title: Evidence that the active centre of chymopapain A is different from the active centres of some other cysteine proteinases and that the Brønsted coefficient (beta nuc.) for the reactions of thiolate anions with 2,2'-dipyridyl disulphide may be decreased by reagent protonation.
abstract: The active centres of chymopapains A and B (jointly designated EC 3.4.22.6) and papaya (Carica papaya L.) peptidase A were investigated by using 2,2'-dipyridyl disulphide and 5,5'-dithiobis-(2-nitrobenzoic acid) as thiol-specific reactivity probes. Whereas the first active-centre pKa values for chymopapain B and papaya peptidase A are less than 5, is as the case for papain (EC 3.4.22.2) and ficin (EC 3.4.22.3), that for chymopapain A is about 6.8. The reason why the reactions of thiols of pKa approx. 6.5 with 2.2'-dipyridyl disulphide are essentially pH-independent in the pH range around the thiol pKa is delineated. The value of the Brønsted coefficient (beta nuc.) for the reactions of thiolate ions with the 2,2'-dipyridyl disulphide monocation appears to be smaller than its value for the corresponding reactions with the neutral disulphide.
mesh_terms: D001665:Binding Sites; D002917:Chymopapain; D004220:Disulfides; D004228:Dithionitrobenzoic Acid; D010450:Endopeptidases; D005361:Ficain; D007700:Kinetics; D010206:Papain; D013438:Sulfhydryl Compounds
chemical_list: D004220:Disulfides; D013438:Sulfhydryl Compounds; D004228:Dithionitrobenzoic Acid; D010450:Endopeptidases; D010206:Papain; D005361:Ficain; D002917:Chymopapain