Influence of antifreeze proteins on the ice/water interface

The low-entropy hydration shell mediated ice-binding mechanism of antifreeze proteins

Antifreeze proteins (AFPs) can inhibit ice crystal growth. The ice-binding mechanism of AFPs remains unclear, yet the hydration shells of AFPs are thought to play an important role in modulating the binding of AFPs and ice. Here, we performed all-atom molecular dynamics simulations of an AFP from Choristoneura fumiferana (CfAFP) at four different temperatures, with a focus on analysis at 240 and 300 K, to investigate the dynamic and thermodynamic characteristics of hydration shells around ice-binding surfaces (IBS) and non-ice-binding surfaces (NIBS). Our results revealed that the dynamics of CfAFP hydration shells were highly heterogeneous, with its IBS favoring a less dense and more tetrahedral solvation shell, and NIBS hydration shells having opposite features to those of the IBS. The IBS of nine typical hyperactive AFPs were found to be in pure low-entropy hydration shell region, indicating that low-entropy hydration shell region of IBS and the tetrahedral arrangements of water molecules around them mediate the ice-binding mechanism of AFPs. It is because the entropy increase of the low-entropy hydration shell around IBS, while the higher entropy water molecules at NIBS most likely prevent ice crystal growth. These findings provide new mechanistic insights into the ice-binding of AFPs.

Factors affecting cryotolerance of mammalian oocytes

Cryopreservation of oocytes is an important tool for preserving genetic resources and for farm animals breeding. Processes taking place during vitrification affect oocytes and result in their reduced developmental capacity and lower fertilisation rates of cryopreserved oocytes. Further improvement in cryopreservation techniques is still required. Several authors already summarized the actual state and perspectives of oocyte cryopreservation as well as potential approaches to improve their development after thawing. The aim of this review is to specify factors affecting cryotolerance of mammalian oocytes, especially bovine in vitro matured oocytes, and to identify the areas, where more efforts were made to improve the success of oocyte cryopreservation. These factors include oocyte lipid content, membrane composition, mRNA protection, cytoskeleton stabilization and application of such potential stimulators of cell cryotolerance as antioxidants, growth factors or antifreeze proteins.

Do antifreeze proteins generally possess the potential to promote ice growth?

The binding of antifreeze proteins (AFPs) to ice needs to be mediated by interfacial water molecules. Our previous study of the effect of AFPs on the dynamics of the interfacial water of freezing at its initial stage has shown that AFPs can promote the growth of ice before binding to it. However, whether different AFPs can promote the freezing of water molecules on the basal and the prismatic surfaces of ice still needs further study. In the present contribution, five representative natural AFPs with different structures and different activities that can be adsorbed on the basal and/or prismatic surfaces of ice are investigated at the atomic level. Our results show that the phenomenon of promoting the growth of ice crystals is not universal. Only hyperactive AFPs (hypAFPs) can promote the growth of the basal plane of ice, while moderately active AFPs cannot. Moreover, this significant promotion is not observed on the prismatic plane regardless of their activity. Further analysis indicates that this promotion may result from the thicker ice/water interface of the basal plane, and the synergy of hypAFPs with ice crystals.

Overexpression of BrAFP1 gene from winter rapeseed (Brassica rapa) confers cold tolerance in Arabidopsis

Antifreeze proteins (AFPs) can bind to ice crystals and restrain the formation of larger crystals, a strategy vital to the survival of plants in freezing environments. The BrAFP1 from winter rapeseed cultivars 'Longyou 7' with high cold tolerance was cloned and overexpressed in Arabidopsis. BrAFP1 was localized in the cytoplasm and nucleus. Under cold stress, SOD activity and free proline content were higher, MDA content and relative conductivity were lower in transgenic lines than those in wide-type Arabidopsis. Frostbite of transgenic plants was minimized, whereas frostbite of the Arabidopsis afp1 mutant was severe. Transition of the amino acid at position 17 of BrAFP1 was related to the increased winter survival of the rapeseed cultivar. Cultivars with higher survival rates had a predilection for tyrosine, not tryptophan, at the 17th site in the amino sequence of BrAFP1. Transcription of BrAFP1 was induced more rapidly, and the expression of the gene was also higher, in Longyou 7 than that in Tianyou 4 under cold stress. Overall, the high expression of BrAPF1 confers more cold-tolerance in Longyou 7.

The influence of a type III antifreeze protein and its mutants on methane hydrate adsorption-inhibition: a molecular dynamics simulation study

Antifreeze proteins (AFPs) inhibit ice growth in various organisms at subzero temperature. Recently, AFPs as a hydrate inhibitor have been a topic of intense discussion, while the detailed mechanism remains obscure. The present work aims to explore molecular insight into the adsorption and inhibition of an AFP III on methane hydrate. Three polar, hydrophilic, and neutral amino acids (Asn14, Thr18, and Gln44) are mutated to elucidate the molecular mechanism of AFP III antifreeze activity. Another triple mutation is also designed to investigate the effect of the side chain. Atomistic molecular dynamics simulations provide detailed structural and dynamical aspects of protein residues and water molecules at the hydrate/water interface. Initially, it was proposed that the AFP III operates by the adsorption-inhibition mechanism on hydrates, almost similar to that of ice. The exchange of amide and hydroxyl groups by mutagenesis alters the shape of the side chain and the capability of hydrogen bonding and demonstrates that hydrogen bonds are not directly responsible for the AFP III antifreeze activity. Moreover, we deciphered that the length of the pendant group is an important factor in the entrapment of the AFP III on the hydrate cages, which is compatible with van der Waals interactions between the side chains and hydrate surface. The results suggest that this interaction is sensitive to the geometry and shape of the hydrate-binding surface (HBS) of the AFP, which implies that the interface between hydrates and the AFP is relatively rigid.

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.

Molecular Dynamics Analysis of Synergistic Effects of Ions and Winter Flounder Antifreeze Protein Adjacent to Ice-Solution Surfaces

The control of freezing saline water at the micrometer level has become very important in cryosurgery and cryopreservation of stem cells and foods. Adding antifreeze protein to saline water is a promising method for controlling the freezing because the protein produces a gap between the melting point and the freezing point. Furthermore, a synergistic effect of the solutes occurs in which the freezing point depression of a mixed solution is more noticeable than the sum of two freezing point depressions of single-solute solutions. However, the mechanism of this effect has not yet been clarified. Thus, we have carried out a molecular dynamics simulation on aqueous solutions of winter flounder antifreeze protein and sodium chloride or calcium chloride with an ice layer. The results show that the cations inhibit the hydrogen bond among water molecules not only in the salt solutions but also in the mixed solutions. This inhibition depends on the local number of ions and the valence of cations. The space for water molecules to form the hydrogen bonds becomes small in the case of the mixed solution of the protein and calcium chloride. These findings are consistent with the synergistic effect. In addition, it is found that the diffusion of ions near positively-charged residues is attenuated. This attenuation causes an increase in the possibility of water molecules staying near or inside the hydration shells of the ions. Furthermore, the first hydration shells of the cations become weak in the vicinity of the arginine, lysine and glutamic-acid residues. These factors can be considered to be possible mechanisms of the synergistic effect.

Influence of Adsorption Orientation on the Statistical Mechanics Model of Type I Antifreeze Protein: The Thermal Hysteresis Temperature

The antifreeze activity of type I antifreeze proteins (AFPIs) is studied on the basis of the statistical mechanics theory, by taking the AFP's adsorption orientation into account. The thermal hysteresis temperatures are calculated by determining the system Gibbs function as well as the AFP molecule coverage rate on the ice-crystal surface. The numerical results for the thermal hysteresis temperatures of AFP9, HPLC-6, and AAAA2kE are obtained for both of the cases with and without inclusion of the adsorption orientation. The results show that the influence of the adsorption orientation on the thermal hysteresis temperature cannot be neglected. The theoretical results are coincidental preferably with the experimental data.

Balance between hydration enthalpy and entropy is important for ice binding surfaces in Antifreeze Proteins

Antifreeze Proteins (AFPs) inhibit the growth of an ice crystal by binding to it. The detailed binding mechanism is, however, still not fully understood. We investigated three AFPs using Molecular Dynamics simulations in combination with Grid Inhomogeneous Solvation Theory, exploring their hydration thermodynamics. The observed enthalpic and entropic differences between the ice-binding sites and the inactive surface reveal key properties essential for proteins in order to bind ice: While entropic contributions are similar for all sites, the enthalpic gain for all ice-binding sites is lower than for the rest of the protein surface. In contrast to most of the recently published studies, our analyses show that enthalpic interactions are as important as an ice-like pre-ordering. Based on these observations, we propose a new, thermodynamically more refined mechanism of the ice recognition process showing that the appropriate balance between entropy and enthalpy facilitates ice-binding of proteins. Especially, high enthalpic interactions between the protein surface and water can hinder the ice-binding activity.

Conformational and hydration properties modulate ice recognition by type I antifreeze protein and its mutants

Mutation of wfAFP changes the intrinsic dynamics in such a way that it significantly influences water mediated AFP adsorption on ice.

The use of antifreeze protein type III for vitrification of in vitro matured bovine oocytes

The aim of this study was to evaluate the use of antifreeze protein type III (AFP III) into vitrification medium on meiotic spindle morphology of in vitro matured bovine oocytes as well as the fertilization and blastocyst rates. Mature cumulus-oocyte complexes (COC) were distributed in four groups: control (untreated), vitrified without supplementation (AFP₀) or supplemented with 500 (AFP₅₀₀) or 1000 ng/mL (AFP₁₀₀₀) into vitrification solutions. Samples from each group were used to analyze the organization of meiotic spindle by confocal microscopy and the remaining COC were submitted to in vitro fertilization and culture for eight days. Control group exhibited only 15% of abnormal spindle. However, the spindle morphology was affected in all vitrified groups regardless to AFP concentration: 75.8%, 76.1% and 69.2% (P > 0.05) for AFP₀, AFP₅₀₀ and AFP₁₀₀₀, respectively. Similar cleavage rate was obtained among the vitrified groups (AFP₀ = 17.9%, AFP₅₀₀ = 16.9% and AFP₁₀₀₀ = 17.8%), but lower (P < 0.05) compared with control group (68.7%). At Day 5 of culture, embryo production rate of AFP₅₀₀ (30.8%) and AFP₁₀₀₀ (25.0%) were similar to control group (49.4%). However, at Day 8 of culture, AFP_0, AFP₅₀₀ and AFP₁₀₀₀ groups exhibited lower (P < 0.05) blastocyst rates (10.0%, 3.8% and 9.4%, respectively) when compared to control (41.1%). In conclusion, AFP III did not preserve meiotic spindle organization against the cryoinjuries. However, the use of AFP III improved embryo development at Day 5 of culture, although this effect was not maintained up to the blastocyst formation.

Interaction of ice binding proteins with ice, water and ions

Ice binding proteins (IBPs) are produced by various cold-adapted organisms to protect their body tissues against freeze damage. First discovered in Antarctic fish living in shallow waters, IBPs were later found in insects, microorganisms, and plants. Despite great structural diversity, all IBPs adhere to growing ice crystals, which is essential for their extensive repertoire of biological functions. Some IBPs maintain liquid inclusions within ice or inhibit recrystallization of ice, while other types suppress freezing by blocking further ice growth. In contrast, ice nucleating proteins stimulate ice nucleation just below 0 °C. Despite huge commercial interest and major scientific breakthroughs, the precise working mechanism of IBPs has not yet been unraveled. In this review, the authors outline the state-of-the-art in experimental and theoretical IBP research and discuss future scientific challenges. The interaction of IBPs with ice, water and ions is examined, focusing in particular on ice growth inhibition mechanisms.

Blocking rapid ice crystal growth through nonbasal plane adsorption of antifreeze proteins

Antifreeze proteins (AFPs) are a unique class of proteins that bind to growing ice crystal surfaces and arrest further ice growth. AFPs have gained a large interest for their use in antifreeze formulations for water-based materials, such as foods, waterborne paints, and organ transplants. Instead of commonly used colligative antifreezes such as salts and alcohols, the advantage of using AFPs as an additive is that they do not alter the physicochemical properties of the water-based material. Here, we report the first comprehensive evaluation of thermal hysteresis (TH) and ice recrystallization inhibition (IRI) activity of all major classes of AFPs using cryoscopy, sonocrystallization, and recrystallization assays. The results show that TH activities determined by cryoscopy and sonocrystallization differ markedly, and that TH and IRI activities are not correlated. The absence of a distinct correlation in antifreeze activity points to a mechanistic difference in ice growth inhibition by the different classes of AFPs: blocking fast ice growth requires rapid nonbasal plane adsorption, whereas basal plane adsorption is only relevant at long annealing times and at small undercooling. These findings clearly demonstrate that biomimetic analogs of antifreeze (glyco)proteins should be tailored to the specific requirements of the targeted application.

Physiological adaptations of yeasts living in cold environments and their potential applications

Yeasts, widely distributed across the Earth, have successfully colonized cold environments despite their adverse conditions for life. Lower eukaryotes play important ecological roles, contributing to nutrient recycling and organic matter mineralization. Yeasts have developed physiological adaptations to optimize their metabolism in low-temperature environments, which affect the rates of biochemical reactions and membrane fluidity. Decreased saturation of fatty acids helps maintain membrane fluidity at low temperatures and the production of compounds that inhibit ice crystallization, such as antifreeze proteins, helps microorganisms survive at temperatures around the freezing point of water. Furthermore, the production of hydrolytic extracellular enzymes active at low temperatures allows consumption of available carbon sources. Beyond their ecological importance, interest in psychrophilic yeasts has increased because of their biotechnological potential and industrial uses. Long-chain polyunsaturated fatty acids have beneficial effects on human health, and antifreeze proteins are attractive for food industries to maintain texture in food preserved at low temperatures. Furthermore, extracellular cold-active enzymes display unusual substrate specificities with higher catalytic efficiency at low temperatures than their mesophilic counterparts, making them attractive for industrial processes requiring high enzymatic activity at low temperatures. In this minireview, we describe the physiological adaptations of several psychrophilic yeasts and their possible biotechnological applications.