PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

The reaction center associated triheme cytochrome of Rhodovulum sulfidophilum, structural analysis and evolutionary aspects. Jean Alric^{*,1, 4}, Yusuke Tsukatani², Makoto Yoshida², Katsumi Matsuura², Keizo Shimada², Wolfgang Nitschke³, Kenji Nagashima², André Verméglio⁴, ¹ Laboratoire de Génétique et Biophysique des Plantes, Marseille, France⁴ CEA-Cadarache DSV-DEVM Laboratoire de Bioénergétique Cellulaire, Saint Paul lez Durance, France² Department of Biology, Tokyo, Japan³ Laboratoire de Bioénergétique et Ingéniérie des Protéines UPR 9036, Marseille, France

ABSTRACT- The triheme cytochrome bound to the bacterial reaction center of Rhodovulum sulfidophilum distinguishes from its common tetraheme counterpart by three sequence differences: the lack of heme I binding site, the replacement of the usual methionine by a cysteine as 6^th axial ligand of heme II, and the loss of three of the four main solvent-exposed amino acids involved in the docking of soluble electron donors. These mutational events result in the loss of the outermost accessible heme I accepting electrons from periplasmic electron carriers, and the conversion of the typical high potential heme II to a very low potential (- 160 mV). In addition, the relative orientation of the hemes measured by EPR show that heme II protoporphyrin ring reorients from a direction perpendicular to the membrane plane in usual RC-bound cytochromes to 35 degrees in the triheme cytochrome. The requirement of a new docking site at the vicinity of heme II or IV, in addition to large sequence deletions and replacements in crucial building blocks of the RC-bound cytochrome, let suppose that major structural rearrangements occurred in the most distal parts of the regular tetraheme to figure out what the triheme looks like. Different scenarios for the evolution of the triheme cytochrome are discussed in light of structural and functional constraints inherited from ordinary RC-bound cytochromes.

KEY WORDS: photosynthetic reaction center, electron transfer, axial ligand, triheme cytochrome