title: pH-dependent salting-out effect in enzyme-catalyzed reaction kinetics.
abstract: The influence of KCl on the second-order rate constant kII = k2/Ks of acetylcholinesterase-catalyzed hydrolysis of butyl acetate is quantitatively described by the linear equation log kII = log k0II + delta kappa c, where c is the salt concentration and delta kappa = kappa E - kappa X* + kappa s is the difference between the salting-out coefficients of the initial reagents (E and S) and of the transition state (X*). The salting-out parameter delta kappa increases with increasing pH (pH range 4.8-6.0) up to the constant value at pH 6.0-8.0, where delta kappa = kappa s. The pH-dependent delta kappa is discussed as an indication of the change in the volume and/or solvation of the enzyme upon the formation of the transition state. The data show that while the pKa1 = 5.6 +/- 0.1 and pKa2 = 8.0 +/- 0.1 of kII are independent of the KCl concentration, the apparent pKa shift can be observed at high salt concentrations due to a pH-dependent salt effect.
authors.affiliation: Institute of Chemical Physics and Biophysics of the Estonian Academy of Sciences, Tallinn, U.S.S.R.
authors.forename: T
authors.initials: T
authors.lastname: Kesvatera
mesh_terms: D000085:Acetates / Q000378:metabolism; D000110:Acetylcholinesterase / Q000378:metabolism*; D002384:Catalysis; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D011189:Potassium Chloride / Q000494:pharmacology*
chemical_list: D000085:Acetates; C006848:butyl acetate; D011189:Potassium Chloride; D000110:Acetylcholinesterase