InterPro domain: IPR000104

Marine teleosts from polar oceans can be protected from freezing in icy sea-water by serum antifreeze proteins (AFPs) or glycoproteins (AFGPs) [ 1 ]: these function by binding to, and preventing the growth of, ice crystals within the fish and depressing the non-equilibrium freezing point to below that of the melting point.

Despite functional similarity, antifreeze proteins are structurally diverse and include glycosylated and at least 3 non-glycosylated forms: the AFGP of nototheniids and cods are polymers of a tripeptide repeat, Ala-Ala-Thr, with a disaccharide attached to the threonine residue; type I AFPs are found in flounder and sculpin; type II AFPs of sea-raven, smelt and herring are Cys-rich proteins; and type III AFPs, found in eel pouts, are rich in beta-structure. Non-homologous antifreeze proteins have also been identified in insects and plants [ 2 ].

Type I AFPs are Ala-rich, amphiphilic, alpha-helical proteins [ 3 ]. The ice-binding sites of all AFPs are relatively flat and hydrophobic and have an uninterupted section of alanines running the length of the approximately 16.5A helix repeat. Basedon the energy-minimised structure [ 4 ], a model has been proposed todescribe the binding of the protein to ice crystals, whereby the proteinbinds to an ice nucleation structure, in a zipper-like fashion, viahydrogen bonding of the methyl-group of threonine side chains (with an 11-residue period) tooxygen atoms in the ice lattice. The growth of ice crystals is thusstopped, or retarded, and the freezing point depressed. The high lysince content of these peptides may serve to promote the solubility of these proteins.

1. Comparative modeling of the three-dimensional structure of type II antifreeze protein. Protein Sci. 4, 460-71
2. Structure and function of antifreeze proteins. Philos. Trans. R. Soc. Lond., B, Biol. Sci. 357, 927-35
3. Structure-function relationships in an antifreeze polypeptide. The role of charged amino acids. J. Biol. Chem. 268, 16396-400
4. Energy-optimized structure of antifreeze protein and its binding mechanism. J. Mol. Biol. 223, 509-17