Abstract
When the competitive inhibitor benzeneboronic acid (BBA) forms a complex with α-chymotrypsin [EC 3.4.21.1] protons are released in the acidic pH region. The proton release can be measured by a difference potentiometric technique. The proton release is also observed in chymotrypsinogen A but not in TRCK-, DIP-, and anhydro-chymotrypsins. Based on these observations, a simple procedure to estimate the equilibrium constants of the trigonal-tetrahedral interconversion of BBA is proposed. Thermodynamic parameters of the ionization of His 57 and of each step involved in BBA binding can be estimated from the temperature dependence of the proton release. Those of His 57 are essentially the same as those of imidazole in water. Regarding the interconversion of BBA on the enzyme, the value of ΔS is similar to ΔS_??_of the deacylation step of nonspecific substrates, and ΔH is remarkably reduced from that for the ionization of BBA in water. The enthalpic gain of the enzymic process is suggested to be due to the change of the proton acceptor, which is water in the case of the ionization of BBA in water, to imidazole on the enzyme.
