Water binding by antifreeze glycoproteins from Antarctic fish

Abstract

Low molecular weight glycoproteins isolated from serum of Antarctic fish have been shown¹ to be responsible for freezing point depression which permits survival at temperatures down to −1.9 °C. The principal active antifreeze glycoprotein (AFGP) of several Antarctic species consists of a repeating tripeptide Ala-Thr-Ala with a disaccharide galactosyl-N-acetylgalactosamine linked to the threonine residue^2–4. The AFGP is more effective in freezing-point depression on a weight basis than NaCl and several hundredfold more effective than ordinary proteins². Melting point, however, is virtually unaffected by the AFGP⁵. We have investigated the possible role of proteinbound water in the unusual antifreeze activity of the AFGP. Ordinary proteins such as bovine serum albumin, haemoglobin, and lysozyme show hydration levels of about 0.4 g H₂O per g protein⁶. This water does not freeze at temperatures down to −35 °C, but is sufficiently mobile to produce a measurable proton magnetic resonance signal. Quantitative determination of the amount of this ‘bound’ water is therefore possible in frozen protein solutions⁷. We have used this technique to examine water binding by the AFGP over a range of temperatures below the freezing point of bulk water.