title: Variation of Ser-L223 hydrogen bonding with the QB redox state in reaction centers from Rhodobacter sphaeroides.
abstract: Ser-L223 is close to ubiquinone (Q(B)) in the B-branch of the bacterial photosynthetic reaction center (bRC) from Rhodobacter (Rb) sphaeroides. Therefore, the presence of a hydrogen bond (H bond) between the two was naturally proposed from the crystal structure. The hydrogen bonding pattern of Q(B) from the light-exposed structure was studied by generating hydrogen atom coordinates based on the CHARMM force field. In the Q(B) neutral charge state (Q(B)(0)), no H bond was found between the oxygen of the OH group from Ser-L223 and the carbonyl oxygen of Q(B) that is distal to the non-heme iron. In the reduced state (Q(B)(-)), however, Ser-L213 was found to form an H bond with Q(B) only when Asp-L213 is protonated by more than 0.75 H(+). This indicates the significance of the protonation of Asp-L213 in forming an H bond between Ser-L223 and Q(B). We found that the driving force to form the H bond between Ser-L223 and Q(B) is enhanced by the positively charged Arg-L217. The calculated Q(B) redox potentials with or without this H bond discriminated two ET rates, which are close to the faster and slower time phases observed in UV-Vis and FTIR studies. Together with the calculated redox potential of the quinones, this H-bond formation could play a key role in conformational gating for the ET process from Q(A) to Q(B).
authors.affiliation: Institute of Chemistry, Department of Biology, Chemistry, and Pharmacy, Free University of Berlin, Takustrasse 6, D-14195 Berlin, Germany.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ishikita
authors.forename: Ernst-Walter
authors.initials: EW
authors.lastname: Knapp
mesh_terms: D001224:Aspartic Acid / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D006860:Hydrogen Bonding; D008027:Light; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D017785:Photons; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000378:metabolism* / Q000528:radiation effects; D012694:Serine / Q000737:chemistry / Q000378:metabolism*; D013053:Spectrophotometry; D014451:Ubiquinone / Q000737:chemistry / Q000378:metabolism*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D014451:Ubiquinone; D001224:Aspartic Acid; D012694:Serine; D010100:Oxygen