Kinetics of Papain Action

Mechanisms of Catalysis of Nucleophilic Reactions of Carboxylic Acid Derivatives.

“…These two p-KVs have been assigned to the carboxylate ion and an ammonium ion (192). These results lead to the postulation of two (or three) groups in the active site, the sulfhydryl group from chemical studies and the carboxylate ion (and ammonium ion) from pH studies (192,349,350,368). Presumably the sulfhydryl group participates in some (fast) step which is kinetically unimportant, while the carboxylate ion and ammonium ion participate in the slow step of the reaction.…”

Demonstration of a change in the rate-determining step in papain- and ficin-catalyzed acyl-transfer reactions

“…The reasons for this conclusion are as follows. Reported values of Km for the papain-catalyzed hydrolysis of the ethyl and methyl esters and amides of ZGly and hippuric acid are between 5 and 270 him (Smith et al, 1958;Kirsch and Igelstrom, 1966;Sun and Tsou, 1963;Henry and Kirsch, 1967;Lucas and Williams, 1969). Since under all conditions Ks > Am, these values of Km are lower limits for Kr.…”

Ficin- and papain-catalyzed reactions. Changes in reactivity of the essential sulfhydryl group in the presence of substrates and competitive inhibitors

“…The essential sulfhydryl group of papain was found to react with chloroacetic acid some 30,000 times faster than does the sulfhydryl group of cysteine (Sluyterman, 1968). The marked reactivity of the chloroacetic acid with the essential sulfhydryl group is even more surprising since negatively charged compounds bind very poorly with papain and ficin (Stockell and Smith, 1957;Smith et al, 1958;Sluyterman, 1964;Whitaker and Bender, 1965;Whitaker, 1969). For example, the Ki the a-A-benzoyl-L-arginine increases 7-to 20-fold when the carboxyl group is ionized (Sluyterman, 1964;Whitaker and Bender, 1965).…”

“…The sequence of amino acids around this essential sulfhydryl group is similar for papain and ficin (Light et al, 1964;Wong and Liener, 1964). Several investigators have concluded that papain catalyzes hydrolytic reactions by the mechanism shown in equation 1 (Smith, 1958;Whitaker and Bender, 1965; Lowe and Williams, 1965a; Kirsch and Igelstrom, 1966;Bender and Brubacher, 1966) E + S ES -X ES' -E + P2…”