Modulation of Heme Redox Potential in the Cytochrome c_6 Family

摘要

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 c_6 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 c_6 despite having His and Met as axial heme-iron ligands. To identify the molecular origins of the difference in potential, the structure of cytochrome c_6 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 c_6 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 c_6 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 c_6 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 c_6, inhibiting reduction by the cytochrome b_6f complex and facilitating establishment of a new function.