Regulating Antifreeze Activity through Water: Latent Functions of the Sugars of Antifreeze Glycoprotein Revealed by Total Chemical Synthesis

Abstract

AbstractAntifreeze glycoprotein (AFGP), which inhibits the freezing of water, is highly O‐glycosylated with a disaccharide, d‐Galβ1‐3‐d‐GalNAcα (GalGalNAc). To elucidate the function of the sugar residues for antifreeze activity at the molecular level, we conducted a total chemical synthesis of partially sugar deleted AFGP derivatives, and unnatural forms of AFGPs incorporating glucose (Glc)‐type sugars instead of galactose (Gal)‐type sugars. These elaborated AFGP derivatives demonstrated that the stereochemistry of each sugar residue on AFGPs precisely correlates with the antifreeze activity. A hydrogen–deuterium exchange experiment using synthetic AFGPs revealed a different dynamic behavior of water around sugar residues depending on the sugar structures. These results indicate that sugar residues on AFGP form a unique dynamic water phase that disturbs the absorbance of water molecules onto the ice surface, thereby inhibiting freezing.