The effect of deuterium substitution on hydrogen bonds in redox proteins

Recently,l we investigated the possibility that NH.. .Sr(Cys) and NH.. .S* hydrogen bonds (backbone amide NH, ligand sulfur) may help raise the redox potential of iron-sulfur proteins by differentially stabilizing the reduced (Cys-S)4Fe4S; cluster. By using molecular-orbital calculations on hydrogen-bonded model systems, we were able to show that the changes in hydrogen-bond energy as a function of charge and geometry were large enough to account for a significant fraction of the differences in redox potential among iron-sulfur proteins and model compounds. Also, we showed that the conformational shifts observed in HiPIP2 upon reduction can be accounted for by changes in hydrogen bonding. In sum, it is our contention that the reduced state results in more highly charged, and therefore stronger, hydrogen bonds, and that this is the principal influence of the protein in controlling the redox potential.