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 rubredoxin variant (PFRD-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 stabilize PFRD-XC4. The main chain to side chain salt bridge between the N-terminus 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 upon protein folding. ; © 2000 American Chemical Society. Received September 27, 1999; Revised Manuscript Received November 16, 1999. Published on Web 01/19/2000. This work was supported by the Howard Huges Medical Institute (S.L.M.) and a NSF fellowship (P.S.). The coordinates for the structure have been deposited in the Protein Data Bank (file 1qcv). We thank M. K. Eidsness for the wild-type P. furiosus rubredoxin gene used in this study, S. Ross for NMR spectroscopy, and S. Ross and C. Sarisky for help with the NMR structure determination. ; Accepted Version - Strop_2000_Biochemistry_Contribution_of_surface_salt_bridges.pdf