title: Quinone (QB) reduction by B-branch electron transfer in mutant bacterial reaction centers from Rhodobacter sphaeroides: quantum efficiency and X-ray structure.
abstract: The photosynthetic reaction center (RC) from purple bacteria converts light into chemical energy. Although the RC shows two nearly structurally symmetric branches, A and B, light-induced electron transfer in the native RC occurs almost exclusively along the A-branch to a primary quinone electron acceptor Q(A). Subsequent electron and proton transfer to a mobile quinone molecule Q(B) converts it to a quinol, Q(B)H(2). We report the construction and characterization of a series of mutants in Rhodobacter sphaeroides designed to reduce Q(B) via the B-branch. The quantum efficiency to Q(B) via the B-branch Phi(B) ranged from 0.4% in an RC containing the single mutation Ala-M260 --> Trp to 5% in a quintuple mutant which includes in addition three mutations to inhibit transfer along the A-branch (Gly-M203 --> Asp, Tyr-M210 --> Phe, Leu-M214 --> His) and one to promote transfer along the B-branch (Phe-L181 --> Tyr). Comparing the value of 0.4% for Phi(B) obtained in the AW(M260) mutant, which lacks Q(A), to the 100% quantum efficiency for Phi(A) along the A-branch in the native RC, we obtain a ratio for A-branch to B-branch electron transfer of 250:1. We determined the structure of the most effective (quintuple) mutant RC at 2.25 A (R-factor = 19.6%). The Q(A) site did not contain a quinone but was occupied by the side chain of Trp-M260 and a Cl(-). In this structure a nonfunctional quinone was found to occupy a new site near M258 and M268. The implications of this work to trap intermediate states are discussed.
authors.affiliation: Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA. mpaddock@physics.ucsd.edu
authors.forename: M L
authors.initials: ML
authors.lastname: Paddock
authors.forename: C
authors.initials: C
authors.lastname: Chang
authors.forename: Q
authors.initials: Q
authors.lastname: Xu
authors.forename: E C
authors.initials: EC
authors.lastname: Abresch
authors.forename: H L
authors.initials: HL
authors.lastname: Axelrod
authors.forename: G
authors.initials: G
authors.lastname: Feher
authors.forename: M Y
authors.initials: MY
authors.lastname: Okamura
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D016227:Benzoquinones / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism*; D001665:Binding Sites / Q000235:genetics; D003460:Crystallization; D018360:Crystallography, X-Ray; D004579:Electron Transport / Q000235:genetics; D007700:Kinetics; D007834:Lasers; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D010674:Pheophytins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D010782:Photolysis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism*; D013053:Spectrophotometry
chemical_list: D001429:Bacteriochlorophylls; D016227:Benzoquinones; D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; C006678:bacteriopheophytin; C004532:quinone