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Antifreeze Proteins

Dateline: 07/31/00

By Alan Bruzel

What Are Antifreeze Proteins?

One way living things master new habitats is by evolving means of defending themselves from the challenges of extreme environmental conditions. Antifreeze proteins provide a notable example of protection against the ravages of cold temperatures. These proteins were first isolated from Antarctic fish, but have now been found in fish from both poles, as well as in bacteria, insects, and plants that must survive cold winter temperatures. Classified by molecular architecture, these proteins are separated into two groups: antifreeze glycoproteins and antifreeze proteins.

Antifreeze Glycoproteins

These proteins, first isolated from Antarctic fish in 1969, contain a three amino acid motif, alanine-alanine-threonine, repeated throughout the protein molecule. Because they have a bond between threonine and the disaccharide moiety galactosyl-N-acetyl galactosamine, these antifreeze proteins are known as glycoproteins: proteins with covalent bonds to carbohydrates.

Antifreeze glycoproteins provide an instance of convergent evolution. (Unrelated organisms that use similar mechanisms to adapt to similar environmental conditions, for instance flippers in whales and fins in fish, demonstrate convergent evolution. Whales and fish are not related, but they do use comparable means of locomotion.) The Antarctic notothenioid fishes and the Arctic cod both live in freezing water and both have similar antifreeze glycoproteins. They were thus believed to have once shared a common ancestor. However, the antifreeze glycoprotein gene in the Antarctic fish seems to have arisen from that fish's trypsinogen gene. (This is a fact interesting in its own right: development of a new protein by duplication of regions of a digestive enzyme gene). When the Arctic cod's antifreeze glycoprotein gene was examined, it was found to be structurally quite different from that of the Antarctic fish. These findings suggest that antifreeze glycoproteins evolved independently in these two fish, and that the fish exhibit a convergence at the level of molecular function.

Antifreeze Proteins

Unlike the antifreeze glycoproteins, the antifreeze proteins are not bound to carbohydrate. Thus far, four types have been identified: Type I, rich in alanine, Type II, cysteine-rich, Type III, with no partiality toward any one amino acid, and Type IV, rich in glutamic acid.

How Do Antifreeze Proteins Work?

To live in frigid climes successfully, one must prevent the formation of ice crystals in one's body. Antifreeze proteins are handy to have around because they lower the freezing point of water more than they affect the melting point of ice, a phenomenon known as thermal hysteresis. Of all antifreeze proteins examined thus far, those isolated from insects have the largest thermal hysteresis activity. They lower the freezing point of water to -5.5^oC while keeping the melting point of ice at its usual value of 0^oC.

Most antifreeze proteins possess a planar face able to form regularly spaced hydrogen bonding patterns with the neatly ordered water molecules of nascent ice crystals. Potential sites for hydrogen bonding are the hydroxyl groups of the carbohydrates in the antifreeze glycoproteins and the polar amino acids (such as aspartic acid, asparagine, glutamic acid, glutamine, serine, and threonine) in the antifreeze proteins. This hydrogen bonding of the antifreeze protein to ice crystals – including potential hydrophobic effects from other regions of the protein – may lead to the observed freezing point depression of water.

What the Web Has to Say about:
Antifreeze Proteins

A/F Protein
Manufacturers of antifreeze proteins derived from natural sources.

Antifreeze Genes Converge
Genes coding for Antarctic antifreeze glycoproteins evolved from trypsinogen gene. From Sean Henahan, Access Excellence.

Antifreeze Protein
Report on fish antifreeze protein from About.com's Biology Guide.

Antifreeze Proteins
Mechanisms of ice-antifreeze protein interactions. From Frank Sönnichsen, Case Western Reserve University.

Antifreeze Proteins
Lecture notes from the Department of Integrative Biology, University of California at Berkeley.

Antifreeze Proteins and Their Genes: From Basic Research to Business Opportunity
Potential uses for antifreeze proteins and their marketability. From Garth L. Fletcher et al. in Chemtech.

Biochemical Adaptation
Peter W. Hochachka and George N. Somero examine cryoprotectant strategies used by different species. Excerpt provided by Eugen Leitl.

Effects of Steric Mutations on the Structure of Type III Antifreeze Protein
Replacing alanine with larger amino acids inhibits antifreeze protein activity. Abstract by C. I. DeLuca et al. in Journal of Molecular Biology Online.

Enhancement of Insect Antifreeze Protein Activity by Solutes of Low Molecular Mass
Effects seen with some organic acids and alcohols, and inorganic salts. Abstract by Ning Li et al. reprinted from the Journal of Experimental Biology.

Flounder Antifreeze Protein
Structure of antifreeze proteins and ice binding mechanisms. Term paper from Carol Halama, University of Wisconsin at Eau Claire.

Freeze! Insect Proteins Halt Ice Growth
Some insect antifreeze proteins show better protective effects than fish antifreeze proteins. From C. Wu, Science News Online.

Freezing Stress
Ice formation, its consequences, and adaptations of plants to freezing conditions. From Bryan D. McKersie, University of Guelph.

How Do Antifreeze Proteins Work?
Several views of their mechanisms of action. From Paul Smaglik in The Scientist.

Hyperthermophiles
An article from this Web site describing how some molecular adaptations allow organisms to survive at the boiling point of water.

Ice-Binding Mechanism of Winter Flounder Antifreeze Proteins
Hydrogen bonding between amino acids and ice surface. From Ailan Cheng and Kenneth M. Merz, Jr., Pennsylvania State University.

Molecular Genetics of the Antifreeze Glycoprotein
Evolutionary connection between antifreeze glycoprotein of Antarctic fish and trypsinogen. From Teachers Experiencing Antarctica and the Arctic.

Solution Structure of Antarctic Eel Pout Antifreeze Protein RD3
Tertiary structure solved using NMR spectroscopy. Article from Kazunori Miura et al., Journal of Biochemistry (Japan).

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