The substituent effect on complex formation between alfa-trypsin and para-substituted benzamidinium ions: A thermodynamic study

RESUMO

Dissociation constants, Kj, were determined spectrophotometrically by measuring the absorbance at 410 nm, using N alfa-benzoyl-D,L-argomome-para-nitroanilide (Bz-D,L-Arg-Nan) as substrate. The Ki values for the complexes of alfa-trypsin with each of the para-derivatives of the benzamidinium ion -NH2, -CH3, -H, -F, -Cl, -Br, -COOEt, and -NO2 were measured at six temperatures (8,15,20,25, 29 and 33§C), in order to determine the thermodynamic parameters for complex formation. The standard enthalpy change was constant and all other parameters were also negative. The large negative values obtained for the standard heat capacity change suggest that the process occurs with a conformational adaptation in the enzyme structure. The apparent partial specife volumes of free alfa-trypsin and alfa-trypsin bound to benzamidinium ion indicated that there is a decrease of approximatelly 0.10 cm**3/g in the enzyme volume when the inhibitor binds. This contraction is consistent with the release of about 130 water molecules per enzyme molecule