title: Ligand binding and covalent structure of an oxygen-binding heme protein from Rhodobacter sphaeroides, a representative of a new structural family of c-type cytochromes.
abstract: The amino acid sequence of an oxygen-binding heme protein (SHP) from Rhodobacter sphaeroides has been determined. The cysteines, which bind the single heme group in the 112-residue protein, are located at positions 43 and 46. SHP is similar in size to the large membrane-bound form of the class I cytochrome c5 of Azotobacter vinelandii (116 residues) and in the location of the heme binding site at positions 48 and 51. Two extra cysteines in SHP (residues 89 and 97) are located in positions similar to those of cytochrome c5 (residues 98 and 101) and form a disulfide bridge in both proteins. In total, four regions of alpha-helix are predicted, covering 46% of the protein, which is comparable to that in other small cytochromes. SHP is thus distantly related to small class I c-type cytochromes but is representative of a distinct family by virtue of its high-spin nature, the lack of a strong sixth ligand, and its capacity to bind oxygen. Potentially, the most important characteristic of SHP is its ability to transiently bind oxygen during autoxidation, which occurs with a half-life of 3 min with a 4-fold excess of O2. SHP also binds carbon monoxide, azide, and cyanide. The kinetics of reduction by free flavins indicate that SHP is less reactive than other class I cytochromes c and that the heme is less accessible to solvent. There is localized positive charge (+3) at the site of reduction of SHP, although the overall protein charge is -2. This may account in part for the ability of SHP to bind anions.
authors.affiliation: Department of Biochemistry, Physiology and Microbiology, Laboratory of Protein Biochemistry and Protein Engineering, State University of Gent, Belgium.
authors.forename: K
authors.initials: K
authors.lastname: Klarskov
authors.forename: G
authors.initials: G
authors.lastname: Van Driessche
authors.forename: K
authors.initials: K
authors.lastname: Backers
authors.forename: C
authors.initials: C
authors.lastname: Dumortier
authors.forename: T E
authors.initials: TE
authors.lastname: Meyer
authors.forename: G
authors.initials: G
authors.lastname: Tollin
authors.forename: M A
authors.initials: MA
authors.lastname: Cusanovich
authors.forename: J J
authors.initials: JJ
authors.lastname: Van Beeumen
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D016227:Benzoquinones / Q000378:metabolism; D003574:Cytochrome c Group / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D005415:Flavins / Q000378:metabolism; D005609:Free Radicals / Q000378:metabolism; D006418:Heme / Q000378:metabolism; D008024:Ligands; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D011485:Protein Binding; D017433:Protein Structure, Secondary; D012242:Rhodobacter sphaeroides / Q000201:enzymology*; D017386:Sequence Homology, Amino Acid
chemical_list: D001426:Bacterial Proteins; D016227:Benzoquinones; D003574:Cytochrome c Group; D005415:Flavins; D005609:Free Radicals; D008024:Ligands; C111967:SHP protein, Rhodobacter sphaeroides; C025232:semiquinone radicals; D006418:Heme; D010100:Oxygen