Structural insights into a single semi-clathrate hydrate formed in a confined environment of porous protein crystal.

Water behavior on protein surfaces influences protein structure and function. Antifreeze Proteins (AFPs) have been intensively studied in context of biological cytotechnology. AFPs inhibit growth of ice microcrystals by forming unique water-cluster networks which are influenced by protein surface morphology and hydrophobicity. Such unique water-cluster networks have been identified as semi-clathrate structures in crystals and are believed to be stabilized by intermolecular interactions within the confined environment. However, there is little atomic-level information about the process of formation of semi-clathrates and the structural units of water-clathrate networks. We identified a single semi-clathrate formed on the pore surface of ferritin crystal which has a structure similar to that of a natural AFP. Comparison of ferritin mutants and determination of temperature-dependent structures revealed that semi-clathrate water molecules on an -helix undergo structural alterations with increasing temperature. Lowering the temperature regenerates the semi-clathrate structure. Water molecules hydrogen-bonded to main chain carbonyl groups are stably immobilized at room temperature and serve as starting points for clathrate formation. These findings provide a mechanistic understanding of water networks in AFPs and guidelines for designing new cryomaterials.