Cytochromes, Iron-Sulfur, and Copper Proteins Mediating Electron Transfer from the Cyt bc1 Complex to Photosynthetic Reaction Center Complexes

Abstract

SummaryElectron transfer between the photosynthetic reaction center and the cytochrome bc1 complexes is often mediated by a high redox potential soluble cytochrome. In purple non-sulfur bacteria, this electron donor is usually cytochrome c2 (Cyt c2), while cyanobacteria and green algae can use the distantly related cytochrome c6 protein. Genetic and biochemical support for the role of these soluble mediators has been obtained in purple bacteria Rhodobacter sphaeroides and Rb. capsulatus, and in several cyanobacteria and green algae. However, recent experiments indicate that photosynthetic organisms often contain redundant electron donors to light-oxidized reaction center complexes. For example, soluble or membrane-bound cytochromes can substitute for the normal electron donor, Cyt c2, in purple non-sulfur bacteria. In addition, the soluble copper protein, plastocyanin, is interchangeable with cytochrome c6 in cyanobacteria and green algae even though it is apparently the sole electron donor in green plants.In contrast, the soluble electron donor is either unknown or presumed to be different in photosynthetic bacteria that lack soluble high potential proteins in the Cyt c2 family. Spectroscopic analysis indicates this donor could be the high redox potential ferredoxin, HiPIP, in Rhodocyclus sp. and members of the Chromatiaceae and Ectothiorhodospiraceae families. In addition, cytochromes other than Cyt c2 may reduce reaction center complexes in some photosynthetic bacteria. For example, Cyt c-551 may be the donor in Ectothiorhodospira sp., while related cytochromes could function in Rc. purpureus and green bacteria. Finally, the membrane bound copper protein, auracyanin, from the green bacterium Chloroflexus aurantiacus could also be responsible for reaction center reduction. In these latter species, the combination of biochemical and genetic analyses have not been used to identify the actual electron donor(s) to reaction center complexes.