Remarkable phylum selectivity of a Schistocerca gregaria trypsin inhibitor: The possible role of enzyme-inhibitor flexibility

A 35-mer polypeptide isolated from the hemolymph of desert locust Schistocerca gregaria (SG) proved to be a canonical inhibitor of bovine trypsin (K_i = 0.2 μM). Despite having a trypsin-specific arginine at the primary specificity P₁ site, it inhibits bovine chymotrypsin almost as well (K_i = 2 μM). Furthermore, while the latter reactivity improves 10⁴-fold by the single replacement of P₁ Arg by Leu, changing P₁ from Lys to Met only moderately improves trypsin affinity (K_i = 30 nM). The apparent low compatibility to trypsin, however, is not observed vs two arthropodal trypsins: SG peptides with P₁ Arg inhibit crayfish and shrimp trypsins with K_i values in the picomolar range. This unprecedented high discrimination between orthologous enzymes is postulated to derive from flexibility differences in the protein-protein interaction. The more than four orders of magnitude phylum selectivity makes these peptides prospective candidates for agricultural use.