EVIDENCE FOR A CLOSE SIMILARITY IN THE CATALYTIC SITES OF PAPAIN AND FICIN IN NEAR-NEUTRAL MEDIA DESPITE DIFFERENCES IN ACIDIC AND ALKALINE MEDIA - KINETICS OF THE REACTIONS OF PAPAIN AND FICIN WITH CHLOROACETATE

The pH-dependences of the 2nd-order rate constant (k) for the alkylation by chloroacetate of the active-center thiol groups of papain (EC 3.4.22.2) [from Carica papaya] and ficin (EC 3.4.22.3) [from Ficus glabrata] were determined over a wide range of pH at 25.degree. C. The main feature of both pH-k profiles is a striking rate maximum at pH 6. The profile for the ficin reaction contains a plateau at high pH. If an analogous plateau exists in the papain reaction, ~kx is much lower. Both enzymes probably contain closely similar thiolate-imidazolium interactive systems at pH 6, but differences in their behavior in more-acidic media and in alkaline media suggest differences in interaction with the postulated carboxylate component of the putative catalytic triad.