Impact of Borate on Structure of Antifreeze Glycoproteins

Antifreeze glycoproteins (AFGPs) facilitate the survival of various organisms in the polar region by preventing internal ice accumulation via an adsorption-inhibition mechanism. Inhibition of AFGP antifreeze activity by the borate buffers has been widely acknowledged as the direct experimental evidence supporting the hydroxyl, rather than methyl, binding mechanism. On the other hand, perturbation of borate binding on the AFGP configuration, which might have considerable influence on the binding efficiency of not only the hydroxyl but also the methyl groups, has rarely been quantitatively examined. Herein we studied, using molecular dynamics simulations, the perturbation on the configuration of a solvated AFGP8 protein induced by the binding of one single borate anion. Near the freezing point, this binding not only makes the disaccharide groups adjacent to the borate-binding disaccharide close to each other but also affects the entire AFGP8 conformation. The structural changes induced by the binding of borate on different disaccharide sidechains exhibit clear site-specificities and the effect of borate binding on the structural changes is significantly reduced at higher temperatures. Our study is valuable for further understanding the relationship between the structure and antifreeze activity of these antifreeze glycoproteins.