Effect of the immediate environment on the reactivity of the essential -SH group of papain

The effect of the microenvironment on the reactivity of the essential -- SH group of papain was studied by alkylation with methyl iodide and with the more polar iodoacetamide. Rate and activation parameters for these reactions were determined with two 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 two enzymes suggests 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 sulfur 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 hydrogen-bond formation between the enzyme and the amide group of the alkylating agent.