Hydrogen bond analysis of Type 1 antifreeze protein in water and the ice/water interface

Antifreeze proteins (AFPs) are a group of structurally very diverse proteins with the unique capability of inhibiting ice crystal growth. Although significant progress has been made in the identification of different families of these proteins, the molecular mechanism of their action is unclear. The previously postulated mechanism of hydrogen bonding between the threonine residues of AFP and the water molecules in the ice surface has been disproved by mutation studies with non-polar residues. Currently, the mechanism of antifreeze activity cannot be fully understood from experimental or computational studies. Computational modeling studies have examined protein–ice interactions, mostly in vacuo. These studies have neglected the effects of the water phase. It has been shown that the vacuum is a very poor approximation for the water properties. Thus, to gain an insight into the molecular mechanism of these proteins we have computationally modeled a more realistic system comprising of AFP Type I from winter flounder (HPLC6), water and ice without any constraints. The results from this study show that the protein forms hydrogen bonds with the water molecules in the ice/water interfacial region. However, a comparison of the results with the protein in water simulations shows that there is no significant gain of hydrogen bonds for protein in the interfacial region compared to in the solvent. These results support the hypothesis that hydrogen bonding is not the primary reason for interaction of HPLC6 with the ice/water interfacial region.