STRUCTURAL AND FUNCTIONAL ASPECTS OF PROTEIN-PROTEIN INTERACTION AS STUDIED THROUGH CRYSTAL STRUCTURE OF SUBTILISIN COMPLEXED WITH ITS TRAPPED SUBSTRATE SSI (STREPTOMYCES SUBTILISIN INHIBITOR)

Abstract

The crystal structure of Streptomyces Subtilisin Inhibitor (SSI) was partially refined by restrained least-squares methods to a conventional R value of 24 % employing rotating anode data to 1.85 A resolution range. The Sstructines of the complex of a bacterial alkaline serine proteinase, subtilisin BPN’, with its proteinaceous inhibitor SSI was partially refined to&® the (sR valuesgot 216% seupkoying lag RR synchrotron data. Comparing the B-factors between free SSI and complexed SSI, the marked rigidification of polypeptide chain segments occurred not only in the “reactive site segment” which is in direct contact with the enzyme but also in those segments which are closely connected with the reactive site segment through either covalent linkage or non-covalent interactions. Moreover the structure of the complex of subtilisin with genetically engineered mutant SSI was solved by ( F mutant - F wild ) difference Fourier syntheses.