C-13 NMR study of how the oxyanion pK(a) values of subtilisin and chymotrypsin tetrahedral adducts are affected by different amino acid residues binding in enzyme subsites S-1-S-4

A range of substrate-derived chloromethane inhibitors have been synthesized which have one to four amino acid residues. These have been used to inhibi t both subtilisin and chymotrypsin. Using C-13 NMR, we have shown that all except one of these inhibitors forms a tetrahedral adduct with chymotrypsin , subtilisin, and trypsin. From the pH-dependent changes in the chemical sh ift of the hemiketal carbon of the tetrahedral adduct, we are able to deter mine the oxyanion pK(a) in the different inhibitor derivatives. Our results suggest that in both the subtilisin and chymotrypsin chloromethane derivat ives the oxyanion pK(a) is largely determined by the type of amino acid res idue occupying the S-1, subsite while binding in the S-2-S-4 subsites only has minor effects on oxyanion pK(a) values. Using free energy relationships , we determine that the different R groups of the amino acid residues bindi ng in the S1 subsite only have minor effects on the oxyanion pK(a) values. We propose that the lower polarity of the chymotrypsin active site relative to that of the subtilisin active site explains why the oxyanion pK(a) is h igher and more sensitive to the type of chloromethane inhibitor used in the chymotrypsin derivatives than in the subtilisin derivatives.