ELECTROSTATIC EFFECTS IN TRYPSIN REACTIONS : INFLUENCE OF SALTS

The influence of inorganic salts on trypsin-catalyzed reactions has been studied. It is shown that: (a) monovalent cations are reversible competitive inhibitors of tryptic hydrolysis of cationic substrates, whereas their binding has no effect on the reaction of neutral substrates; (b) a nonelectrostatic salt effect on the binding of both cationic and non-ionic substrates is caused by changes in the thermodynamic activity coefficient of the substrate; (c) the rate of trypsin active-site acylation is not affected by inorganic salts with monovalent cations. The data suggest that low-molecular-mass substrates are extracted into the enzyme microphase during substrate binding and further chemical transformations proceed without an access from surrounding medium. It is proposed that formation of a properly oriented dipole in the trypsin binding pocket by the cationic group of the substrate and Asp189 carboxyl is responsible for the elevated acylation rate of trypsin active site by substrates containing lysine and arginine. Introduction of additional negative charges into the enzyme molecule by chemical modification of lysyl residues by pyromellitic anhydride increased the specificity of trypsin towards cationic substrates and inhibitors. Lysine residues are therefore considered as suitable targets for site-directed mutagenesis aimed at the improvement of selectivity and catalytic properties of trypsin.