Some cold water marine fishes avoid cellular damage because of freezing by expressing antifreeze proteins (AFPs) that bind to ice and inhibit its growth; one such protein is the globular type III AFP from eel pout. Despite several studies, the mechanism of ice binding remains unclear because of the difficulty in modeling the AFP-ice interaction. To further explore the mechanism, we have determined the x-ray crystallographic structure of 10 type III AFP mutants and combined that information with 7 previously determined structures to mainly analyze specific AFP-ice interactions such as hydrogen bonds. Quantitative assessment of binding was performed using a neural network with properties of the structure as input and predicted antifreeze activity as output. Using the cross-validation method, a correlation coefficient of 0.60 was obtained between measured and predicted activity, indicating successful learning and good predictive power. A large loss in the predictive power of the neural network occurred after properties related to the hydrophobic surface were left out, suggesting that van der Waal's interactions make a significant contribution to ice binding. By combining the analysis of the neural network with antifreeze activity and x-ray crystallographic structures of the mutants, we extend the existing ice-binding model to a two-step process: 1) probing of the surface for the correct ice-binding plane by hydrogen-bonding side chains and 2) attractive van der Waal's interactions between the other residues of the ice-binding surface and the ice, which increases the strength of the protein-ice interaction.
Many poikilothermic organisms have developed antifreeze proteins (AFPs)1 to resist freezing. Five classes of structurally diverse antifreeze proteins have been found in fish (for a review, see Ref. 1). These proteins act by adsorbing to the surface of ice and increasing the curvature of ice fronts between the bound AFPs (2). The freezing point at the surface is depressed, which in turn inhibits the growth of ice crystals. The difference between the temperature at which the ice begins to grow (burst point) and the temperature at which the ice crystal melts is known as thermal hysteresis (TH) and is used as a measure of AFP activity.Recently, several structures of type III AFP have been determined (4 -6). Based on the high-resolution x-ray structure (4), a model was proposed whereby surface adsorption occurs through a hydrogen bond match between the side chains of Gln-9, Asn-14, Thr-15, Thr-18, Gln-44, and the ice prism plane {1010}. These polar residues form part of a flat, amphipathic face that is thought to be the ice-binding surface (Fig. 1). However, the significance of the contribution from hydrogen bonds to the AFP-ice interaction has been questioned by several studies since then. A supposedly conservative change of Thr to Ser in type I AFP led to a large loss of TH activity, whereas a change to the hydrophobic residue valine, which is a better space-filling match, caused only a small loss (7,8). In the stud...