The structure of an insect chymotrypsin

The South American imported fire ant (Solenopsis invicta), without natural enemies in the United States, widely infests the southern United States, ca using more than a half billion dollars in health and agriculture-related da mage annually in Texas alone. Fire ants are resistant to most insecticides, so control will require a more fundamental understanding of their biochemi stry and metabolism leading to the design of selective, ecologically safe i nsecticides. The 4th instar larvae play a crucial role in the nutrition of the colony by secreting proteinases (especially chymotrypsin) which digest food products for the entire colony. The first structure of an ant proteoly tic enzyme, fire ant chymotrypsin, was determined to atomic resolution (1.7 Angstrom). A structural comparison of the ant and mammalian structures con firms the "universality" of the serine proteinase motif and reveals a diffe rence at residues 147-148, which are proteolytically removed in the bovine enzyme but are firmly intact in the ant chymotrypsin, suggesting a differen t activation mechanism for the latter. Likewise, the absence of the covalen tly attached propeptide domain (1-15) further suggests an uncharacteristic activation mechanism. The presence of Gly189 in the S1 site is an atypical feature of this chymotrypsin and is comparable only to human leukocyte elas tase, hornet chymo-trypsin and fiddler crab collagenase. Binding studies co nfirm the chymotrypsin nature of this novel enzyme. (C) 2000 Academic Press .