The BPTI/Kunitz-type inhibitor family includes several extremely potent serine protease inhibitors. To date, the inhibitory mechanisms have only been studied for mammalian inhibitors. Here, the first crystal structure of a BPTI/Kunitz-type inhibitor from a marine invertebrate (rShPI-1A) is reported to 2.5?Å resolution. Crystallization of recombinant rShPI-1A required the salt-induced dissociation of a trypsin complex that was previously formed to avoid intrinsic inhibitor aggregates in solution. The rShPI-1A structure is similar to the NMR structure of the molecule purified from the natural source, but allowed the assignment of disulfide-bridge chiralities and the detection of an internal stabilizing water network. A structural comparison with other BPTI/Kunitz-type canonical inhibitors revealed unusual ? angles at positions 17 and 30 to be a particular characteristic of the family. A significant clustering of ? and ? angle values in the glycine-rich remote fragment near the secondary binding loop was additionally identified, but its impact on the specificity of rShPI-1A and similar molecules requires further study.
