EPR PROPERTIES OF THE 4 HEMES IN THE CYTOCHROME SUBUNIT OF REACTION CENTERS FROM RHODOPSEUDOMONAS-VIRIDIS - CHARACTERIZATION OF THE INDIVIDUAL HEMES

Individual EPR properties of the four hemes in the cytochrome subunit of the Rhodopseudomonas viridis reaction centre have been studied. EPR signals of the four hemes were resolved and assigned in the isolated reaction centres: g(x) = 3.05 and g(y) = 2.20, E(m) = 394 mV; g(x) = 3.39, E(m) = 325 mV; g(x) = 3.31, E(m) = 5 mV; g(z) = 3.28, E(m) = -96 mV. It has been found that the highest potential heme may appear in two interconvertible EPR forms with g(z) = 3.05, g(y) = 2.20 and g(z) = 3.16, g(y) = 2.09; the latter dominates in glycerol-containing medium. Methionine ligand replacement by exogenous imidazole has been performed in the methionine-histidine coordinated lowest potential heme. This gives a three component EPR spectrum characteristic of bis-imidazole complexes. It is suggested that the distinctive EPR signal of the other low potential heme with two histidine ligands (and only the g, component detectable at g = 3.31) is determined by the relative orientation of the coordinating histidine imidazole ring planes which make an angle of 64 degrees. Direct spin-lattice relaxation rates measurement in the cytochrome subunit showed that the heme with E(m) = 394 mV is characterized with T-1 of 49 mu s at 9.6 K while the other three hemes relax as one component with T-1 of approximate to 15 mu s; this implies that the highest potential heme is magnetically isolated in the protein.