title: 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.
abstract: 1. The pH-dependences of the second-order rate constants (k) for the alkylation by chloroacetate of the active-centre thiol groups of papain (EC 3.4.22.2) and ficin (EC 3.4.22.3) were determined over a wide range of pH at 25 degrees C at I 0.1. 2. The main feature of both pH-k profiles is a striking rate maximum at pH6 (characterizing parameters in both cases pKI approx. 3.5, pKII approx. 8.4 and pH-independent rate constant approximately kXH 2.5-3.0 M-1 . s-1). 3. The profile for the ficin reaction contains a plateau at high pH, with approximately kX 0.10 M-1 . s-1; if an analogous plateau exists in the papain reaction, approximately kX ix much lower, less than 0.02 M-1 . s-1. 4. Both enzymes appear to contain closely similar thiolate-imidazolium interactive systems at pH6, but differences in their behaviour in more-acidic media and in alkaline media suggest differences in interaction with the postulated carboxylate component of the putative catalytic triad.
mesh_terms: D000085:Acetates; D000478:Alkylation; D001665:Binding Sites; D002384:Catalysis; D010450:Endopeptidases / Q000378:metabolism*; D005361:Ficain / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010206:Papain / Q000378:metabolism*
chemical_list: D000085:Acetates; C006972:chloroacetic acid; D010450:Endopeptidases; D010206:Papain; D005361:Ficain