Structural and Functional Characterization of the Unusual Triheme Cytochrome Bound to the Reaction Center of Rhodovulum sulfidophilum

Jean Alric,Yusuke Tsukatani,Makoto Yoshida,Katsumi Matsuura,Keizo Shimada,Rainer Hienerwadel,Barbara Schoepp-Cothenet,Wolfgang Nitschke,Kenji V.P Nagashima,André Verméglio

doi:10.1074/jbc.m400361200

Abstract

The cytochrome bound to the photosynthetic reaction center of Rhodovulum sulfidophilum presents two unusual characteristics with respect to the well characterized tetraheme cytochromes. This cytochrome contains only three hemes because it lacks the peptide motif CXXCH, which binds the most distal fourth heme. In addition, we show that the sixth axial ligand of the third heme is a cysteine (Cys-148) instead of the usual methionine ligand. This ligand exchange results in a very low midpoint potential (-160 +/- 10 mV). The influence of the unusual cysteine ligand on the midpoint potential of this distal heme was further investigated by site-directed mutagenesis. The midpoint potential of this heme is upshifted to +310 mV when cysteine 148 is replaced by methionine, in agreement with the typical redox properties of a His/Met coordinated heme. Because of the large increase in the midpoint potential of the distal heme in the mutant, both the native and modified high potential hemes are photooxidized at a redox poise where only the former is photooxidizable in the wild type. The relative orientation of the three hemes, determined by EPR measurements, is shown different from tetraheme cytochromes. The evolutionary basis of the concomitant loss of the fourth heme and the down-conversion of the third heme is discussed in light of phylogenetic relationships of the Rhodovulum species triheme cytochromes to other reaction center-associated tetraheme cytochromes.

Highlights

In photosynthetic organisms, the first step in the conversion of light energy into chemical energy occurs in the reaction center (RC).1 Upon absorption of a photon by the RC, an electron is transferred from a primary electron donor, located on the periplasmic side of the membrane, to an acceptor on the cytoplasmic side
A tetraheme cytochrome c operates as secondary electron donor to the photooxidized primary donor
The fast kinetics of electron transfer, from the tetraheme cytochrome to Pϩ were extensively studied in B. viridis (8)

Summary

Introduction

The first step in the conversion of light energy into chemical energy occurs in the reaction center (RC).1 Upon absorption of a photon by the RC, an electron is transferred from a primary electron donor, located on the periplasmic side of the membrane, to an acceptor on the cytoplasmic side. Unusual RC-bound Triheme Cytochrome of R. sulfidophilum point potentials, and fast interheme electron transfer have been determined for the WT and a mutant of the axial ligand of heme 3(II).

Results

Conclusion