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Self association of Streptomyces subtilisin inhibitor: sedimentation equilibrium and 1H NMR studies.

The self association of Streptomyces subtilisin inhibitor, a dimeric protein molecule of MW 23,000 in an aqueous environment has been investigated by the combined use of sedimentation equilibrium analysis and 1H nuclear magnetic resonance (NMR) spectroscopy. A significant degree of self association was found using the sedimentation equilibrium method in the concentration range between 5 and 20 mg/ml. Furthermore, the use of 1H NMR spectroscopy in conjunction with the sedimentation equilibrium method enabled us to study the self association in a much higher concentration range (up to 60 mg/ml or more). The self association reaction in the concentration range of 10-40 mg/ml fits reasonably well a "successive polymerization model" in which each step of the polymerization reaction consists of the addition of one intrinsic dimer of Streptomyces subtilisin inhibitor to the previously formed polymeric species, with an equilibrium constant common to all the steps of 400 M-1. The 1H NMR chemical shift and line broadening data show that the dimer-dimer interaction probably involves the reactive-site segment of this inhibitor without a significant conformational change in the major part of the protein structure.