Which structural characteristics make glycoproteins good antifreezes?

09-Feb-2004

Many animals and plants in very cold regions have to keep their bodily fluids from freezing in order to avoid the destruction of cell membranes and other heavy damage. Their bodies therefore synthesize their own antifreezes. Among the first antifreeze molecules to be discovered are certain glycoproteins (proteins bound to sugars) from polar fish. Japanese researchers have now systematically examined these glycoproteins. The antifreeze glycoproteins isolated from the blood of polar fish are structurally very varied but amazingly consist of a single motif repeated again and again: a tripeptide with the amino acid sequence alanine-threonine-alanine. Each threosine is equipped with a disaccharide (a "double sugar") made of galactose and N-acetylgalactosamine. The glycoproteins bind to the surface of tiny ICE crystal nuclei and prevent them from growing into larger crystals. In this way they lower the freezing point of water without changing its melting point. This difference is known as thermal hysteresis and is a measure of the antifreeze activity of a substance. In addition, the fish glycoproteins change the morphology of the ice crystals; instead of ordinary ice crystals, hexagonal bipyramids are formed. The team led by Shin-Ichiro Nishimura found proteins made of anywhere from four to fifty tripeptide units in fish blood. In order to find out what's going on with this motif, the researchers prepared chains of up to seven tripeptide units. Even the monomer forces ice crystals into the hexagonal structure. Thermal hysteresis is observed from the dimer up. The effect initially increases with the number of tripeptide building blocks. A maximum is reached at five units; hysteresis cannot be raised by the addition of the sixth and seventh tripeptide. Based on spectrometric data, the researchers determined the probable 3-D structure of the trimer. Its peptide backbone is twisted into a left-handed spiral. The sugar units all protrude on the same side, forming a hydrophilic (water-friendly) front. The hydrophobic (water-repellant) side chains of the molecule form a hydrophobic front. This structural principle seems to be common to all antifreeze glycoproteins. In addition, the structure of the sugar unit and the methyl group of the threonine also play an important role in protection from freezing.

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