Inter-monomer electron transfer is too slow to compete with monomeric turnover in bc1 complex

The homodimeric bc1 complexes are membrane proteins essential in respiration and photosynthesis. The ~ 11 Å distance between the two bL-hemes of the dimer opens the possibility of electron transfer between them, but contradictory reports make such inter-monomer electron transfer controversial. We have constructed in Rhodobacter sphaeroides a heterodimeric expression system similar to those used before, in which the bc1 complex can be mutated differentially in the two copies of cyt b to test for inter-monomer electron transfer, but found that genetic recombination by cross-over then occurs to produce wild-type homodimer. Selection pressure under photosynthetic growth always favored the homodimer over heterodimeric variants enforcing inter-monomer electron transfer, showing that the latter are not competitive. These results, together with kinetic analysis of myxothiazol titrations, demonstrate that inter-monomer electron transfer does not occur at rates competitive with monomeric turnover. We examine the results from other groups interpreted as demonstrating rapid inter-monomer electron transfer, conclude that similar mechanisms are likely to be in play, and suggest that such claims might need to be re-examined.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (135 K)Download as PowerPoint slideHighlights► Linear titration of myxothiazol inhibition is diagnostic of a monomeric mechanism. ► Heterodimeric expression systems generate wild-type homodimer by recombination. ► Selection for survival of functional bc1 complex variants occurs during growth. ► Intermonomer electron transfer is not competitive with monomeric turnover.