Preferential Ordering and Organization of Hydration Water Favor Nucleation of Ice by Ice-Nucleating Proteins over Antifreeze Proteins

Bacteria containing ice-nucleating proteins (INPs) evolvedin natureto nucleate ice at the high sub-zero ambiance. The ability of theINPs to induce order in the hydration layer and their aggregationpropensity appear to be key factors of their ice nucleation abilities.However, the mechanism of the process of ice nucleation by INPs isyet to be understood clearly. Here, we have performed all-atom moleculardynamics simulations and analyzed the structure and dynamics of thehydration layer around the proposed ice-nucleating surface of a modelINP. Results are compared with the hydration of a topologically similarnon-ice-binding protein (non-IBP) and another ice-growth inhibitoryantifreeze protein (sbwAFP). We observed that the hydration structurearound the ice-nucleating surface of INP is highly ordered and thedynamics of the hydration water are slower, compared to the non-IBP.Even the ordering of the hydration layer is more evident around theice-binding surface of INP, compared to the antifreeze protein sbwAFP.Particularly with increasing repeat units of INP, we observe an increasedpopulation of ice-like water. Interestingly, the distances betweenthe hydroxyl groups of the threonine ladder and its associated channelwater of the ice-binding surface (IBS) of INP in the X and Y direction mimic the oxygen atom distancesof the basal plane of hexagonal ice. However, the structural synergiesbetween the hydroxyl group distances of the threonine ladder and itsassociated channel water of the IBS of sbwAFP and oxygen atom distancesof the basal plane are less evident. This difference makes the IBSof the INP a better template for ice nucleation than AFP, althoughboth of them bind to the ice surface efficiently.

Automated summary

This study investigates the mechanism of ice nucleation by ice-nucleating proteins (INPs) using molecular dynamics simulations. The hydration layer around the ice-nucleating surface of an INP is found to be highly ordered, with slower dynamics compared to a non-ice-binding protein. The ice-binding surface of INP shows more evident ordering of the hydration layer compared to an antifreeze protein. The structural synergies observed in the INP make it a better template for ice nucleation than AFP, although both efficiently bind to the ice surface.