Disaccharide Residues are Required for Native Antifreeze Glycoprotein Activity

Composition-antifreeze property relationships of gelatin and the corresponding mechanisms

The inherent functional fractions (gelation and ice-affinitive fractions) of gelatin enable it as a promising cryoprotectant alternative. However, the composition-antifreeze property relationships of gelatin remain to be investigated. In this study, the HW-PSG and LW-PSG fractions of gelatin from fish scales were obtained, according to the critical gelation conditions and ice-binding measurements, respectively. Thermal hysteresis (THA) value, associated with ice nucleation, of LW-PSG was higher than that of HW-PSG. Besides, the relatively low-sized ice crystals (210-550 μm2) indicated that HW-PSG showed strong ice recrystallization inhibition (IRI) ability, compared to other groups. These results suggested that LW-PSG inhibited ice nucleation, while HW-PSG displayed the strong IRI ability. Furthermore, the antifreeze mechanisms were clarified through IRI measurements and molecular dynamics simulation. The minimum size of ice crystals was found for HW-PSG gels with dense microstructure, suggesting the HW-PSG retarded the growth of ice crystals by restricting the migration and phase transformation of water molecules. The hydrogen bond interactions between the ice crystal surface and ASN1294 and PRO1433 residues of LW-PSG, and hydrophobic interactions contributed to inhibiting the nucleation of ice crystals. This study provided some references to further enhance antifreeze performance of gelatin by modulating fragment composition.

Synthetic Antifreeze Glycoproteins with Potent Ice-Binding Activity

Antifreeze glycoproteins (AFGPs) are produced by extremophiles to defend against tissue damage in freezing climates. Cumbersome isolation from polar fish has limited probing AFGP molecular mechanisms of action and limited development of bioinspired cryoprotectants for application in agriculture, foods, coatings, and biomedicine. Here, we present a rapid, scalable, and tunable route to synthetic AFGPs (sAFGPs) using N-carboxyanhydride polymerization. Our materials are the first mimics to harness the molecular size, chemical motifs, and long-range conformation of native AFGPs. We found that ice-binding activity increases with chain length, Ala is a key residue, and the native protein sequence is not required. The glycan structure had only minor effects, and all glycans examined displayed antifreeze activity. The sAFGPs are biodegradable, nontoxic, internalized into endocytosing cells, and bystanders in cryopreservation of human red blood cells. Overall, our sAFGPs functioned as surrogates for bona fide AFGPs, solving a long-standing challenge in accessing natural antifreeze materials.

The Nanoliter Osmometer: Thermal Hysteresis Measurement

The nanoliter osmometer is one of the most common tools in the study of ice-binding proteins (IBPs). It is used not only to measure the thermal hysteresis activity of IBPs but also to explore ice shaping, ice adhesion, and ice growth and melting rates and patterns. The advantage of the nanoliter osmometer for the IBP study and for studying single ice crystals lies in the small sample volume, in the range of nanoliters. Such a small volume enables precise determination and control of the temperature with precision in the range of millidegrees. This chapter describes in detail the process of determination of thermal hysteresis using a nanoliter osmometer operated by a LabVIEW interface. We describe the preparation of suitable capillaries and sample injection, which is a challenging step in the measurement. We then describe the procedure of single crystal formation and the determination of the melting and freezing temperatures. Insights on crucial parameters are emphasized.

Functional aggregation of cell-free proteins enables fungal ice nucleation

Biological ice nucleation plays a key role in the survival of cold-adapted organisms. Several species of bacteria, fungi, and insects produce ice nucleators (INs) that enable ice formation at temperatures above -10 °C. Bacteria and fungi produce particularly potent INs that can promote water crystallization above -5 °C. Bacterial INs consist of extended protein units that aggregate to achieve superior functionality. Despite decades of research, the nature and identity of fungal INs remain elusive. Here, we combine ice nucleation measurements, physicochemical characterization, numerical modeling, and nucleation theory to shed light on the size and nature of the INs from the fungus Fusarium acuminatum. We find ice-binding and ice-shaping activity of Fusarium IN, suggesting a potential connection between ice growth promotion and inhibition. We demonstrate that fungal INs are composed of small 5.3 kDa protein subunits that assemble into ice-nucleating complexes that can contain more than 100 subunits. Fusarium INs retain high ice-nucleation activity even when only the ~12 kDa fraction of size-excluded proteins are initially present, suggesting robust pathways for their functional aggregation in cell-free aqueous environments. We conclude that the use of small proteins to build large assemblies is a common strategy among organisms to create potent biological INs.

Mechanism of Binding of Polyproline to Ice via Interfacial Water: An Experimental and Theoretical Study

The molecular mechanism underlying inhibition of ice growth by polyproline (PPro), a minimal antifreeze glycoprotein mimic, remains unclear. In this work, the change in the structure of water during the growth of ice in PPro solutions was investigated using a combination of near-infrared spectroscopy and molecular dynamics (MD) simulations. The results show that only high concentrations of PPro solutions can effectively inhibit ice growth, as indicated by the variation in the spectral intensity of ice with time. When PPro exhibits an antifreeze effect, the spectral intensity of hydrated water associated with PPro in a solution is weakened. The experiments and MD simulations reveal that the quantity of the interfacial water between the ice crystal and the hydrophobic groups of PPro progressively reaches a plateau. Most significantly, we present clear evidence that the stable existence of this interfacial water is critical for the antifreeze activity of PPro.

Revealing the Mechanism of Irreversible Binding of Antifreeze Glycoproteins to Ice

Antifreeze glycoproteins (AFGPs) are a special kind of antifreeze proteins with strong flexibility. Whether their antifreeze activity is achieved by reversibly or irreversibly binding to ice is widely debated, and the molecular mechanism of irreversible binding remains unclear. In this work, the antifreeze mechanism of the smallest AFGP isoform, AFGP8, is investigated at the atomic level. The results indicate that AFGP8 can bind to ice both reversibly through its hydrophobic methyl groups (peptide binding) and irreversibly through its hydrophilic disaccharide moieties (saccharide binding). Although peptide binding occurs faster than saccharide binding, free-energy calculations indicate that the latter is energetically more favorable. In saccharide binding, at least one disaccharide moiety is frozen in the grown ice, resulting in irreversible binding, while the other moieties significantly perturb the water hydrogen-bonding network, thus inhibiting ice growth more effectively. The present study reveals the coexistence of reversible and irreversible bindings of AFGP8, both contributing to the inhibition of ice growth and further provides molecular mechanism of irreversible binding.

Cold adaptation strategies in plants—An emerging role of epigenetics and antifreeze proteins to engineer cold resilient plants

Cold stress adversely affects plant growth, development, and yield. Also, the spatial and geographical distribution of plant species is influenced by low temperatures. Cold stress includes chilling and/or freezing temperatures, which trigger entirely different plant responses. Freezing tolerance is acquired via the cold acclimation process, which involves prior exposure to non-lethal low temperatures followed by profound alterations in cell membrane rigidity, transcriptome, compatible solutes, pigments and cold-responsive proteins such as antifreeze proteins. Moreover, epigenetic mechanisms such as DNA methylation, histone modifications, chromatin dynamics and small non-coding RNAs play a crucial role in cold stress adaptation. Here, we provide a recent update on cold-induced signaling and regulatory mechanisms. Emphasis is given to the role of epigenetic mechanisms and antifreeze proteins in imparting cold stress tolerance in plants. Lastly, we discuss genetic manipulation strategies to improve cold tolerance and develop cold-resistant plants.

Recent advances of bio-inspired anti-icing surfaces

The need for improved anti-icing surfaces is the demand of the time and closely related to many important aspects of our lives as surface icing threatens not only industrial production but also human safety. Freezing on a cold surface is usually a heterogeneous nucleation process induced by the substrate. Creating an anti-icing surface is mainly achieved by changing surface morphology and chemistry to regulate the interaction between the surface and the water/ice to inhibit freezing on the surface. In this paper, recent research progress in the creation of biomimetic anti-icing surfaces is reviewed. Firstly, basic strategies of bionic anti-icing are introduced, and then bionic anti-icing surface strategies are reviewed according to four aspects: the process of ice formation, including condensate self-removing, inhibiting ice nucleation, reducing ice adhesion, and melting accumulated ice on the surface. The remaining challenges and the direction of future development of biomimetic anti-icing surfaces are also discussed.

Antifreezing Hydroxyl Monolayer of Small Molecules on a Nanogold Surface

The rational design of ice recrystallization inhibition (IRI) materials is challenging due to the poor understanding of the IRI mechanism at the molecular level. Here we report several new findings about IRI. (1) A dense hydroxyl monolayer of small molecules, e.g. 6-aza-2-thiothymine (ATT), adsorbed on a nanogold surface was demonstrated, for the first time, to have IRI activity. Five structural analogues adsorbed on groups nanogold with outward hydroxyl or methyl were created to evidence the origin of IRI activity. (2) Their IRI mechanism is closely related to the density of hydroxyls on a nanogold surface. However, the hydrophobic interaction in our model is not essential for macroscopic IRI activity. (3) A molecular dynamics simulation elucidates the hydroxyl density dependent IRI trajectories underlying the experimental observations, and the radial distribution function reveals that the methyl even slightly hinders the formation of hydrogen bonding due to a hydrophobic interaction. This work sheds more light on the IRI mechanism that should help in the customization of novel IRI materials.

Ice Recrystallization Inhibition Is Insufficient to Explain Cryopreservation Abilities of Antifreeze Proteins

Antifreeze proteins (AFPs) and glycoproteins (AFGPs) are exemplary at modifying ice crystal growth and at inhibiting ice recrystallization (IRI) in frozen solutions. These properties make them highly attractive for cold storage and cryopreservation applications of biological tissue, food, and other water-based materials. The specific requirements for optimal cryostorage remain unknown, but high IRI activity has been proposed to be crucial. Here, we show that high IRI activity alone is insufficient to explain the beneficial effects of AF(G)Ps on human red blood cell (hRBC) survival. We show that AF(G)Ps with different IRI activities cause similar cell recoveries of hRBCs and that a modified AFGP variant with decreased IRI activity shows increased cell recovery. The AFGP variant was found to have enhanced interactions with a hRBC model membrane, indicating that the capability to stabilize cell membranes is another important factor for increasing the survival of cells after cryostorage. This information should be considered when designing novel synthetic cryoprotectants.

Site-Specific Water Dynamics in the First Hydration Layer of an Anti-Freeze Glyco-Protein: A Simulation Study

Antifreeze glycoproteins (AFGPs) inhibit ice recrystallization by a mechanism remaining largely elusive. Dynamics of AFGPs’ hydration water and its involvement in the antifreeze activity, for instance, have not been identified...