Jürgen Wastl

Cytochrome c_6A is a unique dithio-cytochrome of green algae and plants. It has a very similar core structure to that of bacterial and algal cytochromes c₆ but is unable to fulfill the same function of transferring electrons from cytochrome f to photosystem I. A key feature is that its heme midpoint potential is more than 200 mV below that of cytochrome c₆ despite having His and Met as axial heme-iron ligands. To identify the molecular origins of the difference in potential, the structure of cytochrome c₆ from the cyanobacterium Phormidium laminosum has been determined by X-ray crystallography and compared with the known structure of cytochrome c_6A. One salient difference of the heme pockets is that a highly conserved Gln (Q51) in cytochrome c₆ is replaced by Val (V52) in c_6A. Using protein film voltammetry, we found that swapping these residues raised the c_6A potential by +109 mV and decreased that of c₆ by almost the same extent, −100 mV. X-ray crystallography of the V52Q protein showed that the Gln residue adopts the same configuration relative to the heme as in cytochrome c₆ and we propose that this stereochemistry destabilizes the oxidized form of the heme. Consequently, replacement of Gln by Val was probably a key step in the evolution of cytochrome c_6A from cytochrome c₆, inhibiting reduction by the cytochrome b₆f complex and facilitating establishment of a new function.

Cytochrome c₆ has long been known as a redox carrier of the thylakoid lumen of cyanobacteria and some eukaryotic algae that can substitute for plastocyanin in electron transfer. Until recently, it was widely accepted that land plants lack a cytochrome c₆. However, a homologue of the protein has now been identified in several plant species together with an additional isoform in the green alga Chlamydomonas reinhardtii. This form of the protein, designated cytochrome c_6A, differs from the ‘conventional’ cytochrome c₆ in possessing a conserved insertion of 12 amino acids that includes two absolutely conserved cysteine residues. There are conflicting reports of whether cytochrome c_6A can substitute for plastocyanin in photosynthetic electron transfer. The evidence for and against this is reviewed and the likely evolutionary history of cytochrome c_6A is discussed. It is suggested that it has been converted from a primary role in electron transfer to one in regulation within the chloroplast, and is an example of evolutionary ‘bricolage’.