Altering the specificity of subtilisin Bacillus lentus through the introduction of positive charge at single amino acid sites

The use of methanethiosulfonates as thiol-specific modifying reagents in the strategy of combined site-directed mutagenesis and chemical modification allows virtually unlimited opportunities for creating new protein surface environments. As a consequence of our interest in electrostatic manipulation as a means of tailoring enzyme activity and specificity, we have recently adopted this approach for the controlled incorporation of multiple negative charges at single sites in the representative serine protease, subtilisin Bacillus lentus (SBL). We now describe the use of this strategy to introduce multiple positive charges. A series of mono-, di- and triammonium methanethiosulfonates were synthesized and used to modify cysteine mutants of SBL at positions 62 in the S-2 site, 156 and 166 in the S-1 site and 217 in the S-1' site. Kinetic parameters for these chemically modified mutants (CMM) enzymes were determined at pH 8.6. The presence of up to three positive charges in the S-1, S-1' and S-2 subsites of SBL resulted in up to 77-fold lowered activity, possibly due to interference with the histidinium ion formed in the transition state of the hydrolytic reactions catalyzed. (C) 1999 Elsevier Science Ltd. All rights reserved.