Reaction of the sulfhydryl group of papain with chloroacetic acid.

The kinetics of the alkylation of the essential sulfhydryl group of papain with chloroacetic acid was studied. The variation of the second order rate constant with pH at 4.5” and 30.5’, r/2 = 0.07, is bell-shaped, with rate constants at a maximum near neutrality, approaching zero at low pH, and approaching a positive minimum at high pH. These profiles are described by a formulation which relates the apparent rate constants both with the states of dissociation of the sulfhydryl and other groups in papain and with the absolute rate constants for sulfhydryl reaction under specific conditions of pH. This formulation, as well as the absolute rate constants and the dissociation constants, have been used to suggest the presence of groups near the active center sulfhydry1 group which affect the pH dependence of both the alkylation and the catalytic reactions of papain.