SERINE-PROTEASE INHIBITION BY INSECT PEPTIDES CONTAINING A CYSTEINE KNOT AND A TRIPLE-STRANDED BETA-SHEET

Three insect peptides showing high sequence similarity and belonging to the same structural family incorporating a cysteine knot and a short three-stranded antiparallel beta-sheet were studied. Their inhibitory effect on two serine proteases (bovine alpha-chymotrypsin and human leukocyte elastase) is reported. One of them, PMP-C, is a strong alpha-chymotrypsin inhibitor (K-i = 0.2 nM) and interacts with leukocyte elastase with a K-i of 0.12 mu M. The other two peptides, PMP-D2 and HI, interact only weakly with alpha-chymotrypsin and do not inhibit leukocyte elastase. Synthetic variants of these peptides were prepared by solid-phase synthesis, and their action toward serine proteases was evaluated. This enabled us to locate the P1 residues within the reactive sites (Leu-30 for PMP-C and Arg-29 for PMP-D2 and HI), and, interestingly, variants of PMP-DB and RI were converted into powerful inhibitors of both alpha-chymotrypsin and leukocyte elastase, the most potent elastase inhibitor obtained in this study having a K-i of 3 nM.