title: Cytochrome c6A: discovery, structure and properties responsible for its low haem redox potential.
abstract: 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).
authors.affiliation: Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester, UK. jworrall@essex.ac.uk
authors.forename: Jonathan A R
authors.initials: JA
authors.lastname: Worrall
authors.forename: Ben F
authors.initials: BF
authors.lastname: Luisi
authors.forename: Beatrix G
authors.initials: BG
authors.lastname: Schlarb-Ridley
authors.forename: Derek S
authors.initials: DS
authors.lastname: Bendall
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Howe
mesh_terms: D017360:Arabidopsis / Q000737:chemistry; D045365:Cytochromes c6 / Q000737:chemistry* / Q000378:metabolism*; D004220:Disulfides / Q000378:metabolism; D006418:Heme / Q000378:metabolism*; D010084:Oxidation-Reduction; D010455:Peptides / Q000378:metabolism
chemical_list: D045365:Cytochromes c6; D004220:Disulfides; D010455:Peptides; D006418:Heme