title: Reactivities of neutral and cationic forms of 2,2'-dipyridyl disulphide towards thiolate anions. Detection of differences between the active centres of actinidin, papain and ficin by a three-protonic-state reactivity probe.
abstract: The second-order rate constants (k) for the reactions of 2,2'-dipyridyl disulphide (pKa2,45) with 2-mercaptoethanol (pKa9.6) and with benzimidazol-2-ylmethanethiol (pKa values 5.6 and 8.3) were determined at 25 degrees C at I 0.1 by stopped-flow spectral analysis over a wide range of pH. These were used to calculate the pH-independent second-order rate constants (k) for the reactions of neutral 2,2'-dipyridyl disulphide and of its monocation with the 2-mercaptoethanol thiolate anion (associated pKa9.6) and with the benzimidazol-2-ylmethanethiol zwitterion (associated pKa5.6). For both thiolate ions, the rate-enhancement factor (kmonocation/kneutral disulphide) is about 1.5x10(3). The dependence on pH in acidic media of k for the reaction of 2,2'-dipyridyl disulphide with actinidin, the thiol proteinase from Actinidia chinensis, was shown to differ from the forms of pH-dependence observed for the analogous reactions with papain (EC 3.4.22.2) and ficin (3.4.22.3). The reactivity of the 2,2'-dipyridyl disulphide dication and its apparent sensitivity to the presence and location of a positive charge in the attacking thiol are discussed.
mesh_terms: D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives; D001562:Benzimidazoles; D001665:Binding Sites; D055598:Chemical Phenomena; D002621:Chemistry; D003546:Cysteine Endopeptidases; D004220:Disulfides; D010450:Endopeptidases; D005361:Ficain; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008623:Mercaptoethanol; D010206:Papain; D010944:Plants / Q000201:enzymology; D011725:Pyridines / Q000031:analogs & derivatives; D013438:Sulfhydryl Compounds
chemical_list: D001562:Benzimidazoles; D004220:Disulfides; D011725:Pyridines; D013438:Sulfhydryl Compounds; D015082:2,2'-Dipyridyl; D008623:Mercaptoethanol; D010450:Endopeptidases; D003546:Cysteine Endopeptidases; C018745:actinidain; D010206:Papain; D005361:Ficain