Binding of the trypsin inhibitor from white mustard (Sinapis alba L.) seeds to bovine beta-trypsin: thermodynamic study

The effect of pH and temperature on the association equilibrium constant (Ka) for the binding of the trypsin inhibitor from white mustard (Sinapis alba L.) seeds (MTI) to bovine beta-trypsin (EC 3.4.21.4) has been investigated. On lowering the pH from 9 to 3, values of Ka for MTI binding to bovine beta-trypsin decrease thus reflecting the acid-pK and -midpoint shifts, upon inhibitor association, of two independent ionizable groups, and of a three-proton transition, respectively. At pH 8.0, values of thermodynamic parameters for MTI binding to bovine beta-trypsin are: Ka = 4.5 X 10(8)M-1, delta G0 = -11.6 kcal/mol, and delta S0 = +53 entropy units (all at 21 degrees C); and delta H0 = +4.1 kcal/mol (temperature independent between 5 degrees C and 45 degrees C). Binding properties of MTI to bovine beta-trypsin have been analyzed in parallel with those concerning macromolecular inhibitor association to serine (pro)enzymes.