Calcium Ion Implicitly Modulates the Adsorption Ability of Ion-Dependent Type II Antifreeze Proteins on Ice/Water Interface: A Structural Insight

Ion dependent type II antifreeze proteins (AFPs) are unusual design of natural evolution for cold-acclimatization of fishes in Antarctic region. This class of protein requires Ca2+ to perform an unusual biological recognition, binding to specific ice plane. However, the ice-protein complex is yet to be characterized at molecular-scale. Here, using equilibrium simulations, free energy calculation and metadynamics, we have elucidated this unusual ice recognition phenomenon at atomistic level. Origin of ion selectivity has been critically investigated to identify the role of different ions on the dynamics and ice binding ability of the protein. We have demonstrated that within type II protein matrix, the preferred coordination number of Ca2+ is seven involving five protein atoms and two water molecules. Due to this coordination geometry, the ion binding loop adopts a flat solvent exposed conformation which helps the AFP to efficiently adsorb on prism plane. The ice binding surface (IBS) adsorbs on the ice surface mediated by a layer of ordered water. Structural synergy between the ice/water interface of prism plane and water structure around the IBS makes the adsorption highly favorable. On the other hand, the preferred geometry of the Zn2+ coordination sphere within the AFP matrix is tetrahedral. The numbers of the coordination as well as the coordination bond lengths both are smaller for Zn2+ in comparison to Ca2+. Thus to optimize the coordination sphere for Zn2+ within the protein matrix, a kink is introduced in the ion binding loop, a part of the IBS. Therefore, the IBS and ice surface complementarity is greatly perturbed which leads to less effective adsorption.