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

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 degrees) with pK(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 degrees 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 (Delta Delta H degrees = 7.4 kJ mol(-1)), although the entropic term is not negligible.