(53a) Engulfment of Antifreeze Proteins By Ice

Antifreeze proteins (AFPs) are a remarkable class of proteins, which enable diverse organisms, from fishes and insects to plants and bacteria, to survive in frigid polar environments. Despite being present at millimolar concentrations, AFPs help the host organisms survive at sub-freezing temperatures by suppressing ice formation. To function, AFPs must perform one of the most challenging recognition tasks in all of biology: - binding an ice nucleus in a vast excess of water even though there are no chemical and few structural differences between ice and water. Once AFPs bind the ice nucleus, ice growth can occur only if ice is successful in engulfing the bound AFPs. At a sufficiently low temperatures (below the melting point), the driving force for ice growth wins over the unfavorable process of AFP engulfment and AFPs are no longer able to resist the ice growth. The range of temperatures over which AFPs are able to suppress ice growth, known as the thermal hysteresis activity of the AFP, varies from one AFP to another. To uncover how the molecular characteristics of an AFP influence its activity, we employ molecular dynamics simulations and enhanced sampling techniques. We characterize the free energetics of engulfment of AFP by ice to understand how the thermal hysteresis activity of an AFP depends not only on the separation between bound AFPs, but also on AFP size, shape, and chemistry. We find that the thermal hysteresis activity of AFPs scales with their size and that diverse AFPs sourced from a variety of organisms have evolved optimally to resist the engulfment by ice.