title: Tryptophan-heme pi-electrostatic interactions in cytochrome f of oxygenic photosynthesis.
abstract: Cytochrome f of oxygenic photosynthesis has an unprecedented structure, including the N-terminus being a heme ligand. The adjacent N-terminal heme-shielding domain is enriched in aromatic amino acids. The atomic structures of the chloroplast and cyanobacterial cytochromes f were compared to explain spectral and redox differences between them. The conserved aromatic side chain in the N-terminal heme-shielding peptide at position 4, Phe and Tyr in plants and algae, respectively, and Trp in cyanobacteria, is in contact with the heme. Mutagenesis of cytochrome f from the eukaryotic green alga Chlamydomonas reinhardtii showed that a Phe4 --> Trp substitution in the N-terminal domain was unique in causing a red shift of 1 and 2 nm in the cytochrome Soret (gamma) and Q (alpha) visible absorption bands, respectively. The resulting alpha band peak at 556 nm is characteristic of the cyanobacterial cytochrome. Conversely, a Trp4 --> Phe mutation in the expressed cytochrome from the cyanobacterium Phormidium laminosum caused a blue shift to the 554 nm alpha band peak diagnostic of the chloroplast cytochrome. Residue 4 was found to be the sole determinant of this 60 cm(-)(1) spectral shift, and of approximately one-half of the 70 mV redox potential difference between cytochrome f of P. laminosum and C. reinhardtii (E(m7) = 297 and 370 mV, respectively). The proximity of Trp-4 to the heme implies that the spectral and redox potential shifts arise through differential interaction of its sigma- or pi-electrostatic potential with the heme ring and of the pi-potential with the heme Fe orbitals, respectively. The dependence of the visible spectrum and redox potential of cytochrome f on the identity of aromatic residue 4 provides an example of the use of the relatively sharp cytochrome spectrum as a "spectral fingerprint", and of the novel structural connection between the heme and a single nonliganding residue.
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, USA.
authors.forename: M V
authors.initials: MV
authors.lastname: Ponamarev
authors.forename: B G
authors.initials: BG
authors.lastname: Schlarb
authors.forename: C J
authors.initials: CJ
authors.lastname: Howe
authors.forename: C J
authors.initials: CJ
authors.lastname: Carrell
authors.forename: J L
authors.initials: JL
authors.lastname: Smith
authors.forename: D S
authors.initials: DS
authors.lastname: Bendall
authors.forename: W A
authors.initials: WA
authors.lastname: Cramer
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D016825:Chlamydomonas reinhardtii / Q000201:enzymology / Q000235:genetics; D002736:Chloroplasts / Q000201:enzymology / Q000235:genetics; D000458:Cyanobacteria / Q000201:enzymology / Q000235:genetics; D003580:Cytochromes / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D045348:Cytochromes f; D006418:Heme / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D010649:Phenylalanine / Q000235:genetics; D010788:Photosynthesis / Q000235:genetics; D055672:Static Electricity; D014364:Tryptophan / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D014443:Tyrosine / Q000235:genetics
chemical_list: D003580:Cytochromes; D014443:Tyrosine; D006418:Heme; D010649:Phenylalanine; D014364:Tryptophan; D045348:Cytochromes f