Redox thermodynamics of low-potential iron-sulfur proteins

The enthalpy and entropy changes associated with protein reduction (ΔH°'(rc), ΔS°'(rc)) were determined for a number of low-potential iron-sulfur proteins through variable temperature direct electrochemical experiments. These data add to previous estimates making available, overall, the reduction thermodynamics for twenty species from various sources containing all the different types of metal centers. These parameters are discussed with reference to structural data and calculated electrostatic metal-environment interaction energies, and redox properties of model complexes. This work, which is the first systematic investigation on the reduction thermodynamics of Fe-S proteins, contributes to the comprehension of the determinants of the differences in reduction potential among different protein families within a novel perspective. Moreover, comparison with analogous data obtained previously for electron transport (ET) metalloproteins with positive reduction potentials, i.e., cytochromes c, blue copper proteins, and HiPIPs, helps our understanding of the factors controlling the reduction potential in ET species containing different metal cofactors. The main results of this work can be summarized as follows. HiPIPa as compared t the low-potential 4Fe Fds is entirely due to enthalpic effects related to the electrostatic interactions of the cluster with the proteins matrix and the solvent.