Salt bridges stabilize the folded structure of barnase

Formation of salt bridges entails desolvation, and whether they stabilize protein structures is an open question. In this paper, the role of three Arg-Asp salt bridges in barnase was studied by extensive continuum-electrostatics calculations. Twelve mutations of the salt bridges were built on the X-ray structure of barnase. The electrostatic component, ¢¢Gel, of the mutations ’ effects on the folding stability was found to make the protein less stable by 0.5-5.4 kcal/mol. These agreed well with experimental results of Fersht and co-workers for the mutations ’ overall effects, ¢¢G, on the folding stability (RMS deviation) 1.0 kcal/mol). The coupling energy for the Arg69-Asp93 salt bridge,-3.2 kcal/mol, was reproduced. Apparently, the three salt bridges stabilize the structure of barnase because the electrostatic interactions involving the ionic partners overcome the desolvating costs. We suggest that earlier work of Tidor and co-workers indicating a destabilizing role for salt bridges is based on a protocol that significantly overestimates desolvation costs. That protocol also tends to give unreasonably large coupling energies for salt bridges and predicts a-7.7 kcal/mol coupling energy for the Arg69-Asp93 salt bridge