title: Modification of a hydrogen bond to a bacteriochlorophyll a molecule in the light-harvesting 1 antenna of Rhodobacter sphaeroides.
abstract: Site-directed mutagenesis has been used to examine the function of a highly conserved aromatic residue, alpha Trp43, in the light-harvesting 1 antenna of the photosynthetic bacterium Rhodobacter sphaeroides. In this antenna alpha Trp43 is thought to be located near the putative binding site for bacteriochlorophyll; in this work it was changed to both Tyr and Phe, and in each case the main near-infrared absorbance peak was shifted to the blue, from 876 nm to 865 nm and then to 853 nm, respectively. Resonance Raman spectroscopy of the resulting complexes shows a shift of one component of the 1640-cm-1 peak to 1632 cm-1 for the Tyr mutant and to 1660 cm-1 for the Phe mutant. This demonstrates a strengthening of an existing H bond for the Tyr change and a breakage of this bond for the change to Phe. The 1640-cm-1 peak has been previously assigned to H-bonded C2 acetyl carbonyl groups of both bacteriochlorophylls in the light-harvesting 1 antenna dimer [Robert, B. & Lutz, M. (1985) Biochim. Biophys. Acta 807, 10-21]. These results indicate that one of these H bonds is to alpha Trp43, placing this residue in close proximity to the bacteriochlorophyll a macrocycle with which it interacts. The existence of this bond places constraints on the conformation of the alpha polypeptide, and a model of an alpha beta heterodimer is presented incorporating these data.
authors.affiliation: Krebs Institute for Biomolecular Research, University of Sheffield, United Kingdom.
authors.forename: J D
authors.initials: JD
authors.lastname: Olsen
authors.forename: G D
authors.initials: GD
authors.lastname: Sockalingum
authors.forename: B
authors.initials: B
authors.lastname: Robert
authors.forename: C N
authors.initials: CN
authors.lastname: Hunter
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry* / Q000235:genetics; D001665:Binding Sites; D006860:Hydrogen Bonding; D045342:Light-Harvesting Protein Complexes; D016297:Mutagenesis, Site-Directed; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000235:genetics; D013059:Spectrum Analysis, Raman; D014364:Tryptophan / Q000737:chemistry
chemical_list: D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D014364:Tryptophan