How Arctic Fish Avoid Freezing

Scientists have determined the three-dimensional structure responsible for the antifreeze action of a class of proteins in arctic fish.

Known as antifreeze glycoproteins (AFGPs), the molecules have untold potential medical and industrial uses, but until now they have resisted scrutiny.

AFGPs are one of two classes of antifreeze proteins that prevent marine teleost, or bony fish, from freezing in subzero waters by binding to and inhibiting the growth of ice crystals. One class, known simply as antifreeze proteins (AFPs), has been studied extensively. Researchers have been able to explore AFPs' mechanisms of ice-crystal inhibition and have produced the proteins through solid-phase synthesis and gene expression.

AFGPs, in contrast, contain repeating units of an alanine-threonine-alanine tripeptide, with a disaccharide attached to the threonyl residue. The AFGPs in the blood of arctic fish contain innumerable combinations and numbers of these glycoprotein units, making the molecules difficult to isolate and purify. This difficulty has frustrated researchers, as AFGPs are less likely than AFPs to prompt an immune response and so have more promise for medical applications.

Now, chemistry professor Shin-Ichiro Nishimura at Hokkaido University in Japan and colleagues there and at other institutions have developed a method to synthetically string together individual glycoprotein units to make different antifreeze molecules. They have also been able to test the antifreeze properties of each of these molecules, measuring how much they depress the freezing point of water [Angew. Chem. Int. Ed., 43, 856, (2004)].

Remarkably, the group found that even a molecule consisting of a single glycoprotein subunit affects the structure of ice crystals, forcing them into hexagonal bipyramids. A molecule consisting of only two subunits already begins to show antifreeze properties. The effect increases with the number of units, maxing out at five.

Using nuclear magnetic resonance spectroscopy and computational modeling, the group deduced the long-sought 3-D structure of the glycoproteins: a left-handed helix, with all the sugars facing one side, forming a hydrophilic front. The hydrophobic peptide backbone and methyl groups form the other side of the unit.

Margaret M. Harding, a chemistry professor at the University of Sydney, Australia, who studies antifreeze proteins, calls the work "timely and significant," saying it "reports the first detailed structure-activity studies on AFGPs."

Nishimura's group is now working to understand the mechanism by which the glycoproteins inhibit ice growth. And with this new knowledge, Nishimura says, chemists now have a jumping-off point from which to finally explore the usefulness of some of nature's most elusive antifreeze molecules.