PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

Structure and function of the photosynthetic reaction center from the aerobic bacterium Roseobacter denitrificans. Andreas Labahn^*,1, Oliver Hucke¹, Claudia Schwarze¹, Gerhart Drews², ¹ Albert-Ludwigs-Universität Freiburg, Freiburg, Germany² Albert-Ludwigs-Universität Freiburg, Freiburg, Germany

ABSTRACT- Roseobacter (Rs.)denitrificans belongs to the obligate aerobic, bacteriochlorophyll a (BChl) containing anoxygenic proteobacteria which are unable to grow in the absence of oxygen. A cyclic, light-driven electron transfer via a cytochrom bc₁ complex is active within a small redox range [1]. The puhH and the pufC genes [2] have been sequenced. In this work a structural model of the RC from Rs. denitrificans containing the L-, M-, H- and C-subunits, 13 cofactors and 47 conserved water molecules was calculated by homology modeling based on the X-ray structures of the reaction centers (RC) from Thermochromatium tepidum, Blastochloris viridis and Rhodobacter sphaeroides. To probe the interaction of the primary donor with the protein light-induced FTIR difference spectroscopy was performed. The corresponding spectrum exhibits two hydrogen bonds between the C₂-acetyl groups of the two BChls in the primary donor, D_L and D_M, and the side chains of His L168 and Tyr M200, respectively. The cytochrome subunit is anchored in the membrane by a N-terminal, hydrophobic -helix which may have a PufX function. From a detailed analysis of the charged amino acid residues surrounding the hemes it follows that the midpoint potentials of the heme groups were 240 mV, 90 mV, 290 mV and 90 mV in order of ascending distance to the primary donor. The need of oxygen for light-induced charge separation in the RC will be discussed. [1] Schwarze C. et al. (2000) Eur. J. Biochem. 267:422. [2] Kortlüke et al. (1997) J. Bacteriol. 179:5247.

KEY WORDS: reaction center, H-subunit , aerobic photosynthetic bacteria, FTIR spectroscopy