Mechanism of the Qo-site of the bc1-complex,
shown as an annotated QuickTime movie (coutesy, John Allen).
The model shows aspects of the mechanism which have emerged from
consideration of the structures from Ed Berry's laboratory.
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.
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 2Fe2S center. This domain is occupied by stigmatellin
in one of Berry's structures, and we suggest it is the site 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. Electrons
are shown as small light blue discs, and protons as smaller white discs,
leaving the site for the aqueous interface.
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.).
©Copyright 1996, Antony
Crofts, University of Illinois at Urbana-Champaign,