Team:KULeuven/Afp

Anti-freeze protein (Afp)

Have you ever wondered how certain fishes survive in the subzero temperatures in Antarctica? Did you know that bacteria could thrive on the Sea Ice without freezing?

The fascinating adaptation in these living creatures lies in their ability to produce special proteins called ‘Antifreeze Proteins’ (AFP) that help them to maintain their body fluid in a liquid state at subzero temperatures. They are also referred to as antifreeze glycoproteins or antifreeze polypeptides to denote the structural features of these cryoprotectants.

The lowering of the freezing point caused by the AFPs, is noncolligative in nature. Although the freezing point is decreased, there is no significant alteration in the melting point. This difference between melting and freezing point is called Thermal Hysteresis (TH) and is a measure of the antifreeze activity [1]. Raymond and DeVries [2] proposed an adsorption inhibition mechanism, according to which the adsorption of AFPs on the ice crystal surface triggers the formation of ice in convex fronts between protein molecules which is thermodynamically unfavourable. AFPs also act by a mechanism called freeze tolerance, which inhibits the formation of bigger ice crystals at the expense of smaller ones; this mechanism is known as ‘re-crystallization inhibition’ (RI). This RI helps in lowering the freezing point of the body fluid to prevent re-crystallization damage. In some cases, AFPs are said to be the reason to protect membranes from cold-induced damages by preventing thermotropic phase transitions and inhibiting leakage through ion channels, by blocking them. Thus, their cryoprotective potential is contributed by a number of factors.

AFPs can be categorized from Type I-IV, Type I-Hyp AFP, Plant, Insect and Sea Organisms AFP [3]. The Protein structure of Winter Flounder Type1 AFP and some other commonly known AFPs is shown here.

Figure 1 (A) Winter flounder type I AFP structure. The wild-type protein is shown, with the location of the mutated residues shown in a blue (Ala17) oryellow (Thr13/Thr24) stick representation. (B) The known structures of several AFPs. [4]

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References

1. Chattopadhyay M. K; (2007); Antifreeze Proteins of Bacteria; Resonance p 25-30
   
2. J A Raymond and A L DeVries, Proc. Natl. Acad. Sci., USA, Vol.74, No.6, pp.2589–2593, 1977.
   
3. Antifreeze protein
   
4. Steffen P. Graether et. al.; Structure of Type I Antifreeze Protein and Mutants in Supercooled Water, Biophysical Journal, Volume 81, September 2001 1677–1683