ABSTRACT

This chapter examines the dynamic structure of Streptomyces subtilisin inhibitor (SSI) in solution using 1H nuclear magnetic resonance and its relationship with the inhibitory activity. The selective spin diffusion experiments in SSI worked beautifully to show signals from several groups of protons that have escaped from spin diffusion, meaning that there are exposed and/or highly mobile segments in SSI. The chapter suggests that the N-terminal segment of SSI is exposed and/or mobile. A potent protease inhibitor, must have the specificity of binding with the target enzyme and yet a strong resistivity against protease digestion. Whereas the enzyme binding region consists of only about ten amino acid residues, the subunit of SSI has in total 113 amino acids. Plasminostreptin in particular has only 69% sequence homology with SSI, but independent crystallographic analysis shows that its three dimensional structure is almost identical to that of SSI.