Salt bridges stabilize the folded structure of barnase

Formation of salt bridges entails desolvation. and whether they stabilize p rotein structures is an open question. In this paper, the role of three Arg -Asp salt bridges in barnase was studied by extensive continuum-electrostat ics calculations. Twelve mutations of the salt bridges were built on the X- ray structure of barnase. The electrostatic component, Delta DeltaG(el), of the mutations' effects on the folding stability was found to make the prot ein less stable by 0.5-5.4 kcal/mol. These agreed well with experimental re sults of Fersht and co-workers for the mutations' overall effects. Delta De ltaG, 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. App arently, the three salt bridges stabilize the structure of barnase because the electrostatic interactions involving the ionic partners overcome the de solvating costs. We suggest that earlier work of Tidor and co-workers indic ating a destabilizing role for salt bridges is based on a protocol that sig nificantly overestimates desolvation costs. That protocol also tends to giv e unreasonably large coupling energies for salt bridges and predicts a -7.7 kcal/mol coupling energy for the Arg69-Asp93 salt bridge.