PROTEIN CONTRIBUTIONS TO REDOX POTENTIALS OF HOMOLOGOUS RUBREDOXINS -AN ENERGY MINIMIZATION STUDY

The energetic contributions of the protein to the redox potential in a n iron-sulfur protein are studied via energy minimization, comparing h omologous rubredoxins from Clostridium pasteurianum, Desulfovibrio gig as, Desulfovibrio vulgaris, and Pyrococcus furiosus. The reduction rea ction was divided into 1) the change in the redox site charge without allowing the protein to respond and 2) the relaxation of the protein i n response to the new charge state, focusing on the latter. The energy minimizations predict structural relaxation near the redox site that agrees well with that in crystal structures of oxidized and reduced P. furiosus rubredoxin, but underpredicts it far from the redox site. Ho wever, the relaxation energies from the energy-minimized structures ag ree well with those from the crystal structures, because the polar gro ups near the redox site are the main determinants and the charged grou ps are all located at the surface and thus are screened dielectrically . Relaxation energies are necessary for good agreement with experiment ally observed differences in reduction energies between C. pasteurianu m and the other three rubredoxins. Overall, the relaxation energy is l arge (over 500 mV) from both the energy-minimized and the crystal stru ctures. In addition, the range in the relaxation energy for the differ ent rubredoxins is large (300 mV), because even though the structural perturbations of the polar groups are small, they are very near the re dox site. Thus the relaxation energy is an important factor to conside r in reduction energetics.