Substituent effect on the elementary processes of the interaction between several benzeneboronic acids and substilisin BPN.

Benzeneboronic acid, a transition-state analog for serine proteases, binds to the catalytic center of subtilisin BPN'. The binding mechanism is so-called two-step mechanism; the initial fast association followed by a slow unimolecular process (Nakatani, H., Uehara, Y. and Hiromi, K. (1975) J. Biochem. (Tokyo) 77, 615--616), E + S fast equilibrium ES slow equilibrium ES (E = subtilisin, S = benzenebroonic acid). The structure of the transient complex (ES) at the initial association process was manifested by the substituent effect of benzeneboronic acid on the rate parameters in the elementary processes. The study by the temperature-junp and stopped-flow methods showed that the boron atom in benzeneboronic acid strongly interacts with a nucleophilic site, probably, O gamma of Ser-221 or imidazole of His-64 at the catalytic center, already at the initial fast association.