Studies on the Effect of Number of Sugar Moiety in the Antifreeze Activity of Homodimeric AFGPs

摘要

Antifreeze glycoproteins (AFGPs) are found in the plasma of deep sea polar fish such as the Antarctic notothenioids and the northern cods. The AFGPs consist of repeating tripeptide units, Alanyl-Alanyl-Threonyl (Ala-Ala-Thr)_(n=4-50) units, connected with the disaccharide β-D-galactosyl-(1→3)-α-D-N-acetylgalactosamine through a glycosidic bond at the secondary hydroxyl group of the threonine residue. Eight distinct AFGP subtypes are exist in nature. Among them, AFGP 1 is the largest weight (33.7 kDa) and AFGP 8 is the lowest molecular weight (2.6 kDa). AFGPs are able to depress the freezing temperature of the blood serum in fish enough to keep them from freezing in their sub-zero environment while the melting temperature remains unchanged. The gap between freezing point and melting point was referred to as thermal hysteresis (TH) and is taken as a primary manifestation of antifreeze activity by AFGPs. Within the hysteresis temperature gap, AFGPs modify the ice crystal habit, in that the AFGPs saturated ice crystal forms a unique shape, such as a hexagonal bipyramid. These effects prevent ice crystals from growing to a size where they would damage the fish's body.