‘Antifreeze’ glycoproteins from polar fish

Abstract

Antifreeze glycoproteins (AFGPs) constitute the major fraction of protein in the blood serum of Antarctic notothenioids and Arctic cod. Each AFGP consists of a varying number of repeating units of (Ala-Ala-Thr) n , with minor sequence variations, and the disaccharide b-D-galactosyl-(1fi3)-a-N-acetyl-D-galactosamine joined as a glycoside to the hydroxyl oxygen of the Thr residues. These compounds allow the fish to survive in subzero ice-laden polar oceans by kinetically depressing the temperature at which ice grows in a noncolligative manner. In contrast to the more widely studied antifreeze proteins, little is known about the mechanism of ice growth inhibition by AFGPs, and there is no definitive model that explains their properties. This review summarizes the structural and physical properties of AFGPs and advances in the last decade that now provide opportunities for further research in this field.High field NMR spectroscopy and molecular dynamics studies have shown that AFGPs are largely unstructured in aqueous solution. While standard carbohydrate degradation studies confirm the requirement of some of the sugar hydroxyls for antifreeze activity, the importance of following structural elements has not been established: (a) the number of hydroxyls required, (b) the stereochemistry of the sugar hydroxyls (i.e. the requirement of galactose as the sugar), (c) the acetamido group on the first galactose sugar, (d) the stereochemistry of the b-glycosidic linkage between the two sugars and the a-glycosidic linkage to Thr, (e) the requirement of a disaccharide for activity, and (f) the Ala and Thr residues in the polypeptide backbone. The recent successful synthesis of small AFGPs using solution methods and solidphase chemistry provides the opportunity to perform key structure-activity studies that would clarify the important residues and functional groups required for activity.Genetic studies have shown that the AFGPs present in the two geographically and phylogenetically distinct Antarctic notothenioids and Arctic cod have evolved independently, in a rare example of convergent molecular evolution. The AFGPs exhibit concentration dependent thermal hysteresis with maximum hysteresis (1.2°C at 40 mgAEmL )1 ) observed with the higher molecular mass glycoproteins. The ability to modify the rate and shape of crystal growth and protect cellular membranes during lipid-phase transitions have resulted in identification of a number of potential applications of AFGPs as food additives, and in the cryopreservation and hypothermal storage of cells and tissues.

A Unique Capsular Polysaccharide Structure from the Psychrophilic Marine Bacterium Colwellia psychrerythraea 34H That Mimics Antifreeze (Glyco)proteins

“…[55][56][57] For example, antifreeze glycoproteins (AFGPs) isolated from fish blood plasma consist of a regular tripeptide sequence of Ala-Ala-Thr with a disaccharide fragment (-D-galactosyl-(1-3)--D-galactosamine) linked to the threonine residue. [58][59][60][61] It is noteworthy that the polysaccharide described here displays a structural feature very close to that present in AFGPs. Actually, a -N-acetyl-galactosamine is substituted by an -galacturonosyl residue bearing an amide-linked threonine.…”

Interaction of ice binding proteins with ice, water and ions

“…31,42,44 AFGPs consist of n ¼ 4-50 tripeptide repeats of (Ala-AlaThr) n with the disaccharide galactose-N-acetylgalactosamine attached to each hydroxyl oxygen atom of the Thr residues. 45,46 AFGPs are categorized into eight classes of isoforms with AFGP1 corresponding to the largest glycoproteins (M w ¼ 34 kDa) and AFGP8 to the smallest (M w ¼ 2.6 kDa). 4 No solution structure is available for AFGPs, since they are polydisperse, flexible, and rather disordered.…”

“…4 No solution structure is available for AFGPs, since they are polydisperse, flexible, and rather disordered. 46 Fish type I AFPs also have a highly repetitive amino acid sequence. The two type I AFPs from winter flounder, for example, are rich in alanine and have an a-helical fold with 11-residue periodicity [Figs.…”

State diagrams for improving processing and storage of foods, biological materials, and pharmaceuticals (IUPAC Technical Report)

“…Although such proteins were first thought to act via a colligative effect, by decreasing the meting point of water, it was later observed that they act by preventing the growth of ice crystals, having only a small effect on colligative properties [237]. The ability to modify the rate and shape of ice-crystal growth and protect cellular membranes during lipid-phase transitions has resulted in identification of a number of potential applications of AFGPs as food additives, and in applications for the cryopreservation and hypothermal storage of cells and tissues [238,239].…”