title: Structural and kinetic properties of Rhodobacter sphaeroides photosynthetic reaction centers containing exclusively Zn-coordinated bacteriochlorophyll as bacteriochlorin cofactors.
abstract: The Zn-BChl-containing reaction center (RC) produced in a bchD (magnesium chelatase) mutant of Rhodobacter sphaeroides assembles with six Zn-bacteriochlorophylls (Zn-BChls) in place of four Mg-containing bacteriochlorophylls (BChls) and two bacteriopheophytins (BPhes). This protein presents unique opportunities for studying biological electron transfer, as Zn-containing chlorins can exist in 4-, 5-, and (theoretically) 6-coordinate states within the RC. In this paper, the electron transfer perturbations attributed exclusively to coordination state effects are separated from those attributed to the presence, absence, or type of metal in the bacteriochlorin at the HA pocket of the RC. The presence of a 4-coordinate Zn(2+) ion in the HA bacteriochlorin instead of BPhe results in a small decrease in the rates of the P*→P(+)HA(-)→P(+)QA(-) electron transfer, and the charge separation yield is not greatly perturbed; however coordination of the Zn(2+) by a fifth ligand provided by a histidine residue results in a larger rate decrease and yield loss. We also report the first crystal structure of a Zn-BChl-containing RC, confirming that the HA Zn-BChl was either 4- or 5-coordinate in the two types of Zn-BChl-containing RCs studied here. Interestingly, a large degree of disorder, in combination with a relatively weak anomalous difference electron density was found in the HB pocket. These data, in combination with spectroscopic results, indicate partial occupancy of this binding pocket. These findings provide insights into the use of BPhe as the bacteriochlorin pigment of choice at HA in both BChl- and Zn-BChl-containing RCs found in nature.
authors: Department of Microbiology and Immunology, The University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC V6T 1Z3, Canada.
authors: Rafael G
authors: RG
authors: Saer
authors: Department of Chemistry and Biochemistry and the Biodesign Institute, Arizona State University, Tempe, AZ 85287-1604, USA.
authors: Jie
authors: J
authors: Pan
authors: Department of Microbiology and Immunology, The University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC V6T 1Z3, Canada.
authors: Amelia
authors: A
authors: Hardjasa
authors: Department of Chemistry and Biochemistry and the Biodesign Institute, Arizona State University, Tempe, AZ 85287-1604, USA.
authors: Su
authors: S
authors: Lin
authors: Department of Biochemistry and Molecular Biology and Centre for Blood Research, The University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC V6T 1Z3, Canada.
authors: Federico
authors: F
authors: Rosell
authors: Department of Biochemistry and Molecular Biology and Centre for Blood Research, The University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC V6T 1Z3, Canada.
authors: A Grant
authors: AG
authors: Mauk
authors: Department of Chemistry and Biochemistry and the Biodesign Institute, Arizona State University, Tempe, AZ 85287-1604, USA.
authors: Neal W
authors: NW
authors: Woodbury
authors: Department of Microbiology and Immunology, The University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC V6T 1Z3, Canada.
authors: Michael E P
authors: ME
authors: Murphy
authors: Department of Microbiology and Immunology, The University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC V6T 1Z3, Canada. Electronic address: j.beatty@ubc.ca.
authors: J Thomas
authors: JT
authors: Beatty
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001429:Bacteriochlorophylls / Q000737:chemistry / Q000378:metabolism*; D018360:Crystallography, X-Ray; D007700:Kinetics; D008958:Models, Molecular; D009154:Mutation; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011166:Porphyrins / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000235:genetics / Q000378:metabolism*; D015032:Zinc / Q000737:chemistry / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; D045322:Photosynthetic Reaction Center Complex Proteins; D011166:Porphyrins; C452433:bacteriochlorin; D015032:Zinc
keywords: Bacteriochlorophyll; Bacteriopheophytin; Electron transfer; Reaction center; Rhodobacter sphaeroides; Spectroscopy