Ligand bindings of bovine carboxypeptidase B. III. Hydrophobic activators in dipeptide hydrolysis.

Several hydrophobic compounds acted as activators in dipeptide (Bz-Gly-L-Arg-OH, Z-Gly-L-Phe-OH) hydrolysis by bovine carboxypeptidase B. These hydrophobic compounds include Bz-Gly-OH, Z-Gly-OH, Z-L-Phe-OH, and Z-L-Phe-GLy-OH. These compounds were indicated to bind to the secondary substrate binding sites which is proposed to be responsible for substrate activation kinetics in dipeptide hydrolysis. Of the compounds Z-L-Phe-OH alone acted also as a inhibitor at higher concentrations, indicating that it binds to both primary and secondary sites as the dipeptide substrates do. Comparison of the activation effects of the compounds employed indicated that hydrophobic interaction played an important role in binding to the secondary site. Substrate and modifier binding constants were also determined and the results indicated that modifier binding increased both affinity and catalytic rate constant of the primary site. On the other hand, Z-Gly-OH and Z-L-Phe-Gly-OH inhibited the hydrolyses of tri and tetrapeptide substrates. This observation suggests that the secondary site is contained in the extended active center which the enzyme possibly has.