Pathways of Transmembrane Electron Transfer in Cytochrome bc Complexes: Dielectric Heterogeneity and Interheme...

The intramembrane cytochrome bc₁ complex of the photosynthetic bacterium Rhodobacter capsulatus and the cytochrome b₆f complex, which functions in oxygenic photosynthesis, utilize two pairs of b-hemes in a symmetric dimer to accomplish proton-coupled electron transfer. The transmembrane electron transfer pathway in each complex was identified through the novel use of heme Soret band excitonic circular dichroism (CD) spectra, for which the responsible heme-heme interactions were determined from crystal structures. Kinetics of heme reduction and CD amplitude change were measured simultaneously. For bc₁, in which the redox potentials of the transmembrane heme pair are separated by 160 mV, heme reduction occurs preferentially to the higher-potential intermonomer heme pair on the electronegative (n) side of the complex. This contrasts with the b₆f complex, where the redox potential difference between transmembrane intramonomer p- and n-side hemes is substantially smaller and the n-p pair is preferentially reduced. Limits on the dielectric constant between intramonomer hemes were calculated from the interheme distance and the redox potential difference, ΔE_m. The difference in preferred reduction pathway is a consequence of the larger ΔE_m between n- and p-side hemes in bc₁, which favors the reduction of n-side hemes and cannot be offset by decreased repulsive Coulombic interactions between intramonomer hemes.