Thermodynamics of reduction of Chromatium vinosum high-potential iron-sulfur protein and its histidine depleted H42Q mutant

Abstract

The thermodynamic parameters for the reduction reaction of the high-potential iron-sulfur protein from Chromatium vinosum, and its H42Q mutant, have been measured by direct cyclic voltammetry using a ‘non-isothermal’ reference electrode. Histidine 42 is mainly responsible for the pH dependence of the reduction potential ( E°) with p K a values equal to 5.9 and 6.3 for the oxidized and reduced proteins, respectively. The H42Q mutant does not present this pH dependence, although an increasing E° at the lowest pH values is still present. The main contribution to the difference in free energy between the protonated and the neutral His42 forms is given by the enthalpic term ( ΔΔH° = 7.4 kmol −1), although the entropic term is not negligible.