High water mobility on the ice-binding surface of a hyperactive antifreeze protein

Antifreeze proteins (AFPs) prevent uncontrolled ice formation in organisms exposed to subzero temperatures by binding irreversibly to specific planes of nascent ice crystals. To understand the thermodynamic driving forces and kinetic mechanism of AFP activity, it is necessary to characterize the hydration behavior of these proteins in solution. With this aim, we have studied the hyperactive insect AFP from Tenebrio molitor (TmAFP) with the O-17 magnetic relaxation dispersion (MRD) method, which selectively monitors the rotational motion and exchange kinetics of water molecules on picosecond-microsecond time scales. The global hydration behavior of TmAFP is found to be similar to non-antifreeze proteins, with no evidence of ice-like or long-ranged modifications of the solvent. However, two sets of structural water molecules, located within the core and on the ice-binding face in the crystal structure of TmAFP, may have functional significance. We find that 2 of the 5 internal water molecules exchange with a residence time of 8 +/- 1 mu s at 300 K and a large activation energy of similar to 50 kJ mol(-1), reflecting intermittent large-scale conformational fluctuations in this exceptionally dense and rigid protein. Six water molecules arrayed with ice-like spacing in the central trough on the ice-binding face exchange with bulk water on a sub-nanosecond time scale. The combination of high order and fast exchange may allow these water molecules to contribute entropically to the ice-binding affinity without limiting the absorption rate.