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 fulfil the same function of transferring electrons from cytochrome f to Photosystem I. A key feature of cytochrome c(6A) is that its haem midpoint potential is more than 200 mV below that of cytochrome c(6) (E(m) approximately +340 mV) despite both cytochromes having histidine and methionine residues as axial haem-iron ligands. One salient difference between the haem pockets is that a valine residue in cytochrome c(6A) replaces a highly conserved glutamine residue in cytochrome c(6). This difference has been probed using site-directed mutagenesis, X-ray crystallography and protein film voltammetry studies. It has been found that the stereochemistry of the glutamine residue within the haem pocket has a destabilizing effect and is responsible for tuning the haem's midpoint potential by over 100 mV. This large effect may have contributed to the evolution of a new biological function for cytochrome c(6A).
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