Organic Chemical Models for Proteinase Inhibitors

Abstract

The research to be reviewed briefly here began as an attempt to find a way to introduce new nucleophiles at enzyme active sites and to study the chemical behavior of these nucleophiles. Earlier work by Bender [1–4] and Koshland [5–6] seemed to indicate that the prospects for observing effective nucleophilic catalysis by newly introduced nucleophiles at enzyme active sites were not good. Specifically, they had carried out the conversion of the serine hydroxyl present at the active site of the well known proteinase, subtilisin, to a sulfhydryl group. The question which this experiment was designed to answer was whether the newly introduced sulfhydryl group would act as an efficient catalytic participant in the enzymatic hydrolysis of esters and peptides. Certainly, there are numerous proteinases like papain, for instance, where in the native form of the enzyme an active site sulfhydryl group is acylated and deacy-lated in the course of the enzyme-catalyzed hydrolysis of esters and peptides, and the expectation that a synthetically produced active site sulfhydryl group in thiol-subtilisin could play a similar role was reasonable. Nevertheless, Bender and Koshland found that thiolsubtilisin was a poor enzymatic catalyst, reacting primarily with nitrophenyl esters of carboxylic acids and then not particularly effectively. While the observations they made did not appear to augur well for proposals to study newly introduced nucleophiles at enzyme active sites, we decided to try a somewhat different course in our work [7]. We felt that by the investigation of the reactions of appropriate labile substrates with enzyme active sites we might be able to elucidate the properties of newly introduced intramolecular nucleophiles formed by the cleavage of the substrate’s reactive bond accompanying enzyme-substrate covalent bond formation as illustrated inEq.(1)below. Here ZH is the enzyme-bound nucleophile in the native form and YH is the new nucleophile generated in the vicinity of the enzyme’s active site by reaction of the inhibitor’s X-Y bond with ZH.

Supported in part by NSF Grant GI-37992.