Mechanism of the Qo-site of the bc1-complex, shown in a 3-D movie
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    The model shows two aspects of the mechanism (1) which have emerged from consideration of the structures from Ed Berry's laboratory.
  1. The mobile head of the iron sulfur protein (ISP) (red spacefilling model) of the complex moves between docking interfaces on cytochrome b (light blue) and cytochrome c1 (green), while the anchor (dark blue) remains stationery. Electron transfer occurs from QH2 at the cytochrome b interface, and to heme c1 at the cytochrome c1 interface, and the ISP moves back and forth between these sites as the enzyme turns over. The docking position on cytochrome b is seen in stigmatellin containing crystals, that near cytochrome c1 in native crystals.
  2. The quinone binding pocket at the Qo-site has two different domains. At one of these (the domain distal from the heme of cyt bL), the occupant is close to the Fe2S2 center. This domain is occupied by stigmatellin in one of Berry's structures, and we suggest it is the site at which the reaction complex between the enzyme, QH2 and the ISP is formed, from which QH2 undergoes oxidation. The movie shows binding of QH2 (yellow) in the pocket, followed by oxidation of QH2 to form the semiquinone (red), in this domain. The second (proximal) domain, nearer to the heme of cyt bL, is occupied by myxothiazol in another of Berry's structures. We suggest that the semiquinone (red) moves to this site on formation, and from this position passes its electron to the heme of cyt bL, forming Q (blue), which then exits the site. Note that because the activation barrier is in the reactions leading to dissociation of the reaction complex into products (2), the movement of the ISPred away from its docking domain on cyt b, and the movement of the semiquinone from distal to proximal lobes, would have to occur simultaneously. For technical reasons, these processes are shown as separate in the movie. The equilibrium constant for semiquinone formation is extremely small (2, 3), so the semiquinone is not detectable, even under conditions in which mass-action would favor its production.
  3. Electrons are shown as small light blue discs, and protons as smaller white discs, leaving the site for the aqueous interface. These are shown for illustrative purposes only. See main text for our suggestions about release of the first proton.
Reference.
  1. Crofts, A.R., Barquera, B., Gennis, R.B., Kuras, R., Guergova-Kuras, M. and Berry, E.A. (1997) Mechanistic aspects of the Qo-site of the bc1-complex as revealed by mutagenesis studies, and the crystallographic structure. Proceedings of the IXth. International Symposium on Phototrophic Prokaryotes, Vienna, Sept. 1997, (Peschek et al., eds.).
  2. Crofts,  A.R. and Wang, Z. (1989) How rapid are the internal reactions of the UQH2:cyt  c2 oxidoreductase? Photosynth. Res. 22, 69-87.
  3. Jünemann, S., Heathcote, P. and Rich, P. (1998) J. Biol. Chem. 273, 21603-21607

©Copyright 1996, Antony Crofts, University of Illinois at Urbana-Champaign, a-crofts@uiuc.edu