Crystal structure of a bacterial protein proteinase inhibitor (Streptomyces subtilisin inhibitor) at 2.6 A resolution

The crystal structure of a bacterial protein proteinase inhibitor (Streptomyces subtilisin inhibitor) was solved at 2·6 Å resolution. Each subunit of the dimeric inhibitor has a five-stranded antiparallel β-sheet and two short α-helices. The subunit-subunit interface formed by a stack of two β-sheets provided by the two subunits resembles the dimer-dimer interface of concanavalin A. Conformation of the reactive site around the scissible bond Met73-Val74 seems very rigid. Between bovine pancreatic trypsin inhibitor (Kunitz) and the Streptomyces inhibitor, the reactive site conformations are almost identical with each other from the P₂ to P₂′ residues, while between the soybean trypsin inhibitor (Kunitz) and the Streptomyces inhibitor they are similar from the P₂ to P₁′ residues. There are overall similarities in conformation extending from the P₃ to P₂′ residues between the Streptomyces inhibitor and a hypothetical substrate presumed (Robertus et al., 1972b) to be bound to subtilisin BPN′ in a productive binding mode. Apart from the reactive site, there seems to be no structural relationship among the Streptomyces, bovine pancreatic and soybean inhibitors, suggesting their convergent evolution from separate ancestral proteins.