EPR study of the redox interactions in cytochrome c3 from Desulfovibrio vulgaris Miyazaki

Abstract

We present a new examination of the EPR redox titration data for the tetraheme cytochrome c3 from Desulfovibrio vulgaris Miyazaki. Our analysis includes the contribution of the interaction potentials between the fur redox sites and is based on the model previously developed for the study of cytochrome c3 from Desulfovibrio desulfuricans Norway. We observed, as for D. desulfuricans Norway cytochrome c3, that the conformation of the heme with the lowest redox potential, heme 4, is sensitive to the redox state of the heme with the highest potenital, heme 1. However in D. vulgaris Miyazaki cytochrome c3 spectral simulations show that heme 4 is present in two conformational states which interconvert partially when heme 1 is reduced. The sets of redox parameters which satisfy the fitting procedure of the titration curves are in the following domain: -250 mV .ltoreq. e14 -220mV, -325 mV .ltoreq. e2 .ltoreq. -320mV, -335mV .ltoreq. e3 .ltoreq. -330mV, -360 mV .ltoreq. e4 .ltoreq.3 -355mV, -5mV .ltoreq. I12 .ltoreq. 10 mV, -10 mV .ltoreq. I13 .ltoreq. 5 mV, -15 mV .ltoreq. I23 .ltoreq. -5 mV, -15 mV .ltoreq. I24 .ltoreq. -10 mV, 025 mV .ltoreq. I34 .ltoreq. -15 mV. As in D. desulfuricans Norway cytochrome c3 the interactions are moderate. Simple electrostatic considerations suggest that these moderate values could be related to the large accessibility of the hemes to the solvent. Our work does not confirm the existence of a cooperative interaction between heme 2 and heme 3 which has been proposed on the basis of electrochemical measurements.