Abstract

The electron transfer reaction is one of the most essential processes of life. Not only does it provide the means of transforming solar and chemical energy into a utilizable form for all living organisms, it also extends into a range of metabolic processes that support the life of a cell. Thus, it is of great interest to understand the physical basis of the rates and reduction potentials of these reactions. To identify the major determinants of reduction potentials in redox proteins, we have chosen the simplest electron transfer protein, rubredoxin, a small (52-54 residue) iron-sulfur protein family, widely distributed in bacteria and archaea. Rubredoxins can be grouped into two classes based on the correlation of their reduction potentials with the identity of residue 44; those with Ala44 (ex: Pyrococcus furiosus) have reduction potentials that are ~50 mV higher than those with Val44 (ex: Clostridium pasteurianum). Based on the crystal structures of rubredoxins from C. pasteurianum and P. furiosus , we propose the identity of residue 44 alone determines the reduction potential by the orientation of the electric dipole moment of the peptide bond between 43 and 44. Based on 1.5 A resolution crystal structures and molecular dynamics simulations of oxidized and reduced rubredoxins from C. pasteurianum, the structural rearrangements upon reduction suggest specific mechanisms by which electron transfer reactions of rubredoxin should be facilitated.

• Keywords: electron transfer; rubredoxin; redox potential; iron-sulfer