title: Kinetics of binding of chicken cystatin to papain.
abstract: The kinetics of binding of chicken cystatin to papain were studied by stopped-flow fluorometry under pseudo-first-order conditions, i.e., with an excess of inhibitor. All reactions showed first-order behavior, and the observed pseudo-first-order rate constant increased linearly with the cystatin concentration up to the highest concentration that could be studied, 35 microM. The analyses thus provided no evidence for a limiting rate resulting from a conformational change stabilizing an initial encounter complex, in contrast with previous studies of reactions between serine proteinases and their protein inhibitors. The second-order association rate constant for complex formation was 9.9 X 10(6) M-1 s-1 at 25 degrees C, pH 7.4, I = 0.15, for both forms of cystatin, 1 and 2. This value approaches that expected for a diffusion-controlled rate. The temperature dependence of the association rate constant gave an enthalpy of activation at 25 degrees C of 31.5 kJ mol-1 and an entropy of activation at 25 degrees C of -7 J K-1 mol-1, compatible with no appreciable conformational change during the reaction. The association rate constant was independent of pH between pH 6 and 8 but decreased at lower and higher pH in a manner consistent with involvement of an unprotonated acid group with a pKa of 4-4.5 and a protonated basic group with a pKa of 9-9.5 in the interaction. The association rate constant was unaffected by ionic strengths between 0.15 and 1.0 but decreased somewhat at lower ionic strengths. Incubation of the complex between cystatin 2 and papain with an excess of cystatin 1 resulted in slow displacement of cystatin 2 from the complex.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Veterinary Medical Chemistry, Swedish University of Agricultural Sciences, Uppsala, Sweden.
authors.forename: I
authors.initials: I
authors.lastname: Björk
authors.forename: E
authors.initials: E
authors.lastname: Alriksson
authors.forename: K
authors.initials: K
authors.lastname: Ylinenjärvi
mesh_terms: D015891:Cystatins; D007700:Kinetics; D008433:Mathematics; D010206:Papain / Q000378:metabolism*; D011480:Protease Inhibitors / Q000378:metabolism*; D011485:Protein Binding; D011506:Proteins / Q000378:metabolism*
chemical_list: D015891:Cystatins; D011480:Protease Inhibitors; D011506:Proteins; C100158:cystatin, egg-white; D010206:Papain