Complexation of two proteic insect inhibitors to the active site of chymotrypsin suggests decoupled roles for binding and selectivity

The crystal structures of two homologous inhibitors (PMP-C and PMP-D2v) fro m the insect Locusta migratoria have been determined in complex with bovine alpha -chymotrypsin at 2.1- and 3.0-Angstrom resolution, respectively. PMP -C is a potent bovine alpha -chymotrypsin inhibitor whereas native PMP-D2 i s a weak inhibitor of bovine trypsin. One unique mutation at the P1 positio n converts PMP-D2 into a potent bovine alpha -chymotrypsin inhibitor. The t wo peptides have a similar overall conformation, which consists of a triple -stranded antiparallel beta -sheet connected by three disulfide bridges, th us defining a novel family of serine protease inhibitors. They have in comm on the protease interaction site, which is composed of the classical protea se binding loop (position P5 to P'4, corresponding to residues 26-34) and o f an internal segment (residues 15-18), held together by two disulfide brid ges. Structural divergences between the two inhibitors result in an additio nal interaction site between PMP-D2v (position P10 to P6, residues 21-25) a nd the residues 172-175 of alpha -chymotrypsin. This unusual interaction ma y be responsible for species selectivity. A careful comparison of data on b ound and free inhibitors (from this study and previous NMR studies, respect ively) suggests that complexation to the protease stabilizes the flexible b inding loop (from P5 to P'4).