Assignment of individual heme EPR signalsof Desulfovibrio baculatus (strain 9974) tetraheme cytochrome c3 A redox equilibria study

Abstract

An EPR redox titration was performed on the tetraheme cytochrome c3 isolated from Desulfovibrio baculatus (strain 9974), a sulfate‐reducer. Using spectral differences at different poised redox states of the protein, it was possible to individualize the EPR g‐values of each of the four hemes and also to determine the mid‐point redox potentials of each individual heme: heme 4 (−70 mV) at gmax= 2.93, gmed= 2.26 and gmin= 1.51; heme 3 (− 280 mV) at gmax= 3.41; heme 2 (−300 mV) at gmax= 3.05, gmed= 2.24 and gmin= 1.34; and heme 1 (−355 mV) at gmx= 3.18. A previously described multi‐redox equilibria model used for the interpretation of NMR data of D. gigas cytochrome c3 [Santos, H., Moura, J. J. G., Moura, I., LeGall, J. & Xavier, A. V. (1984) Eur. J. Biochem. 141, 283‐296] is discussed in terms of the EPR results.