UNTIL now, no one has quite figured out how antifreeze proteins help insects, fish and plants to survive at subzero temperatures. But a detailed computer model has lent strong support to a theory that the antifreeze proteins stop the growth of ice crystals by attaching to them and forcing them to change their shape 鈥� much like a pillow is deformed when stones are placed on it.
Several organisms, including icefish and Antarctic cod, produce antifreeze proteins (AFPs) to help them survive in icy waters. Normally, the formation of ice crystals in their body fluids should rupture delicate membranes and cellular structures, but the AFPs bind to the surface of ice crystals and prevent them growing.
Now, Leonard Sander and Alexei Tkachenko of the University of Michigan in Ann Arbor have shown just how this could happen. When the proteins attach to ice crystals, the ice is forced to grow in a bulge between them. The computer model shows that if the bulge gets large enough, then the ice will engulf the proteins rather than keep on growing. This slows down the expansion of the ice crystals. 鈥淚f the ice grows slowly, more proteins will [attach] than can be absorbed, so the ice can鈥檛 鈥榚at鈥� them fast enough,鈥� says Sander. 鈥淭he process collapses and the ice stops growing.鈥�
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As the ice stops forming, the surrounding fluid can become supercooled to below freezing point. Some animals can survive even if their body fluids are chilled to 鈭�2 掳C. The researchers tested the model with proteins of different shapes to predict how far the water could be supercooled before ice started to form again, and their predictions matched experimental data collected by other researchers almost exactly (Physical Review Letters, vol 93, p 128102). The model also shows that spherical proteins are most effective, because they have no region for the ice to preferentially attack, while rod-shaped proteins are not as good, as the ice can grow over their pointed ends quite easily.
Not everyone is convinced that the theory is accurate, however. One of its requirements is that the proteins must irreversibly attach to the ice. But biophysicist Yin Yeh of the University of California at Davis says this may not happen.
鈥淲e are starting to see phenomena where proteins are not as firmly anchored as postulated in the model,鈥� he says. 鈥淭here are a number of experiments that show these molecules may be coming on and off the surface.鈥� Yeh has detected proteins moving around the surface of ice crystals, even at 鈭�15 掳C. And if the proteins do not bind to the ice permanently, they cannot prevent the ice crystals from forming.
