Molecular Insight into the Adsorption of Spruce Budworm Antifreeze Protein to an Ice Surface: A Clathrate-Mediated Recognition Mechanism

The principal mechanism of ice recognition by antifreeze protein (AFP) has been a topic of intense discussion in recent times. Despite many experimental and theoretical studies, the detailed understanding of the process remains elusive. The present work aims to explore the molecular mechanism of ice recognition by an insect AFP from the spruce budworm, sbwAFP. As evident from our simulation, the water dynamics becomes very sluggish around the ice binding surface (IBS) as a result of the combined effect of confinement and ordering induced by the perfectly aligned methyl side chains of threonine residues, the THR ladder. The hydroxyl groups of threonine form strong hydrogen bonds with few of those highly ordered water molecules that are close to the THR ladder, which is the origin of anchored clathrate water at the IBS of sbwAFP. We propose anchored clathrate-mediated basal plane recognition by sbwAFP. The AFP adsorbed on the basal plane through water clathrate framed around the IBS. The surface of the basal plane and anchored clathrate water completes the caging around the threonine residues, which is the origin of the binding plane specificity of sbwAFP. This adsorbed AFP-ice complex undergoes dynamic crossover to a hydrogen-bonded complex within the thermal hysteresis (TH) regime of this particular AFP. The anchored clathrate water becomes part of the newly grown basal front as a result of the geometrical matches between the basal plane and the anchored clathrate water repeat distance. This observation provides a structural rationale for the experimentally observed time-dependent increase in TH activity for insect AFP. Our study proposes clathrate-mediated ice recognition by AFP and elucidates the dynamic events involved during ice binding by the insect AFP.