Mechanism and Catalytic Site Atlas

Introduction

FMDV Leader protease is a classic cysteine protease with a unique binding domain which allows specific selection of its substrates - its own C terminus and eIF4G. His148 is positioned by Asp163 so as to be able to deprotonate Cys51, increasing its nucleophilicity, prior to Cys51 attacking the carbon of carbonyl group of the peptide bond. This results in the formation of a charged tetrahedral intermediate which susequently collapses with the elimination of the amine group. This is aided by proton donation from His148 to the leaving group. Thus the acyl enzyme is formed which must be cleared before the next catalytic cycle may begin. His148 removes a proton from the active site water, increasing its nucleophilicity, as it attacks the carbonyl group of the acyl enzyme. The resulting tetrahedral intermediate collapses with the formation of a carboxylic acid group and the free enzyme. His148 aids donates a proton it picked up from water to Cys51 thiol group.

The leader protease differs from the papain enzyme with Asn46 replacing Gln19 (in papain) which alongside the main chain amide on the catalytic cysteine, stabilises the negative charge on the oxyanion intermediate - the oxyanion hole. Furthermore, Asp163 replaces His159 in papain to position the nucleophilic cysteine