Structure-Function Relationship in a Winter Flounder Antifreeze Polypeptide
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Using synthetic analogs of an alpha-helical winter flounder antifreeze polypeptide (AFP) we investigated some important molecular details of the mechanism of action of this AFP. Of the seven peptides synthesized, all but one were amino-terminal deletions of the native AFP. Three of the seven synthetic analogs possessed the same antifreeze activity as the native polypeptide; the other analogs were devoid of antifreeze activity. The growth rates along the a and c axes of ice in solutions of varying concentrations of the three active AFP analogs were examined. The a axis growth rates of ice were inversely proportional to the concentration of the active peptides. The c-axis growth rates of ice were also dependent on peptide concentration. The active peptides enhanced c-axis growth at lower concentrations, while at higher concentrations they inhibited c axis growth. The ability of the peptides to develop antifreeze activity and to alter the a and c axis growth rates of ice depended on the presence of appropriately positioned amino acid residues with hydrogen bonding side chains. From these observations we propose that at low concentrations the AFP, through dipolar interactions and hydrogen bonding, interact with the prism faces of ice retarding a axis growth. At these concentrations, the electrical field of the AFP helix-dipole, like an externally applied field (Bartlett, J.T., van der Heuval, A.P., and Mason, B.J. (1963) Z. Angew. Math. Phys. 14, 599-610), can enhance ice c axis growth. At higher concentrations, the AFP interact with all ice crystal planes and retard both a and c axis growth.
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