Crystal Structure of a Protein Proteinase Inhibitor, SSI (Streptomyces Subtilisin Inhibitor), at 4 Å Resolution1

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