CBMS Lecture Series

Antifreeze proteins that protect marines fishes from freezing in icy seawater were discovered in the late 1960's. They function by binding to internal seed ice crystals and preventing their growth. Since then, the variety and roles of ice-binding proteins have increased to include the inhibition of ice recrystallization, ice adhesion, and even ice nucleation. The mechanism by which ice-binding proteins can be freely soluble at millimolar concentrations in water and yet bind irreversibly to the ice – the solid state of water – was a mystery for a long time. But high resolution crystal structures obtained at Brookhaven for several ice-binding proteins showed that they have a clathrate water pattern on their ice-binding sites. We now think that these ice-like waters merge with the quasi-liquid layer of water around an ice crystal and become frozen together at subzero temperatures. Although these proteins are only bound to ice by hydrogen bonds and other weak forces, there are many of them, and almost all would have to break at the same time for the protein to come off the ice. Modeling of ice nucleation protein structures suggests that they resemble much larger versions of ice-binding proteins. In addition, they are also extensively multimerized. Here similar protein surface structures are in place to organize ice-like waters, but in such great numbers that they can nucleate freezing at high sub-zero temperatures up to -2 ºC.