Ribonuclease salt bridge

Katakura Y, Kumamoto T, Iwai Y, et al. Fluorescence polarization study of a salt bridge between a single-chain Fv and its antigen ribonuclease A. Mol. Immunol. 1997 34 731-734. 

Two other types of specific side-chain interactions have been proposed to stabilize the a helix formed by C-peptide (Shoemaker et al., 1987b). A salt bridge between Glu-2 and Arg-10 has been detected in the crystal structure of ribonuclease A (Wlodawer and Sjolin, 1983) as well as in C-peptide in aqueous solution (Rico et al., 1986). This salt bridge also fixes the N-terminal boundary of the helix between Glu-2 and Thr-3. It is not sterically possible to make this ion pair if Glu-2 is part of the helix. Synthetic studies in which either Glu-2 or Arg-10 is replaced by Ala have provided support for the importance of this interaction in stabilizing the a-... 

Ribonuclease S, of knowm crystal structure (10), presents an instructive case. Except in the region of residues 18-23, it is very similar to RNase A (11). CjNj modifies known salt-bridge pairs (5). The distance between the C s of ALA20 and SER 21 is 27 A calculated from the X-ray diffraction based coordinates (12), however, considerable uncertainty exists relative to structural parameters for residues 18-23 (12). Are residues 20 and 21 transiently associated and susceptible to C2N2 driven condensation The answer appears to be yes but not as a reestablished peptide bond. Instead, it appears to be a depsi-pepMe ester link. 

---