Electron transfer in the Rhodobacter sphaeroides reaction center assembled with zinc bacteriochlorophyll

Su Lin, Paul R. Jaschke, Haiyu Wang, Mark Paddock, Aaron Tufts, James P. Allen, Federico I. Rosell, A. Grant Mauk, Neal W. Woodbury, J. Thomas Beatty

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Abstract

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).

Original languageEnglish
Pages (from-to)8537-8542
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume106
Issue number21
DOIs
Publication statusPublished - May 26 2009

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Keywords

  • Magnesium chelatase mutant
  • Photosynthetic bacterial reaction center
  • Photosystems I and II
  • Protein-cofactor interaction

ASJC Scopus subject areas

  • General

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