Crystal structure of a protein proteinase inhibitor, SSI (Streptomyces subtilisin inhibitor), at 4 A resolution.

The crystal structure of a protein proteinase inhibitor, SSI (Streptomyces subtilisin inhibitor), which strongly inhibits bacterial alkaline proteinases specifically, was determined at 4 A resolution using four heavy-atom derivatives. The SSI molecule can be described as an ellipsoid of about 30 X 40 X 65 A composed of two identical subunits each having dimensions of about 35 X 25 X 40 A and a molecular weight of 11,483. The subunit has an extensive beta-sheet structure, but no long alpha-helices are present. Based on the binding sites of platinum reagents known to form coordination complexes with methionine, it is speculated that the P1 residue, Met 73, of the reactive site is at the protruding edge of the subunit. At the subunit-subunit interface, a beta-sheet of one subunit is stacked on top of the corresponding beta-sheet of the other subunit.