Contribution of surface salt bridges to protein stability

The role of surface salt bridges in protein stabilization has been a source of controversy. Here we present the NMR structure of a hyperthermophilic r ubredoxin variant (PFPD-XC4) and the thermodynamic analysis of two surface salt bridges by double mutant cycles. This analysis shows that the surface side chain to side chain salt bridge between Lys 6 and Glu 49 does not stab ilize PFRD-XC4. The main chain to side chain salt bridge between the N-term inus and Glu 14 was, however, found to stabilize PFRD-XC4 by 1.5 kcal mol(- 1). The entropic cost of making a surface salt bridge involving the protein 's backbone is reduced, since the backbone has already been immobilized upo n protein folding.