PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

EPR study on the arrangement of the terminal [4Fe-4S] clusters in the reaction center of green sulfur bacteria. K. Lakshmi^*,1, Wade Johnson², John Golbeck², Gary Brudvig¹, ¹ Department of Chemistry, New Haven, CT, USA² Department of Biochemistry and Molecular Biology, University Park, PA, USA

ABSTRACT- The photosynthetic reaction center (RC) of the anaerobic green sulfur bacterium Chlorobium tepidum (CbRC) is an analog of photosystem I (PSI) in green plants and cyanobacteria. However, there are important functional differences that may stem from structural differences between the two type I RCs.¹ Also, despite the presence of similar arrangement of electron transfer cofactors in the heterodimeric core of aerobic RCs, it is thought that only one branch of the double set is used in electron transfer. In contrast, the CbRC contains a double set of cofactors bound to a homodimeric core. This may, in fact promote electron transfer through both branches of the RC. The study of electron-transfer pathways in the CbRC could provide answers to the selective electron-transfer pathway in aerobic PSI RCs. The PSI RC has been well studied. However, functional and structural details on the green bacterial RC (or the heliobacterial RC) do not exist. Our approach is to obtain structural information on cofactor distances in Chlorobium RCs by using EPR techniques. In this study, we investigate the geometric arrangement of the terminal acceptors in the electron transfer chain by measuring the spin-relaxation rate of a slow relaxer, P₈₄₀⁺, in the presence and absence of one or two reduced iron-sulfur clusters in the Chlorobium RC. Spin-lattice relaxation enhancements from pairwise interactions of paramagnetic centers in proteins have previously been used for distance determinations between redox sites in PSI. We resolved the identity of the F_A and F_B [4Fe-4S] clusters seen on the electron density map of PSI before the X-ray confirmation.² We present a similar spin-lattice relaxation study of pairwise interactions in the Chlorobium RC. The spin-lattice relaxation rates obtained in Chlorobium are compared with known spin-lattice relaxation rates and inter-spin distances in PSI. This work is supported by Grant GM36442 from the NIH (G.W.B.) and Grant MCB-9723661 from the NSF (J.H.G.). References: 1. Vassiliev et al. (2001) Biochim. Biophys. Acta, 1507, 139; 2. Lakshmi et al. (1999) Biochemistry 38, 13210.

KEY WORDS: Photosystem I, Chlorobium reaction center, Iron-sulfur clusters, EPR spectroscopy