton uptake. Rate constants above these limits are suggestive of the function of a proton antenna that increases the local concentration of protons near D132 of the D pathway.

Figure 6

After over a nanosecond of an MD simulation of the R. sphaeroides CcO structure with added water (31, 86), a chain of hydrogen-bonded waters is clearly seen—stretching from E286 through a hydrophobic cavity to Mg. The glutamate has its carboxyl pointed up and is interacting with W172.