Preferential Ordering and Organization of Hydration Water Favor Nucleation of Ice by Ice-Nucleating Proteins over Antifreeze Proteins

Bacteria containing ice-nucleating proteins (INPs) evolved in nature to nucleate ice at the high sub-zero ambiance. The ability of the INPs to induce order in the hydration layer and their aggregation propensity appear to be key factors of their ice nucleation abilities. However, the mechanism of the process of ice nucleation by INPs is yet to be understood clearly. Here, we have performed all-atom molecular dynamics simulations and analyzed the structure and dynamics of the hydration layer around the proposed ice-nucleating surface of a model INP. Results are compared with the hydration of a topologically similar non-ice-binding protein (non-IBP) and another ice-growth inhibitory antifreeze protein (sbwAFP). We observed that the hydration structure around the ice-nucleating surface of INP is highly ordered and the dynamics of the hydration water are slower, compared to the non-IBP. Even the ordering of the hydration layer is more evident around the ice-binding surface of INP, compared to the antifreeze protein sbwAFP. Particularly with increasing repeat units of INP, we observe an increased population of ice-like water. Interestingly, the distances between the hydroxyl groups of the threonine ladder and its associated channel water of the ice-binding surface (IBS) of INP in the X and Y direction mimic the oxygen atom distances of the basal plane of hexagonal ice. However, the structural synergies between the hydroxyl group distances of the threonine ladder and its associated channel water of the IBS of sbwAFP and oxygen atom distances of the basal plane are less evident. This difference makes the IBS of the INP a better template for ice nucleation than AFP, although both of them bind to the ice surface efficiently.

Molecular Factors of Ice Growth Inhibition for Hyperactive and Globular Antifreeze Proteins: Insights from Molecular Dynamics Simulation

The molecular mechanism behind the ice growth inhibition by antifreeze proteins (AFPs) is yet to be understood completely. Also, what physical parameters differentiate between the AFP and non-AFP are largely unknown. Thus, to get an atomistic overview of the differential antifreeze activities of different classes of AFPs, we have studied ice growth from different ice surfaces in the presence of a moderately active globular type III AFP and a hyperactive spruce budworm (sbw) AFP. Results are compared with the observations of ice growth simulations in the presence of topologically similar non-AFPs using all-atom molecular dynamics simulations. Simulation data suggest that the ice surface coverage is a critical factor in ice growth inhibition. Due to the presence of an ice binding surface (IBS), AFPs form a high affinity complex with ice, accompanied by a transition of hydration water around the IBS from clathrate-like to ice-like. Several residues around the periphery of the IBS anchor the AFP to the curved ice surface mediated by multiple strong hydrogen bonds, stabilizing the complex immensely. In the high surface coverage regime, the slow unbinding kinetics dominates over the ice growth kinetics and thus facilitates the ice growth inhibition. Due to the non-availability of a proper IBS, non-AFPs form a low-affinity complex with the growing ice surface. As a result, the non-AFPs are continuously repelled by the surface. If the concentration of AFPs is low, then the effective surface coverage is reduced significantly. In this low surface coverage regime, AFPs can also behave like impurities and are engulfed by the growing ice crystal.

Antifreeze proteins and their biomimetics for cell cryopreservation: Mechanism, function and application-A review

Cell-based therapy is a promising technology for intractable diseases and health care applications, in which cryopreservation has become an essential procedure to realize the production of therapeutic cells. Ice recrystallization is the major factor that affects the post-thaw viability of cells. As a typical series of biomacromolecules with ice recrystallization inhibition (IRI) activity, antifreeze proteins (AFPs) have been employed in cell cryopreservation. Meanwhile, synthesized materials with IRI activity have emerged in the name of biomimetics of AFPs to expand their availability and practicality. However, fabrication of AFPs mimetics is in a chaotic period. There remains little commonality among different AFPs mimetics, then it is difficult to set guidelines on their design. With no doubt, a comprehensive understanding on the antifreezing mechanism of AFPs in molecular level will enable us to rebuild the function of AFPs, and provide convenience to clarify the relationship between structure and function of these early stage biomimetics. In this review, we would discuss those previously reported biomimetics to summarize their structure characteristics concerning the IRI activity and attempt to develop a roadmap for guiding the design of novel AFPs mimetics.

Deciphering the Role of the Non-ice-binding Surface in the Antifreeze Activity of Hyperactive Antifreeze Proteins

Antifreeze proteins (AFPs) show thermal hysteresis through specific interaction with the ice crystal. Hyperactive AFPs interact with the ice surface through a threonine-rich motif present at their ice-binding surface (IBS). Ordering of water around the IBS was extensively investigated. However, the role of non-IBS in ice growth inhibition is yet to be understood completely. The present study explores the nature of hydration and its length-scale evaluation around the non-IBS for hyperactive AFPs. We observed that the hydration layer of non-IBS is liquid-like, even in highly supercooled conditions, and the nature of hydration is drastically different from the hydration pattern of non-AFP surfaces. In similar conditions, the hydration layer around the IBS is ice-like ordered. Non-IBS of the hyperactive AFP exposes toward the bulk and is able to maintain the liquid-like character of its hydration water up to 15 Å. We also find that the amino acid compositions and their spatial distribution on the non-IBS are markedly different from those of the IBS and non-AFP surfaces. These results elucidate the combined role of IBS and non-IBS in ice-growth inhibition. While IBS is required to adsorb on ice efficiently, the exposed non-IBS may prevent ice nucleation/growth on top of the bound AFPs.

Ordered hydration layer mediated ice adsorption of a globular antifreeze protein: mechanistic insight

The ice binding surface of a type III AFP induces water ordering at lower temperature, which mediates its adsorption on the ice surface.