EPR STUDY OF CYTOCHROME IN THE D1D2 CYT B-559 COMPLEX

EPR measurements indicate that the low-potential heme (LP, E(m) similar to + 150 mV) and extra-low-potential heme (XLP, E(m) similar to -45 mV at pH 7.2) of Cyt b-559 observed in the D1D2 Cyt b-559 complex (Shuvalov, V.A., Schreiber, U. and Heber, U. (1994) FEBS Lett. 337, 226-230) both correspond to low-spin Fe(III) with bis-histidine ligation (at pH approximate to 7). The characteristic g values of both species are the same: g(1) = 2.9, g(2) = 2.3 and g(3) = 1.5. At pH 9.4 the LP heme shows similar low-spin Fe(III) signals, whereas the XLP heme is converted into two forms, with E(m) of + 40 mV and -220 mV, which correspond to low-spin Fe(III) and high-spin Fe(III) (g(perpendicular to) = 5.9) forms, respectively. The close similarity of the high-spin EPR spectrum of the XLB form at pH 9.4 with the spectrum of the catalase Cyt b-558 at pH 13.5 suggests that in the lowest potential form of XLP the Fe(III) carries one OH- ligand. The significance of such a ligation of the XLP heme is discussed.