Designing substrate specificity by protein engineering of electrostatic interactions.

Protein engineering of electrostatic interactions between charged substrates and complementary charged amino acids, at two different sites in the substrate binding cleft of the protease subtilisin BPN', increases kcat/Km toward complementary charged substrates (up to 1900 times) and decreases kcat/Km toward similarly charged substrates. From kinetic analysis of 16 mutants of subtilisin and the wild type, the average free energies for enzyme-substrate ion-pair interactions at the two different sites are calculated to be -1.8 +/- 0.5 and -2.3 +/- 0.6 kcal/mol (1 cal = 4.18 J) [at 25 degrees C in 0.1 M Tris X HCl (pH 8.6)]. The combined electrostatic effects are roughly additive. These studies demonstrate the feasibility for rational design of charged ligand binding sites in proteins by tailoring of electrostatic interactions.