Electron transfer in the Rhodobacter sphaeroides reaction center assembled with zinc bacteriochlorophyll.
The cofactor composition and electron-transfer kinetics of the reaction center (RC) from a magnesium chelatase (bchD) mutant of Rhodobacter sphaeroides were characterized. In this RC, the special pair (P) and accessory (B) bacteriochlorophyll (BChl) -binding sites contain Zn-BChl rather than BChl a. Spectroscopic measurements reveal that Zn-BChl also occupies the H sites that are normally occupied by bacteriopheophytin in wild type, and at least 1 of these Zn-BChl molecules is involved in electron transfer in intact Zn-RCs with an efficiency of >95% of the wild-type RC. The absorption spectrum of this Zn-containing RC in the near-infrared region associated with P and B is shifted from 865 to 855 nm and from 802 to 794 nm respectively, compared with wild type. The bands of P and B in the visible region are centered at 600 nm, similar to those of wild type, whereas the H-cofactors have a band at 560 nm, which is a spectral signature of monomeric Zn-BChl in organic solvent. The Zn-BChl H-cofactor spectral differences compared with the P and B positions in the visible region are proposed to be due to a difference in the 5th ligand coordinating the Zn. We suggest that this coordination is a key feature of protein-cofactor interactions, which significantly contributes to the redox midpoint potential of H and the formation of the charge-separated state, and provides a unifying explanation for the properties of the primary acceptor in photosystems I (PS1) and II (PS2).
Lin
Su
S
The Biodesign Institute at Arizona State University, Arizona State University, Tempe, AZ 85287-5201, USA. slin@asu.edu
Jaschke
Paul R
PR
Wang
Haiyu
H
Paddock
Mark
M
Tufts
Aaron
A
Allen
James P
JP
Rosell
Federico I
FI
Mauk
A Grant
AG
Woodbury
Neal W
NW
Beatty
J Thomas
JT
D001429:Bacteriochlorophylls / Q000737:chemistry* / Q000378:metabolism*; D055664:Electrochemical Techniques; D004579:Electron Transport; D019143:Evolution, Molecular; D007700:Kinetics; D055668:Photochemical Processes; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000378:metabolism*; D013053:Spectrophotometry; D015032:Zinc / Q000737:chemistry* / Q000378:metabolism*
D001429:Bacteriochlorophylls; D045322:Photosynthetic Reaction Center Complex Proteins; D015032:Zinc