Fish antifreeze with an electrical twist.

Fish antifreeze with an electrical twist

To survive temperatures at which their body fluids normally would freeze, fish living in the icy waters of polar seas produce special proteins that act as an antifreeze. These proteins inhibit the formation of large ice crystals, which would otherwise damage biological tissue and cause death. Researchers now have worked out the three-dimensional structure of one of these antifreeze proteins. Its coiled rrangement, they say, suggests one way in which such proteins may bind ice nuclei to keep them from growing.

Daniel S.C. Yang of the University of Pittsburgh and his colleagues worked with an antifreeze protein -- rich in the amino acid alanine -- isolated from the winter flounder. Their crystallographic studies reveal that the protein, only 5 nanometers long, looks like a tiny corkscrew. Such a molecular structure is known as an alpha-helix.

"This is the first report of a polypeptide of this size that is a single alpha-helix," the researchers write in the May 19 NATURE. Other, comparable protein molecules have a more complicated structure that often includes less orderly amino-acid arrangements.

Previous research established that an alpha-helix protein is an electric dipole. In other words, te molecule acts as if one end has a negative charge and the other end a positive charge. When a protein is electrostatically attracted to an ice crystal, all its molecules tend to line up in one particular direction with respect to the crystal.

At the same time, side-chain molecular groups along the protein's helical strand can swivel to facilitate bonding with atoms on ice surfaces. "The flexibility of the side chains means hat many patterns of hydrogen bonding can exist," the researchers say.

The possibility of an electrical interaction between a protein molecule and an ice crystal seems to explain why antifreeze proteins bind to certain faces of an ice crystal. Earlier theories, while suggesting that such an alignment happens, had failed to specify why it occurs in a certain orientation (SN: 11/22/86, p.330).

Fish and other organisms resistant to cold temperatures carry a mixture of different antifreeze proteins. Whether a similar binding mechanism applies to all antifreeze proteins remains unknown. Researchers must devise experiments both to test the proposed binding mechanism and to establish the thre-dimensional structures of more antifreeze proteins.