Cytochrome c6A 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 fulfil the same function of transferring electrons from cytochrome f to Photosystem I. A key feature of cytochrome c6A is that its haem midpoint potential is more than 200 mV below that of cytochrome c6 (Em≈+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 c6A replaces a highly conserved glutamine residue in cytochrome c6. 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 c6A.
Cytochrome c6A: discovery, structure and properties responsible for its low haem redox potential
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Jonathan A.R. Worrall, Ben F. Luisi, Beatrix G. Schlarb-Ridley, Derek S. Bendall, Christopher J. Howe; Cytochrome c6A: discovery, structure and properties responsible for its low haem redox potential. Biochem Soc Trans 1 December 2008; 36 (6): 1175–1179. doi: https://doi.org/10.1042/BST0361175
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