title: Temperature-dependences of the kinetics of reactions of papain and actinidin with a series of reactivity probes differing in key molecular recognition features.
abstract: The temperature-dependences of the second-order rate constants (k) of the reactions of the catalytic site thiol groups of two cysteine peptidases papain (EC 3.4.22.2) and actinidin (EC 3.4.22.14) with a series of seven 2-pyridyl disulphide reactivity probes (R-S-S-2-Py, in which R provides variation in recognition features) were determined at pH 6.7 at temperatures in the range 4-30 degrees C by stopped-flow methodology and were used to calculate values of DeltaS++, DeltaH++ and DeltaG++. The marked changes in DeltaS++ from negative to positive in the papain reactions consequent on provision of increase in the opportunities for key non-covalent recognition interactions may implicate microsite desolvation in binding site-catalytic site signalling to provide a catalytically relevant transition state. The substantially different behaviour of actinidin including apparent masking of changes in DeltaH++ by an endothermic conformational change suggests a difference in mechanism involving kinetically significant conformational change.
authors.affiliation: Laboratory of Structural and Mechanistic Enzymology, School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK.
authors.forename: Sheraz
authors.initials: S
authors.lastname: Gul
authors.forename: Geoffrey W
authors.initials: GW
authors.lastname: Mellor
authors.forename: Emrys W
authors.initials: EW
authors.lastname: Thomas
authors.forename: Keith
authors.initials: K
authors.lastname: Brocklehurst
mesh_terms: D002384:Catalysis; D020134:Catalytic Domain; D003546:Cysteine Endopeptidases / Q000737:chemistry / Q000378:metabolism*; D004220:Disulfides / Q000737:chemistry / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010206:Papain / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D013438:Sulfhydryl Compounds / Q000737:chemistry / Q000378:metabolism; D013696:Temperature
chemical_list: D004220:Disulfides; D013438:Sulfhydryl Compounds; D003546:Cysteine Endopeptidases; C018745:actinidain; D010206:Papain