Structure/Function Studies of Insect Antifreeze Proteins

摘要

Antifreeze proteins (AEPs) from overwintering larvae of the beetle Dendroides canadensis are the most active AFPs known. Thirteen similar AFPs were purified and characterized. These consist of varying numbers of 12 and 13 mer repeating units with the consensus sequence Cys-Thr-X3-Ser- x5-x6-Cys-X8-X9-Ala- X11-Th-X13 where X3 and X1 tend toward charged residues, X5 toward threonine or serine, X9 toward asparagine or aspartate, X6 toward asparagine or lysine, and X13 toward alanine. All of the cysteine residues are disulfide bridged, usually to the other cysteine within the repeat unit.% This provides an extremely stable protein structure and probably positions the hydroxyl group of the highly conserved serine and threonine residues so they can hydrogen bond to ice, a requisite for the antifreeze activity. The secondary structure of these AFPs is 46% 6-sleet, 39% turn, 2% helix and 13% random. Several low molecular mass salutes, mostly organic, were shown to enchance the activity of the AFPs several fold. The most active of these is citrate which enchances activity as much as sixfold. Succinate, malate, aspartate, glutamate, ammonium sulfate, glycerol, sorbitol, alanine and ammonium bicarbonate were also very effective. The mechanism of the enhancement is unknown.