Interaction between Trypsin-Like Enzyme from Streptomyces erythraeus and Japanese Quail Ovomucoid

We examined the interaction between a trypsin-like enzyme from Streptomyces erythraeus (TLE-Se) and Japanese quail ovomucoid (QO) as a model of the interaction between a serine protease and a Kazal-type inhibitor. Both the second domain (Domain II) and the third domain (Domain III) of QO inhibited TLE-Se. But there was a great difference in the apparent association rate constant (ka) between Domain II and Domain III at pH 8.0 (the optimum pH of the enzymatic activity), and Domain III associated with TLE-Se about 10⁵ times faster than Domain II. The pH dependency of ka for Domain III and TLE-Se was maximum around pH 8, but in the case of Domain II and TLE-Se the maximum was around pH 4. This suggested that some acidic amino acid residues had some influence upon the association of Domain II with TLE-Se. When we examined the inter action between TLE-Se and amidated Domain II in which most of the aspartic and glutamic acids were amidated for conversion into asparagine and glutamine with NH and carbodiimide, the pH dependence of ka greatly differed from that obtained for TLE-Se and native Domain II, and an almost constant ka value (10³–10⁵ times higher than that of native Domain II) was exhibited between pH 5 and 8. At the same time, the dissociation constant at pH 8.0 became about 10⁻² times smaller, and thus the affinity between TLE-Se and amidated Domain II became stronger. These observations suggest that the electrostatic effect derived from the repulsion between the minus charges which are located at contact area sites of both molecules control the reactivity in the interaction between protease and a protein protease inhibitor.