Secondary structure of antifreeze proteins from overwintering larvae of the beetle Dendroides canadensis

Antifreeze proteins from overwintering larvae of the beetle Dendroides canadensis are among the most active antifreeze proteins known. The Dendroides AFPs (DAFPs) consist of 6 or 7, 12- or 13-mer repeat units with a consensus sequence of -C-T-X3-S-X5-X6-C-X8-X9-A-X11-T-X13-. Nearly all of the Cys residues are in internal disulfide bridges between positions 1 and 7 within the repeats. The study presented here identified the secondary structure of the DAFPs using infrared and circular dichroism (CD) spectroscopies. The eight disulfide bridges impose significant constraints on potential secondary structural features (i.e., a number of three-residue gamma-turns) which may lead to unusual infrared and CD spectra that require special interpretation. At 25 degreesC the DAFPs contain approximately 46% beta-sheet, 39% turn, 2% helix, and 13% random structure. In the presence of ice there is a slight increase in helix and beta-sheet structures and a decrease in both turn and especially random structures. This change in the presence of ice may reflect a certain amount of flexibility in the DAFP structure. These structural changes may permit an improved lattice match between the DAFPs and ice, a requisite for the noncolligative freezing-point-depressing activity of the DAFPs.