Studies on <I>Streptomyces</I> Subtilisin Inhibitor (SSI)Multidisciplinary Approaches to Specific Interaction between Proteins

Abstract

Multidisciplinary studies on Streptomyces subtilisin inhibitor (SSI) have been reviewed. SSI is a protein proteinase inhibitor isolated in a crystalline form from the culture broth of Streptomyces albogriseolus S-3253 by Murao et al. in 1972. This inhibitor exhibits very strong inhibition almost exclusively against alkaline serine proteases such as subtilopeptidases. SSI is composed of two identical subunits of MW 11, 500 (113 amino acid residues), and acts in a dimeric form under the ordinary conditions to inhibit two moles of enzyme per mole of dimer (MW 23, 000).Stability, easy crystallization, molecular size, unique specificity of inhibition, relative easiness in obtaining large amount of preparation due to microbial production, and the fact that three dimensional structure of subtilisin BPN', which is most strongly inhibited by this inhibitor, has been elucidated, all make SSI one of the most suitable subjects for the study of the specific interaction between proteins. Co-operative studies centered onthis inhibitor started in 1973 among several laboratories in Japan. The primary structure of SSI has been elucidated in 1974, and a preliminary report on its three dimensional structure based on 2.3Å-resolution X-ray difraction pattern has appeared in 1977.Research topics reviewed here include: molecular weight determination; amino acid composition and the primary structure; X-ray crystallography; states of mino acid residues (solvent perturbation, spectrophotometric titration); hydrogen-deuterium exchange; optical properties (UV, fluorescence); denaturation (heat, acid); dissociation into monomer; inhibition spectrum; equivalence of inhibition and binding; isolation of the enzyme inhibitor complex; reactive site against the enzyme; affinity to enzymes (inhibitor constants); changes in the states of amino acid residues on the interaction of the enzyme inhibitor complex; reactive site against the enzyme; affinity to enzymes MCD); kinetics of association of the inhibitor and the enzyme; effects of chemical modification of amino acid residues (of SSI, of the enzyme); utilization of SSI (affinity chromatography on immobilized SSI).