Binding of m-nitrobenzeneboronic acid to the active site of subtilisin BPN.

m-Nitrobenzeneboronic acid as a possible transition-state analog for serine proteases was found to cause absorption spectral change from 250 nm 350 nm upon binding with subtilisin BPN' (EC 3.4.21.14) at pH 6.5. Similar difference spectral changes of m-nitrobenzeneboronic acid were also observed at alkaline pH or upon addition of N-methylimidazole at pH 6.5. A characteristic circular dichroism spectrum of m-nitrobenezeneboronic acid was induced upon binding with subtilisin BPN' not only at pH 6.5, but also at alkaline pH. Circular dichroism spectral titration confirmed the stoichiometry of 1 : 1 for the m-nitrobenzeneboronic acid - subtilisin complex. m-Nitrobenzeneboronic acid was shown to be useful as a reversible chromophoric probe for the catalytic site of serine proteases.