Dielectric Heterogeneity in the Cytochrome B6F Complex

Abstract

Electron transfer in the dimeric cytochrome b6f complex, which includes four b-type hemes organized as two pairs in symmetric monomers, was studied by simultaneous measurement of the kinetics of heme reduction by dithionite and an associated amplitude increase of Soret band split circular dichroism (CD) spectra diagnostic of heme-heme exciton interactions, for which similar kinetics were determined. Based on inter-heme distances and orientations from crystal structures of the complex, the increase in the split CD signal is dominated by interaction between the two intra-monomer b-hemes, located on the electrochemically negative and positive sides of the complex, whose midpoint oxidation-reduction potentials, Em, determined by titrations of isolated complex, differ by 75-100 mV. Kinetics are fit best by preferential reduction of the intra-monomer heme pair. Equilibration of transferred electrons would, however, predict preferential reduction of the two higher potential hemes, one in each monomer. Heterogeneity of the dielectric constant is implied, a consequence of structure inhomogeneity, and/or dielectric reorganization in response to electron transfer. The largest dielectric constant exists between the intra-monomer b-hemes, resulting in a lower energy state of the reduced intra-monomer heme pair relative to any other heme pair.View Large Image | View Hi-Res Image | Download PowerPoint Slide