On the enhanced catalytic activity of papain towards amide substrates

According to the scanty literature data papain (EC. 3.4.4.10) reacts with ester and corresponding amide substrates at a similar rate (Glazer, Smith, 1971) despite a considerable difference in the reactivities of the ester and amide bonds. An explanation for the similar rates may be an increased acylation rate of amides relative to that of esters owing to hydrogen bond formation between the amide group of an amide substrate and Asp-158 carbonyl oxygen as it is apparent from the three-dimensional structure of papain. This possibility was confirmed by comparing the second-order rate constants of acylation of papain with the ester and amide derivatives of N-benzoylglycine and O-benzoylglycolic acid. The rate enhancement with amides is not an equally important factor with all substrates of papain: the amides of N-acyl-L-phenylalanylglycine are hydrolyzed at a considerably lower rate than the corresponding esters. It is concluded from the above data that the binding mode is somewhat different with various substrates.