Conformational and Hydration Properties Modulate the Ice Recognition by Type I Antifreeze Protein and its Mutants
Mechanism of ice recognition by antifreeze protein (AFP) is a topic of recent interest. Here, using equilibrium simulation and free energy calculation, we provide structural rationale to the observed experimental anomalies on type I AFP (wfAFP isoform HPLC6) and its mutants as well as probe the molecular origin of ice recognition by them. Our results clearly demonstrate that the interplay between the conformational and hydration properties dictates the ice binding ability of type I AFP and its mutants. We find that the HPLC6 exists as a highly stable long helix which adsorbs on the ice surface through the ordered water cages around the CH3 group of threonine (THR) residues, rather than directly binding to the ice surface by threonine (THR) through hydrogen bonding. Upon mutating THR with serine (SER), the straight helix conformation of the HPLC6 disappears and the most stable conformation is a kinked helix devoid of ice binding ability. Free energy calculation reveals that there is a dynamic equilibrium between straight and bent helical conformations in case of valine (VAL) mutant. The straight long helical form of the VAL mutant also has the ability to form ordered water cage structure around the CH3 groups of VAL residues and thereby efficiently adsorbs on ice plane similar to the wild type AFP.