title: Ubiquinone binding, ubiquinone exclusion, and detailed cofactor conformation in a mutant bacterial reaction center.
abstract: The X-ray crystal structure of a Rhodobacter sphaeroides reaction center with the mutation Ala M260 to Trp (AM260W) has been determined. Diffraction data were collected that were 97.6% complete between 30.0 and 2.1 A resolution. The electron density maps confirm the conclusions of a previous spectroscopic study, that the Q(A) ubiquinone is absent from the AM260W reaction center (Ridge, J. P., van Brederode, M. E., Goodwin, M. G., van Grondelle, R., and Jones, M. R. (1999) Photosynthesis Res. 59, 9-26). Exclusion of the Q(A) ubiquinone caused by the AM260W mutation is accompanied by a change in the packing of amino acids in the vicinity of the Q(A) site that form part of a loop that connects the DE and E helices of the M subunit. This repacking minimizes the volume of the cavity that results from the exclusion of the Q(A) ubiquinone, and further space is taken up by a feature in the electron density maps that has been modeled as a chloride ion. An unexpected finding is that the occupancy of the Q(B) site by ubiquinone appears to be high in the AM260W crystals, and as a result the position of the Q(B) ubiquinone is well-defined. The high quality of the electron density maps also reveals more precise information on the detailed conformation of the reaction center carotenoid, and we discuss the possibility of a bonding interaction between the methoxy group of the carotenoid and residue Trp M75. The conformation of the 2-acetyl carbonyl group in each of the reaction center bacteriochlorins is also discussed.
authors: Division of Biochemistry and Molecular Biology and Department of Chemistry, University of Glasgow, Glasgow, G12 8QQ, U.K.
authors: K E
authors: KE
authors: McAuley
authors: P K
authors: PK
authors: Fyfe
authors: J P
authors: JP
authors: Ridge
authors: R J
authors: RJ
authors: Cogdell
authors: N W
authors: NW
authors: Isaacs
authors: M R
authors: MR
authors: Jones
mesh_terms: D000409:Alanine / Q000235:genetics; D001665:Binding Sites; D002338:Carotenoids / Q000737:chemistry; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008968:Molecular Conformation; D009154:Mutation; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics; D012242:Rhodobacter sphaeroides; D014364:Tryptophan / Q000235:genetics; D014451:Ubiquinone / Q000737:chemistry*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D014451:Ubiquinone; C104880:spheroidenone; D002338:Carotenoids; D014364:Tryptophan; D000409:Alanine