title: A new metal-binding site in photosynthetic bacterial reaction centers that modulates QA to QB electron transfer.
abstract: Isolated reaction centers (RCs) from Rhodobacter sphaeroides were found to bind Zn(II) stoichiometrically and reversibly in addition to the 1 equiv of non-heme Fe(II). Metal and EPR analyses confirm that Zn(II) is ligated to a binding site that is distinct from the Fe site. When Zn(II) is bound to this site, electron transfer between the quinones QA and QB (QA-QB --> QAQB-) is slowed and the room-temperature kinetics become distributed across the microsecond to millisecond time domain. This effect of metal binding on the kinetics is similar to the more global effect of cooling RCs to 2 degreesC in the absence of Zn(II). This suggests that Zn(II) binding alters localized protein motions that are necessary for rapid QA-QB --> QAQB- electron transfer. Inspection of the RC crystal structure suggests a cluster of histidine ligands located beneath the QB binding pocket as a potential binding site.
authors.affiliation: Chemistry Division, Argonne National Laboratory, Illinois 60439, USA. utschig@anlchm.chm.anl.gov
authors.forename: L M
authors.initials: LM
authors.lastname: Utschig
authors.forename: Y
authors.initials: Y
authors.lastname: Ohigashi
authors.forename: M C
authors.initials: MC
authors.lastname: Thurnauer
authors.forename: D M
authors.initials: DM
authors.lastname: Tiede
mesh_terms: D001665:Binding Sites; D002413:Cations, Divalent; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005296:Ferrous Compounds / Q000737:chemistry / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000378:metabolism*; D013054:Spectrophotometry, Atomic; D013696:Temperature; D015032:Zinc / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D002413:Cations, Divalent; D005296:Ferrous Compounds; D045322:Photosynthetic Reaction Center Complex Proteins; D015032:Zinc