Crystal structure of subtilisin complexed with its trapped substrate Streptomyces subtilisin inhibitor—Protein-protein interaction and evolution of serine proteinases and their proteinaceous inhibitors.

The complex of a bacterial alkaline serine proteinase, subtilisin BPN', with its proteinaceous inhibitor Streptomyces subtilisin inhibitor is unique in several respects, compared with other similar complexes containing serine proteinases of trypsin family. In addition to the usual antiparallel β-sheet involving P1–P3 residues of the inhibitor, P4–P6 residues form antiparallel β-sheet with a previously unnoticed chain segment (the ‘S4-6 site’) of subtilisin. The ‘S4-6 site’ does not exist in serine proteinases of trypsin family, whether of mammalian or microbial origin. Global induced-fit movement seems to occur on the ‘trapped substrate’ Streptomyces subtilisin inhibitor: a channel-like structure in SSI remote from the contact region becomes about 2 Å wider upon complexing with subtilisin. Main role of the secondary contact region of Streptomyces subtilisin inhibitor seems to support the reactive site loop (primary contact region). Steric homology for the two contact regions is so high between the inhibitors of Streptomyces subtilisin inhibitor family and those of pancreatic secretory trypsin inhibitor-ovomucoid inhibitor family that it seems to favour a divergent evolution and to support the general notion as to the relationship of prokaryotic and eukaryotic genes put forwarded by Doolittle (Nature (London), 272, 581, 1978).