Dr. Peter L. Davies, Queen's University, Department of Biochemistry
Queen's University Peter L. Davies
Professor of Biochemistry & Biology

ANTIFREEZE PROTEINS

What is an antifreeze protein?

A relatively safe definition of an AFP might be a protein with affinity for ice. But how do these water-soluble proteins bind better to solid H2O than liquid H2O? The vast majority of proteins have no detectable affinity for ice.

AFPs are thought to bind to ice at intervals rather than completely covering the surface (Fig. 1). When this system is slightly undercooled, waters add to the ice front between the bound AFPs forcing these surfaces to have a curvature. The greater the curvature is, the harder it is for waters to join the ice. This results in a lowering of the (non-equilibrium) freezing point below the melting point (= thermal hysteresis). Figure 1 is a two-dimensional representation of this scenario. When considered in three dimensions, AFPs have been likened to buttons on a mattress.

Fig. 1. Adsorption-inhibition of ice growth. AFPs (blue ovals) are shown in solution and bound to the curved ice front (hatched line).


AFP activities

Animals that rely on freeze resistance for survival (like some fishes and insects) often use antifreeze proteins to depress their freezing point below subzero ambient temperature.

Other organisms (like plants and soil bacteria) that tolerate freezing can use antifreeze proteins to inhibit recrystallization of ice, ie. stop the growth of large crystals at the expense of small ones.

Fig. 2. Recrystallization inhibition. Here are two frozen solutions viewed under a microscope in a time-lapse sequence. The ice in the upper panel contains AFP, which keeps the crystals tiny. The lower panel lacks AFP. The number of crystals goes down as their size increases.




How do AFPs bind to ice?

The binding of AFPs to ice can be considered as a receptor:ligand interaction. It can investigated by solving the three-dimensional structure of the AFP using NMR or X-ray crystallography; identifying the ice-binding surface by site-directed mutagenesis, determining which surface of ice is bound by the AFP using ice etching, and then integrating this information by modelling the docking of the AFP to ice.





Fig. 3. Model of shorthorn sculpin (fish) AFP binding to a secondary prism plane of ice. Refer to: Baardsnes et al., “Antifreeze Protein from Shorthorn Sculpin: Identification of the Ice-Binding Surface”. (2001) Protein Science 10, 2566-2576.



AFP diversity

There are five very different types of AFP in fishes. In some cases their molecular ancestors have been identified. Thus the type II AFPs are clearly derived from C-type lectins. Type III AFPs are derived from the C-Terminal domain of sialic acid synthase. Their diversity may reflect recent evolution and the ability of ice to present many different surfaces for binding. To date four different AFPs have been found in insects, at least two different ones in plants and a variety of others in microorganisms.


How we isolate AFPs by ice affinity purification

 











Modeling of repetitive AFPs






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