title: Structural and functional characterization of the unusual triheme cytochrome bound to the reaction center of Rhodovulum sulfidophilum.
abstract: The cytochrome bound to the photosynthetic reaction center of Rhodovulum sulfidophilum presents two unusual characteristics with respect to the well characterized tetraheme cytochromes. This cytochrome contains only three hemes because it lacks the peptide motif CXXCH, which binds the most distal fourth heme. In addition, we show that the sixth axial ligand of the third heme is a cysteine (Cys-148) instead of the usual methionine ligand. This ligand exchange results in a very low midpoint potential (-160 +/- 10 mV). The influence of the unusual cysteine ligand on the midpoint potential of this distal heme was further investigated by site-directed mutagenesis. The midpoint potential of this heme is upshifted to +310 mV when cysteine 148 is replaced by methionine, in agreement with the typical redox properties of a His/Met coordinated heme. Because of the large increase in the midpoint potential of the distal heme in the mutant, both the native and modified high potential hemes are photooxidized at a redox poise where only the former is photooxidizable in the wild type. The relative orientation of the three hemes, determined by EPR measurements, is shown different from tetraheme cytochromes. The evolutionary basis of the concomitant loss of the fourth heme and the down-conversion of the third heme is discussed in light of phylogenetic relationships of the Rhodovulum species triheme cytochromes to other reaction center-associated tetraheme cytochromes.
authors.affiliation: Laboratoire de Genetique et Biophysique des Plantes, UMR 6191 CNRS-CEA-CNRS-Aix-Marseille II, 163 avenue de Luminy, Marseille, France.
authors.forename: Jean
authors.initials: J
authors.lastname: Alric
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Tsukatani
authors.forename: Makoto
authors.initials: M
authors.lastname: Yoshida
authors.forename: Katsumi
authors.initials: K
authors.lastname: Matsuura
authors.forename: Keizo
authors.initials: K
authors.lastname: Shimada
authors.forename: Rainer
authors.initials: R
authors.lastname: Hienerwadel
authors.forename: Barbara
authors.initials: B
authors.lastname: Schoepp-Cothenet
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Nitschke
authors.forename: Kenji V P
authors.initials: KV
authors.lastname: Nagashima
authors.forename: André
authors.initials: A
authors.lastname: Verméglio
mesh_terms: D020816:Amino Acid Motifs; D001665:Binding Sites; D002455:Cell Division; D003545:Cysteine / Q000737:chemistry; D003580:Cytochromes / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme / Q000737:chemistry*; D008024:Ligands; D008954:Models, Biological; D008957:Models, Genetic; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010084:Oxidation-Reduction; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D041903:Rhodovulum / Q000378:metabolism*; D013053:Spectrophotometry; D013329:Structure-Activity Relationship
chemical_list: D003580:Cytochromes; D008024:Ligands; D006418:Heme; D003545:Cysteine