title: The structure of chloroplast cytochrome c6 at 1.9 A resolution: evidence for functional oligomerization.
abstract: The molecular structure of cytochrome c6 from the green alga Chlamydomonas reinhardtii has been determined from two crystal forms and refined to 1.9 A resolution. The two crystal forms are likely the result of different levels of post-translational modification of the protein. This is the first report of a high-resolution structure of a chloroplast-derived class I c-type cytochrome. The overall fold is similar to that of other class I c-type cytochromes, consisting of a series of alpha-helices and turns that envelop the heme prosthetic group. There is also a short two-stranded anti-parallel beta-sheet in the vicinity of the methionine axial ligand to the heme; this region of the molecule is formed by the most highly conserved residues in c6-type cytochromes. Although class I c-type cytochromes are assumed to function as monomers, both crystal forms of cytochrome c6 exhibit oligomerization about the heme crevice that is, in part, mediated by the short anti-parallel beta-sheet. The functional significance of this oligomerization is supported by the appearance of similar interfaces in other electron transfer couples, HPLC and light-scattering data, and is furthermore consistent with kinetic data on electron transfer reactions of c6-type cytochromes.
authors.affiliation: Molecular Biology Institute, University of California, Los Angeles 90024-1570, USA.
authors.forename: C A
authors.initials: CA
authors.lastname: Kerfeld
authors.forename: H P
authors.initials: HP
authors.lastname: Anwar
authors.forename: R
authors.initials: R
authors.lastname: Interrante
authors.forename: S
authors.initials: S
authors.lastname: Merchant
authors.forename: T O
authors.initials: TO
authors.lastname: Yeates
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005075:Biological Evolution; D016825:Chlamydomonas reinhardtii / Q000201:enzymology; D002736:Chloroplasts / Q000201:enzymology*; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D018360:Crystallography, X-Ray; D003580:Cytochromes / Q000737:chemistry*; D045348:Cytochromes f; D006418:Heme / Q000737:chemistry; D008027:Light; D008969:Molecular Sequence Data; D011487:Protein Conformation; D012542:Scattering, Radiation; D012997:Solvents / Q000737:chemistry
chemical_list: D003580:Cytochromes; D012997:Solvents; D006418:Heme; D045348:Cytochromes f