ture required for nucleation is significantly lower, and an existing ice crystal grows much more slowly during cooling than in pure water; the melting point, however, is not affected. This results in hysteresis during freezing and thawing, and is the reason why these proteins are also called THPs (Table 1.3.8).
Antifreeze proteins are a mixture of small to medium-sized proteins which can be extracted from the tissue of frost-hardy leaves, but not of frost-sensitive leaves (Urrutia et al. 1992). They are divided into several groups: Antifreeze proteins and antifreeze glycoproteins are both subdivided into further groups based on the tertiary structures of the two major types of protein. There are clear conceptions as to how these proteins function at the molecular level, although this is not yet true of plant AFPs.
The already classical antifreeze glycoproteins of cold-water fish are built around the repeating tripeptide -[Ala-Ala-Thr]-, where the threonine bears a sugar residue.
The AFP of the winter flounder is also characterised by such repetitive sequences, namely of an undecapeptide. The 11 amino acid units consist mainly of alanine with rhythmically interspaced polar amino acids (Thr, Asp and Asn). These are ordered in such a way that their polar residues are all on one side of the helix (Fig 1.3.24), so that the protein has a hydrophilic and a hydrophobic side. The hydrophilic face of these proteins is thought to attach to the main growing surface of the ice crystal, and the hydrophobic side of the protein, which is now turned to the outside, makes it difficult for further water clusters to associate with the ice
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