Control of Reduction Thermodynamics in [2Fe-2S] Ferredoxins. Entropy-Enthalpy Compensation and the Influence of Surface Mutations

The reaction thermodynamics for the one-electron reduction of the [2Fe–2S] cluster of both human ferredoxin and various surface point mutants, in which each of the negatively charged residues Asp72, Glu73, Asp76, and Asp79 were converted to Ala, have been determined by variable temperature spectroelectrochemical measurements. The above are conserved residues that have been implicated in interactions between the vertebrate-type ferredoxins and their redox partners. In all cases, and similar to other 2Feferredoxins, the reduction potentials are negative as a result of both an enthalpic and entropic stabilization of the oxidized state. Although all Hs Fd mutants, with the exception of Asp72Ala, show slightly higher E°′values than that of wild type Hs Fd, according to expectations for a purely electrostatic model, they exhibit changes in the ΔH°′rc values that are electrostatically counter-intuitive. The observation of enthalpy–entropy compensation within the protein series indicates that the mutation-induced changes in ΔH°′rc and ΔS°′rc are dominated by reduction-induced solvent reorganization effects. Protein-based entropic effects are likely to be responsible for the low E°′ value of D72A.