antifreeze glycoprotein

Antifreeze proteins are found in some fish, insects and plants. They bind to ice crystals and prevent them from growing to a size where they would damage the host.

Specific hydrogen bonds form on the surface where protein meets ice and inhibits crystal growth. Also noted, the antifreeze molecules accumulate at the interface between ice and water, not at the interface between ice and a vacuum. So an hypothesis is that a hydrophobic reaction between the protein and the neighboring water prevents the water from forming ice crystals.

Antifreeze glycoprotein, discovered by Arthur L. DeVries in the 1960's, is also an example of convergent evolution. Scientists have determined that the arctic cod and the antarctic icefish (notothenioids) have nearly identical AFGP. This originally led them to believe the fish might have a common ancestor, but further analysis revealed that the gene expression in the two was completely different.

In the icefish, the gene expression for AFGP is nearly identical with its gene expression for the digestive enzyme trypsinogen; the arctic cod's AFGP gene does not resemble its gene for trypsinogen, nor does it resemble the icefish's gene for AFGP. The only reasonable conclusion then is that they developed AFGP independently - or convergently.

The antarctic icefish are notable for more than AFGP; they are also the only vertebrates lacking red blood cells.

Below Arctic and Antarctic ice caps, the water is in equilibrium with the ice and stays liquid at around -2°C. The fish which live in this water (Antarctic notothenioids and Arctic cod) must keep ice crystals from forming in their bloodstream, since ice crystal catalyzes the formation of more ice crystal, which will eventually block blood flow. As a defense against ice crystals, the fish make antifreeze glycoprotein, or AFGP, and circulate it in their blood.

AFGP doesn't work by stopping ice crystals from forming, the blood already has colligative molecules (salts and amino acids) to do that. In fact, AFGP isn't present in the blood at high enough concentrations to have any colligative action at all. Instead, it works on ice crystals that have already managed to form, by attaching to them in such a way that they can no longer catalyze the formation of more ice crystal. How the AFGP-Ice is dealt with from there is unknown, but there's either a biological mechanism for releasing it from the fish's body, or periodically the fish must move into warm enough waters to melt the ice crystal.

One reason that AFGP's are so interesting is that they present an example of very recent evolutionary change, so they can give a window on the microbiological process of evolution. Up to fifteen million years ago, the earth's gradual cooling period (which has been happening for the past 50 million years) accelerated, and caused ice sheets to form over the the Antarctic continent, and out over the ocean. That same process didn't happen in the Arctic until about two million years ago. Thus, no antifreeze glycoproteins would have evolved until then, their evolution necessitated by the environmental change. Antarctic notothenioids have an AFGP that shares a 93% genetic sequence match with a pancreatic digestive protein named tripsinogen, so it's development probably started from a frame shift in the tripsinogen DNA.

In recent news, a synthetic AFGP has been synthesized that seems to have all of the properties of the natural ones. AFGP's have always been limited by available quantity, as they had to be extracted from fish. With a synthetic AFGP available, mainstream use is coming soon. Food treated with AFGP's is immunized against freezer burn, since it stops the growing ice crystals that do freezing damage. One trial was done injecting lambs with AFGP 24 hours before slaughter, which made the processed meat resistant to freezing damage. Research is even being done to splice AFGP genes into agricultural plants, making them able to withstand a wider range of temperatures and shipping conditions.