Proton pumping in the bc1 complex: A new gating mechanism that prevents short circuits

Abstract

The Q-cycle mechanism of the bc1 complex explains how the electron transfer from ubihydroquinone (quinol, QH2) to cytochrome (cyt) c (or c2 in bacteria) is coupled to the pumping of protons across the membrane. The efficiency of proton pumping depends on the effectiveness of the bifurcated reaction at the Q(o)-site of the complex. This directs the two electrons from QH2 down two different pathways, one to the high potential chain for delivery to an electron acceptor, and the other across the membrane through a chain containing heme bL and bH to the Qi-site, to provide the vectorial charge transfer contributing to the proton gradient. In this review, we discuss problems associated with the turnover of the bc1 complex that center around rates calculated for the normal forward and reverse reactions, and for bypass (or short-circuit) reactions. Based on rate constants given by distances between redox centers in known structures, these appeared to preclude conventional electron transfer mechanisms involving an intermediate semiquinone (SQ) in the Q(o)-site reaction. However, previous research has strongly suggested that SQ is the reductant for O2 in generation of superoxide at the Q(o)-site, introducing an apparent paradox. A simple gating mechanism, in which an intermediate SQ mobile in the volume of the Q(o)-site is a necessary component, can readily account for the observed data through a coulombic interaction that prevents SQ anion from close approach to heme bL when the latter is reduced. This allows rapid and reversible QH2 oxidation, but prevents rapid bypass reactions. The mechanism is quite natural, and is well supported by experiments in which the role of a key residue, Glu-295, which facilitates proton transfer from the site through a rotational displacement, has been tested by mutation.

Overall energy conversion efficiency of a photosynthetic vesicle

“…Induced curvature profiles are known to exert a segregating force between different types of proteins in the membrane (Frese et al, 2008). stoichiometry is 3:1 (Cartron et al, 2014), consistent with earlier observations (Crofts, 2004;Crofts et al, 2006), corresponding to approximately 4 cytbc 1 dimeric complexes per vesicle. Chromatophore vesicles typically contain 1-2 ATP synthases (Feniouk et al, 2002;Cartron et al, 2014).…”

“…The primary components of chromatophore vesicles in purple bacteria, as depicted in Figure 1, are, in order of energy utililization (Cogdell et al, 2006,Cartron et al, 2014: (i) light harvesting complex 2 (LH2) (Koepke et al, 1996;Papiz et al, 2003); (ii) light harvesting complex 1 (LH1 Sener et al, 2009]); (iii) RC (Jamieson et al, 2002;Strümpfer and Schulten, 2012a); (iv) cytbc 1 (Crofts, 2004;Crofts et al, 2006); and (v) ATP synthase (Feniouk and Junge, 2009;Hakobyan et al, 2012). RC-LH1 complexes typically form dimeric RC-LH1-PufX complexes facilitated by the polypeptide PufX Sener et al, 2009), although monomeric complexes are also found in membranes from photosynthetically grown cells at a ratio of approximately 10% .…”

A semiquinone intermediate generated at the Q o site of the cytochrome bc 1 complex: Importance for the Q-cycle and superoxide production

“…In some models the SQ is thermodynamically stabilized (45), thus lowering its reactivity with O 2 ; in other models it is specifically destabilized (3,36), limiting its reactivity with O 2 by lowering its steady-state concentration. Some models posit that the SQ is shielded from O 2 within the Q o site (3), while access to the site or its reactivity is proposed to be allosterically (46,47) or electrostatically (14,27) gated. Still, other models deny the existence of an SQ intermediate altogether (21).…”

“…The key question in cyt bc complexes is how the Q o site minimizes bypass reactions while maintaining high flux through the Q-cycle (14,27,28). The question is profound because the bypass reactions are vastly more thermodynamically favored (21,29).…”

Review and Hypothesis. New insights into the reaction mechanism of transhydrogenase: Swivelling the dIII component may gate the proton channel

“…The chemistry of oxygen reduction by cytochrome c oxidase is used to gate proton channels and alter proton affinities resulting in proton pumping, but conformational changes must be subtle compared to either Complex I or the ATP synthase [61]. Complex III has a unique ''Q-cycle'' to move charges across the membrane and a large conformational change of the Rieske Fe-S subunit to assure the two electrons from quinol are bifurcated, i.e., directed to different electron acceptors [62]. The enzymology of each of these systems is distinct from the others and each is a remarkable testament to the different ways that evolution has solved the problem of both generating and utilizing the proton motive force across a biological membrane.…”