CONTROL OF THE REDOX POTENTIAL IN C-TYPE CYTOCHROMES - IMPORTANCE OF THE ENTROPIC CONTRIBUTION

The enthalpic and entropic components of the redox free energy variation of cytochrome c(553) from Desulfovibrio vulgaris Hildenborough and its mutant Y64V, flavocytochrome b(2) from Saccharomyces cerevisiae, and the different hemes of cytochromes c(3) from Desulfovibrio vulgaris Miyazaki and Desulfovibrio desulfuricans Norway have been determined in 0.1 M Tris-HCl pH 7.0 (7.6 for cytochromes C-3) at 25 degrees C by using nonisothermal potentiometric titrations. The set of available experimental data demonstrates that the entropic component plays an important role in the control of the redox potential in c-type and b-type cytochromes. The variation of the entropic component within the class of cytochromes characterized by a positive value of E degrees' is proposed to be mainly determined by the variation of the exposure of the heme propionates to the solvent. In the case of tetraheme cytochromes c(3), the thermodynamic characteristics vary largely among the hemes belonging to the same molecule, which reflects the environmental peculiarities of each heme and also the heme-heme redox interactions. This study substantiates the existence of compensatory effects between large and opposite contributions to E degrees' predicted by all the current theoretical models which are based on electrostatic free energy calculations.