Increasing protein stability by altering long-range coulombic interactions

It is difficult to increase protein stability by adding hydrogen bonds or burying nonpolar surface. The results described here show that reversing the charge on a side chain on the surface of a protein is a useful way of increasing stability. Ribonuclease T1 is an acidic protein with a pI approximate to 3.5 and a net charge of approximate to -6 at pH 7. The side chain of Asp49 is hyperexposed, not hydrogen bonded, and 8 Angstrom from the nearest charged group. The stability of Asp49Ala is 0.5 kcal/mol greater than wild-type at pH 7 and 0.4 kcal/mol less at pH 2.5. The stability of Asp49His is 1.1 kcal/mol greater than wild-type at pH 6, where the histidine 49 side chain (pK(a) = 7.2) is positively charged. Similar results were obtained with ribonuclease Sa where Asp25Lys is 0.9 kcal/mol and Glu74Lys is 1.1 kcal/mol more stable than the wild-type enzyme. These results suggest that protein stability can be increased by improving the coulombic interactions among charged groups on the protein surface. In addition, the stability of RNase TI decreases as more hydrophobic aromatic residues are substituted for Ala49, indicating a reverse hydrophobic effect.