Ice restructuring inhibition activities in antifreeze proteins with distinct differences in thermal hysteresis

Antifreeze proteins (AFPs) share two related properties: the ability to depress the freezing temperature below the melting point of ice (thermal hysteresis; TH); and the ability to inhibit the restructuring of ice into larger crystals. Since the ‘hyperactive’ AFPs, which have been more recently discovered, show an order of magnitude more TH than previously characterized AFPs, we have now determined their activities in ice restructuring inhibition (IrI) assays. IrI activities of three TH-hyperactive AFPs and three less TH-active AFPs varied over an 8-fold range. There was no obvious correlation between high TH activity and high IrI activity. However, the use of mutant AFPs demonstrated that severe disruption of ice-binding residues diminished both TH and IrI similarly, revealing that that the same ice-binding residues are crucial for both activities. In addition, bicarbonate ions, which are known to enhance the TH activity of AFPs, also enhanced their IrI activity. We suggest that these seemingly contradictory observations can be partially explained by differences in the coverage of ice by TH-hyperactive and non-hyperactive AFPs, and by differences in the stability of AFP-bound ice under supercooled and recrystallization conditions.