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Intermonomer Electron Transfer between the b Hemes of Heterodimeric Cytochrome bc1
journal contribution
posted on 2016-02-18, 15:38 authored by Pascal Lanciano, Bahia Khalfaoui-Hassani, Nur Selamoglu, Fevzi DaldalThe
ubihydroquinone:cytochrome c oxidoreductase,
or cytochrome bc1, is a central component
of respiratory and photosynthetic energy transduction pathways in
many organisms. It contributes to the generation of membrane potential
and proton gradient used for cellular energy (ATP) production. The
three-dimensional structures of cytochrome bc1 show a homodimeric organization of its three catalytic subunits.
The unusual architecture revived the issue of whether the monomers
operate independently or function cooperatively during the catalytic
cycle of the enzyme. In recent years, different genetic approaches
allowed the successful production of heterodimeric cytochrome bc1 variants and evidenced the occurrence of
intermonomer electron transfer between the monomers of this enzyme.
Here we used a version of the “two-plasmid” genetic
system, also described in the preceding paper (DOI:
10.1021/bi400560p), to study a new heterodimeric mutant variant
of cytochrome bc1. The strain producing
this heterodimeric variant sustained photosynthetic growth of Rhodobacter capsulatus and yielded an active heterodimer.
Interestingly, kinetic data showed equilibration of electrons among
the four b heme cofactors of the heterodimer, via
“reverse” intermonomer electron transfer between the bL hemes. Both inactive homodimeric and active
heterodimeric cytochrome bc1 variants
were purified to homogeneity from the same cells, and purified samples
were subjected to mass spectrometry analyses. The data unequivocally
supported the idea that the cytochrome b subunits
carried the expected mutations and their associated epitope tags.
Implications of these findings on our interpretation of light-activated
transient cytochrome b and c redox
kinetics and the mechanism of function of a dimeric cytochrome bc1 are discussed with respect to the previously
proposed heterodimeric Q cycle model.
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Keywords
bL hemesRhodobacter capsulatusintermonomer electron transferheterodimeric Q cycle modelIntermonomer Electron Transferb Hemesepitope tagsphotosynthetic energy transduction pathwaysphotosynthetic growthDOIATPb heme cofactorsmass spectrometry analysesdimeric cytochrome bc 1heterodimeric cytochrome bc 1 variantscytochrome bc 1.c redox kineticsproton gradientheterodimeric varianthomodimeric organizationcytochrome bc 1 showcytochrome bcytochrome b subunits
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