Redox thermodynamics of cytochromes c subjected to urea induced unfolding

Abstract

The thermodynamics of the electron transfer (ET) process for beef heart and yeast cytochromes c and the Lys72Ala/Lys73Ala/Lys79Ala mutant of the latter species subjected to progressive urea-induced unfolding was determined electrochemically. The results indicate the presence of at least three protein forms which were assigned to a low-temperature and a high-temperature His-Met intermediate species and a bis-histidinate form (although the presence of a His-Lys form cannot be excluded). The much lower E°′ value of the bis-histidinate conformer as compared to His-Met ligated species is largely determined by the enthalpic contribution induced by axial ligand substitution. The biphasic E°′ versus T profile for the His-Met species is due to a difference in reduction entropy between the conformers at low and high temperatures. Enthalpy–entropy compensation phenomena for the reduction reaction at varying urea concentration for all the forms of the investigated cytochromes c were addressed and discussed.