title: Evidence for association-activation effects in reactions of papain from studies on its reactivity towards isomeric two-protonic-state reactivity probes.
abstract: 4-(N-Aminoethyl 4-pyridyl disulphide)-7-nitrobenzo-2-oxa-1,3-diazole was synthesized and evaluted as a two-protonic-state reactivity probe by kinetic study of its reactions with papain (EC 3.4.22.2) and with benzimidazol-2-ylmethanethiol. Evidence is presented to suggest that: (i) both this probe molecule and its 2-pyridyl isomer bind to papain; (ii) the binding is followed by a change in the environment of the thiol group of cysteine-25; (iii) the striking rate maximum in neutral media observed in the reaction of papain with the 2-pyridyl isomer but not with the 4-pyridyl isomer arises from association of the 2-pyridyl leaving group with the imidazolium ion of histidine-159.
mesh_terms: D001665:Binding Sites; D055598:Chemical Phenomena; D002621:Chemistry; D004789:Enzyme Activation; D007536:Isomerism; D007700:Kinetics; D008956:Models, Chemical; D010069:Oxadiazoles / Q000138:chemical synthesis; D010206:Papain
chemical_list: D010069:Oxadiazoles; D010206:Papain