Protein Name

Antifreeze protein isoform hplc6


Pseudopleuronectes americanus (winter flounder)

Biological Context

Cells can contain very significant amounts of water. When temperature drops below certain values, water freezes and forms ice. The volume of ice differs from that of liquid water, containing the same amount of water. This is due to the formation of ice crystals, regular arrangements of water molecules. Cooling liquids for car radiators help keep the car's cooling water liquid in the winter and the formation of ice on roads can be prevented by salt. Such additives prevent freezing by interfering with the extensive hydrogen bonding capability of water. Similar strategies are used by nature. Adding salts is obviously not a working idea, because this would upset the balance of ions inside the body. What happens instead in organisms that need to protect themselves against cold temperatures is the addition of proteins that interfere with the formation of ice by providing particular hydrogen bonding capabilities.

Structure Description


Anti-freeze proteins (AFP) come in different shapes, but one class of AFP is particularly simple in design, the alpha-helical AFP of which you see one example here. This protein was extracted from a fish, the winter flounder, that can live in water with sub-freezing temperatures. This alpha-helical AFP consists of just one single helix, a very unusual structure. Two helices come together to form a dimer. Not only is that in itself unusual, the amino acid sequence itself is unusual too. The hydrophobic residues are all alanine with the exception of two leucines. The helix has 36 residues and is about 55 angstrom long. The two leucines are located approximately in the middle of the helix, pointing in the same direction and are about 15 angstrom apart. Charged sidechains form so-called ice-binding motifs (IBM). These IBMs are located at distances which enable them to match the lattice of ice, although the arrangement in this particular examples is not entirely clear.

Protein Data Bank (PDB)



  • Sicheri, F. Yang, D.S.; "Ice-binding structure and mechanism of an antifreeze protein from winter flounder."; Nature; (1995) 375:427-431 PubMed:7760940.


author: Arno Paehler

Japanese version:PDB:1WFB