Effect of the Immediate Environment on the Reactivity of the Essential—SH Group of Papain

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Abstract
The effect of the microenvironment on the reactivity of the essential -SH group of papain (EC-3.4.22.2) was studied by alkylation with methyl iodide and with the more polar iodoacetamide. Rate and activation parameters for these reactions were determined with 2 forms of the -SH group: the free mercaptide ion at pH 10.0, and the mercaptide-imidazolium ion-pair at pH 5.5. The ion-pair of papain reacts with methyl iodide at a rate 1470 times less than that of thiolsubtilisin. This surprising difference between the reactivities of the 2 enzymes suggest that in contrast to thiolsubtilisin, where a non-polar environment enhances the rate, in the case of papain a more polar environment somewhat inhibits the reaction with the non-polar methyl iodide. The positive activation entropy for the papain reaction may indicate an ordered structure of bound water around the S atom. The high rate and the low activation entropy (organized transition state) of the reaction of papain with iodoacetamide can be explained in terms of H-bond formation between the enzyme and the amide group of the alkylating agent.

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