Mechanistic aspects of the Qo-site of the bc1-complex
as revealed by mutagenesis studies, and the crystallographic structure.
By
A.R. Crofts*, Blanca Barquera*, R.B. Gennis*, R. Kuras*,
Mariana Guergova-Kuras* and E.A. Berry#
Center for Biophysics and Computational Biology, U. of Illinois
at Urbana-Champaign*, and Lawrence Berkeley National Laboratory, U. C.
Berkeley#.
Abstract
Solution of the X-ray crystallographic structure of the ubiquinol:cytochrome
c oxidoreductase (the bc1-complex) from several animal sources
has provide a basis for understanding the mechanism, and the effects of
mutation on the function of the complex. In this paper, we review the biophysical
information on the mechanism of the ubihydroquinone (quinol) oxidizing
site (the Qo-site), the information from analysis of mutant
strains which effect function or inhibitor binding at the quinol oxidizing
site, and details of the structure of the complex from avian heart mitochondria
as it pertains to this catalytic site. A paradoxical feature of the quinol
oxidation reaction is the mechanism that ensures that the two electrons
from quinol enter separate electron transfer chains. The crystallographic
data show several features of interest in this mechanism. The iron sulfur
protein is found in several different positions in different structures,
and in structures with inhibitors bound, stigmatellin and myxothiazol occupy
different domains in the Qo-binding pocket, with the former
distal and the latter proximal to heme bL. We suggest a mechanism
with the following novel features: i) movement of the iron sulfur protein
between two separate reaction interfaces on cytochrome c1 and
cytochrome b, and ii) movement of the occupant of the Qo-binding
pocket between a domain distal from the heme of cyt bL, but
close to the iron sulfur protein docking interface, and a domain proximal
to heme bL. We suggest that all interactions with the iron sulfur
protein occur from the distal position. These include a) the binding of
stigmatellin and UHDBT, which lead to a change in mid-point potential and
a shift in spectrum of the reduced 2Fe2S center, b) the formation of a
complex between ubiquinone and the reduced iron sulfur protein which gives
rise to the gx=1.800 EPR band, and c) the oxidation of quinol
to ubisemiquinone by the oxidized 2Fe2S center. We suggest that after formation,
the ubisemiquinone moves to the proximal position before transferring an
electron to heme bL, and that this movement, together with a
change in conformation of the site through displacement of a few
residues, prevents further interaction with the 2Fe2S center.
Reference.
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.). In press