A DIMINISHED ROLE FOR HYDROGEN-BONDS IN ANTIFREEZE PROTEIN-BINDING TOICE

The most abundant isoform (HPLC-6) of type I antifreeze protein (AFP(I )) in winter flounder is a 37-amino-acid-long, alanine-rich, alpha-hel ical peptide, containing four Thr spaced 11 amino acids apart. It is g enerally assumed that HPLC-6 binds ice through a hydrogen-bonding matc h between the Thr and neighboring Asx residues to oxygens atoms on the {2021} plane of the ice lattice. The result is a lowering of the none quilibrium freezing point below the melting point (thermal hysteresis) . HPLC-6, and two variants in which the central two Thr were replaced with either Ser or Val, were synthesized, The Ser variant was virtuall y inactive, while only a minor loss of activity was observed in the Va l variant, CD, ultracentrifugation, and NMR studies indicated no signi ficant structural changes or aggregation of the variants compared to H PLC-6. These results call into question the role of hydrogen bonds and suggest a much more significant role for entropic effects and van der Waals interactions in binding AFP to ice.