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October 5, 2022

Antifreeze Proteins

Discovered in the ocean fish of the polar region, these proteins decreased the freezing point of the fish blood without increasing the plasma osmotic pressure to enable survival at sub-zero cold temperatures. Further, it was observed that the formation/ growth rate of ice crystals was suppressed, though the blood temperature was below the freezing point of seawater. The glycoproteins responsible for lowering the freezing point in a non-colligative manner were named antifreeze proteins (AFPs). AFPs, also known as ice crystal growth modifiers, are the class of biomolecules that play an essential role in preventing the growth of ice crystals and their recrystallization, to protect the cells from mechanical (freezing) damage. Secretion of AFPs is one of the survival strategies adopted by animals inhabiting low ambient temperatures.

AFPs are classified based on the sequence and the structures, but all have similar properties that enable them to bind to the building ice crystals (a pre-requisite of an AFP) under sub-zero conditions to decrease the freezing point. Five biological functions identified for AFPs are: freezing point depression, modification in ice nucleation, holding the liquid pockets, adhering to ice, and preventing/ inhibiting ice crystallization/ recrystallization. Binding to the ice, AFPs generate thermal hysteresis (TH). TH with respect to AFPs means a decrease in the freezing point of the solution without affecting its melting point. The difference in the freezing point (Tf) and the melting point (Tm) creates a gap called the TH gap (Tm - Tf). The magnitude of the gap depends on the type and concentration of AFPs. Under lower concentrations, AFPs prevent the recrystallization of ice, whereas, at higher concentrations generate TH.

The AFPs are adsorbed on the ice surface via an ice-binding site (IBS). IBS is a hydrophobic Threonine rich (T-Rich repeats) motif that aids their binding via hydroxyl groups on T-residues to inhibit ice growth. AFPs binds to the water between ice and IBS to initiate the ice-binding process. To block the ice growth, the adsorption rate of AFPs increases compared to the ice crystallization rate. The crystallization stops at the region where AFPs bind to the ice, but it continues at the unbound region (the region between the two AFPs adsorbed) until the water binding in those regions is energetically unfavoured. Eventually, the ice growth ceases, and the freezing point is depressed until the curvature and the space between the bound AFPs are maintained. Therefore, being a natural ice modulator and its role in cell membranes protection against cold injuries, AFPs are extensively studied for their application in cryopreservation.

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