Hydrogen Bond Strength Modulates the Mechanical Strength of Ferric-Thiolate Bonds in Rubredoxin

It has long been recognized that hydrogen bonds formed by protein backbone amides with cysteinyl S-gamma atoms play important roles in modulating the functional and structural properties of the iron-sulfur centers in proteins. Here we use single molecule atomic force microscopy, cyclic voltammetry, and protein engineering techniques to investigate directly how the strength of N-H center dot center dot center dot S-gamma hydrogen bonds in the secondary coordination sphere affects the mechanical stability of Fe(III)-thiolate bonds of rubredoxin. Our results show that the mechanical stability of Fe(III)-thiolate bonds in rubredoxin correlates with the strength of N-H center dot center dot center dot S-gamma hydrogen bonds as reflected by the midpoint reduction potential, providing direct evidence that N-H center dot center dot center dot S-gamma hydrogen bonds play important roles in modulating the mechanical and kinetic properties of the Fe(III)-thiolate bonds of iron-sulfur proteins and corroborating the important roles of the protein environment in tuning the properties of metal-thiolate bonds.