title: Temperature and cryoprotectant influence secondary quinone binding position in bacterial reaction centers.
abstract: We have determined the first de novo position of the secondary quinone QB in the Rhodobacter sphaeroides reaction center (RC) using phases derived by the single wavelength anomalous dispersion method from crystals with selenomethionine substitution. We found that in frozen RC crystals, QB occupies primarily the proximal binding site. In contrast, our room temperature structure showed that QB is largely in the distal position. Both data sets were collected in dark-adapted conditions. We estimate that the occupancy of the QB site is 80% with a proximal: distal ratio of 4:1 in frozen RC crystals. We could not separate the effect of freezing from the effect of the cryoprotectants ethylene glycol or glycerol. These results could have far-reaching implications in structure/function studies of electron transfer in the acceptor quinone complex because the above are the most commonly used cryoprotectants in spectroscopic experiments.
authors.affiliation: Biosciences Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA.
authors.forename: P Raj
authors.initials: PR
authors.lastname: Pokkuluri
authors.forename: Philip D
authors.initials: PD
authors.lastname: Laible
authors.forename: Adam E
authors.initials: AE
authors.lastname: Crawford
authors.forename: Joy F
authors.initials: JF
authors.lastname: Mayfield
authors.forename: Mohammed A
authors.initials: MA
authors.lastname: Yousef
authors.forename: Stephan L
authors.initials: SL
authors.lastname: Ginell
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Hanson
authors.forename: Marianne
authors.initials: M
authors.lastname: Schiffer
mesh_terms: D016227:Benzoquinones / Q000737:chemistry; D001665:Binding Sites; D003451:Cryoprotective Agents / Q000494:pharmacology*; D018360:Crystallography, X-Ray; D004583:Electrons; D019855:Ethylene Glycol / Q000737:chemistry / Q000494:pharmacology; D005990:Glycerol / Q000737:chemistry / Q000494:pharmacology; D008027:Light; D008956:Models, Chemical; D010710:Phosphates / Q000494:pharmacology; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D017680:Potassium Compounds / Q000494:pharmacology; D011485:Protein Binding; D011487:Protein Conformation; D011809:Quinones / Q000737:chemistry*; D012242:Rhodobacter sphaeroides / Q000378:metabolism; D012645:Selenomethionine / Q000737:chemistry; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D016227:Benzoquinones; D003451:Cryoprotective Agents; D010710:Phosphates; D045322:Photosynthetic Reaction Center Complex Proteins; D017680:Potassium Compounds; D011809:Quinones; C004532:quinone; D012645:Selenomethionine; C013216:potassium phosphate; D019855:Ethylene Glycol; D005990:Glycerol