Contributes to protect fish blood from freezing at subzero sea water temperatures. Lowers the blood freezing point by about 1.1 degrees at a concentration of 0.1 mg/ml, and by about 1.5 degrees at a concentration of 0.2 mg/ml. Binds to nascent ice crystals and prevents further growth. Thermolabile, displays irreversible loss of activity and aggregation at room temperature. Homodimer. Detected in blood serum (at protein level). Detected in liver. The polypeptide has an alpha-hairpin structure. Two subunits dimerize to form a four-helix, rod-like structure. Each subunit is composed of a cap structure and of tandem 11 residue repeats with the sequence [TI]-X-X-X-A-X-X-X-A-X-X. Each repeat forms three helical turns with an average of 3.7 residues per turn, as opposed to 3.6 residues per turn for a classical alpha helix. Contrary to other proteins, where the protein core is stabilized by hydrophobic interactions and contains little water, this protein contains in its interior over 400 ordered water molecules with a semi-clathrate structure. Most likely, the interaction with the surface of ice crystals is mediated by bound water molecules that protrude between the helices. The concentration in blood serum is about 0.2 mg/ml. Belongs to the type-I AFP family. It did it its way - Issue 162 of July 2014