Antifreeze proteins (AFPs) have the unique ability to adsorb to ice and inhibit its growth. Many organisms ranging from fish to bacteria use AFPs to retard freezing or lessen the damage incurred upon freezing and thawing. The ice-binding mechanism of the long linear alpha-helical type I AFPs has been attributed to their regularly spaced polar residues matching the ice lattice along a pyramidal plane. In contrast, it is not known how globular antifreeze proteins such as type III AFP that lack repeating ice-binding residues bind to ice. Here we report the 1.25 A crystal structure of recombinant type III AFP (QAE isoform) from eel pout (Macrozoarces americanus), which reveals a remarkably flat amphipathic ice-binding site where five hydrogen-bonding atoms match two ranks of oxygens on the [1010] ice prism plane in the <0001> direction, giving high ice-binding affinity and specificity. This binding site, substantiated by the structures and properties of several ice-binding site mutants, suggests that the AFP occupies a niche in the ice surface in which it covers the basal plane while binding to the prism face.
The most abundant isoform (HPLC-6) of type I antifreeze protein (AFP 1 ) in winter flounder is a 37-amino-acid-long, alanine-rich, R-helical peptide, containing four Thr spaced 11 amino acids apart. It is generally assumed that HPLC-6 binds ice through a hydrogen-bonding match between the Thr and neighboring Asx residues to oxygens atoms on the {202 h1} plane of the ice lattice. The result is a lowering of the nonequilibrium 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 virtually inactive, while only a minor loss of activity was observed in the Val variant. CD, ultracentrifugation, and NMR studies indicated no significant structural changes or aggregation of the variants compared to HPLC-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.Type I AFP 1 is the smallest and arguably the simplest of the four macromolecular antifreeze types characterized to date (1). It is in effect a single, long R-helix and therefore lacks tertiary structure (2). The most abundant isoform of this AFP (HPLC-6) from winter flounder (Pleuronectes americanus) is 37 amino acids long, contains three complete 11-amino-acid repeats of Thr-X 2 -Asx-X 7 , where X is generally alanine, and ends with the start of a fourth repeat. This helical periodicity that places the Thr and Asx residues on the same face of the helix, suggested a mechanism for adsorption of the AFP to ice in which these regularly spaced hydrophilic groups would hydrogen bond to oxygen atoms in the ice lattice (3). Adsorption leads to inhibition of ice crystal growth (4) because ice is forced to grow with a surface curvature between the bound AFP, which in turn results in a lowering of the nonequilibrium freezing point below the melting point (5, 6). The difference in these two temperatures is termed thermal hysteresis and is used as a measure of antifreeze activity.At very low concentrations, AFP bind to ice but do not stop its growth. Under these conditions bound AFP is frozen into the ice rather than excluded by the advancing ice front. The protein binding planes in these crystals have been made visable by sublimation (ice etching) and determined to be the {202 h1} pyramidal plane of hexagonal ice (I h ) for type I AFP (5). Moreover, because this antifreeze is a nonglobular, extended molecule it was possible to establish a direction 〈011 h2〉 of binding on the plane. An elegant proof of this resulted from the synthesis of an all D-type I AFP, which was shown by the ice etching method to bind to the same plane but in the mirror image direction (7). This information was used to suggest a hydrogen-bonding match between the i, i + 11 threonines spaced 16.5 Å apart along the helix and accessible ice lattice oxygens spaced 16.7 Å apart along the 〈011 h2〉 direction of the {202 h1} binding plane.On the ba...
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