title: Mutation-induced perturbation of the special pair P840 in the homodimeric reaction center in green sulfur bacteria.
abstract: Homodimeric photosynthetic reaction centers (RCs) in green sulfur bacteria and heliobacteria are functional homologs of Photosystem (PS) I in oxygenic phototrophs. They show unique features in their electron transfer reactions; however, detailed structural information has not been available so far. We mutated PscA-Leu688 and PscA-Val689 to cysteine residues in the green sulfur bacterium Chlorobaculum tepidum; these residues were predicted to interact with the special pair P840, based on sequence comparison with PS I. Spectroelectrochemical measurements showed that the L688C and V689C mutations altered a near-infrared difference spectrum upon P840 oxidation, as well as the redox potential of P840. Light-induced Fourier transform infrared difference measurements showed that the L688C mutation induced a differential signal of the S-H stretching vibration in the P840(+)/P840 spectrum, as reported in P800(+)/P800 difference spectrum in a heliobacterial RC. Spectral changes in the 13(1)-keto C=O region, caused by both mutations, revealed corresponding changes in the electronic structure of P840 and in the hydrogen-bonding interaction at the 13(1)-keto C=O group. These results suggest that there is a common spatial configuration around the special pair sites among type 1 RCs. The data also provided evidence that P840 has a symmetric electronic structure, as expected from a homodimeric RC.
authors.affiliation: Division of Material Science (Physics), Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
authors.forename: Chihiro
authors.initials: C
authors.lastname: Azai
authors.affiliation: Division of Material Science (Physics), Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
authors.forename: Yuko
authors.initials: Y
authors.lastname: Sano
authors.affiliation: Division of Material Science (Physics), Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Kato
authors.affiliation: Division of Material Science (Physics), Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
authors.forename: Takumi
authors.initials: T
authors.lastname: Noguchi
authors.affiliation: Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.forename: Hirozo
authors.initials: H
authors.lastname: Oh-oka
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D019414:Chlorobi / Q000737:chemistry / Q000235:genetics*; D006860:Hydrogen Bonding / Q000187:drug effects; D009154:Mutation; D009942:Organometallic Compounds / Q000737:chemistry / Q000494:pharmacology; D010084:Oxidation-Reduction; D045331:Photosystem I Protein Complex / Q000737:chemistry / Q000235:genetics*; D011487:Protein Conformation / Q000187:drug effects*
chemical_list: D001426:Bacterial Proteins; D009942:Organometallic Compounds; C531154:P846 compound; D045331:Photosystem I Protein Complex; C096017:PscA protein, green sulfur bacteria