Synthesis of cysteine protease inhibitors through Darzens reactions

Abstract

In this work, we are going to talk about the cysteine proteases. In the cysteine proteases, or thiol proteases, the mechanism involves a nucleophilic thiol in a catalytic triad or dyad. Cysteine proteases are common in fruits and are used as an ingredient in meat tenderizers. The mechanism of action of a cysteine protease, can be described in detail in this way 1; the thiolate anion attacks the carbonyl carbon of the peptide bond to be cleaved and a tetrahedral intermediate is produced (step a of figure 1). The oxyanion is fixed by an oxyanion hole. Stabilization of this transition state occurs by H bonding to the backbone NH of cysteine and the NH2 group of the histidine. Then, esterification of the thiol makes the imidazolium ion sufficiently acidic to protonate the nitrogen of the leaving group and the acyl enzyme is produced. In order to donate this proton to the amide nitrogen, rotation of the active site histidine is necessary. Deacylation may occur via a general base-catalyzed mechanism, whereby the imidazole nitrogen polarizes a water molecule which then attacks the acyl enzyme at the carbonyl carbon. The cleaved substrate (as free acid) and the regenerated enzyme are produced via a second tetrahedral intermediate.