Ordered hydration layer mediated ice adsorption of a globular antifreeze protein: A mechanistic insight
Ice/water interface recognition mechanism of antifreeze proteins (AFPs) is highly contentious. Conventionally, protein adsorption to a solid surface is primarily driven by the polar interactions between the hydrophilic residues of the protein and interfacial water of the solid surface. Ice surface recognition by type III AFP is surprising in this context where the ice binding surface (IBS) is hydrophobic. Present study provides molecular insight into the unusual interface recognition phenomenon of a type III AFP (QAE isoform) from Macrozoarces americanus. Potential of mean force calculations show that the type III AFP adsorbs on the ice surface mediated through a layer of ordered water. Molecular dynamics simulations at lower than ambient temperature reveal that the flat hydrophobic IBS induces ordering of water. The Excellent geometrical synergy between hydration water structure around the IBS and water arrangements on the pyramidal surface favours the adsorption on the pyramidal plane. Mutations that interrupt the hydration shell water ordering essentially lead to less efficient adsorption which largely reduces the anti-freezing activity of the AFP. Binding free energy calculations of wild-type and several mutant AFPs reveal that binding affinity is linearly correlated with the experimentally observed thermal hysteresis activity. Therefore, binding to the specific ice plane with considerable affinity is the dictating factor of the anti-freeze activity for type III AFP. Mechanistic insights into the ice binding process of wild-type and different mutant AFPs obtained from this study pave the way for rational designing of type III variants with much improved activity which possesses ample industrial applicability, particularly in cryo-preservation.