Reversible binding of the HPLC6 isoform of type I antifreeze proteins to ice surfaces and the antifreeze mechanism studied by multiple quantum filtering-spin exchange NMR experiment

Cryoprotective Polysaccharides with Ordered Gel Structures Induce Ice Growth Anticipation and Survival Enhancement during Cell Cryopreservation

This work cross-correlated rheological, thermodynamic, and conformational features of several natural polysaccharides to their cryoprotective performance. The basis of cryoprotection of FucoPol, pectin, and agar revealed a causal combination of (i) an emerging sol-gel transition (p = 0.014) at near-hypothermia (4 °C), (ii) noncolligative attenuated supercooling of the kinetic freezing point of water (p = 0.026) supporting ice growth anticipation, and (iii) increased conformational order (p < 0.0001), where helix-/sheet-like features boost cryoprotection. FucoPol, of highest cryoprotective performance, revealed a predominantly helical structure (α/β = 1.5) capable of forming a gel state at 4 °C and the highest degree of supercooling attenuation (TH = 6.2 °C). Ice growth anticipation with gel-like polysaccharides suggests that the gel matrix neutralizes elastic deformations and lethal cell volumetric fluctuations during freezing, thus preventing the loss of homeostasis and increasing post-thaw viability. Ultimately, structured gels capable of attenuated supercooling enable cryoprotective action at the polymer-cell interface, in addition to polymer-ice interactions. This rationale potentiates implementing alternative, biobased, noncytotoxic polymers in cryobiology.

Ice Recrystallization Unveils the Binding Mechanism Operating at a Diffused Interface

Recrystallization of ice is a natural phenomenon that causes adverse effects in cryopreservation, agriculture, and in frozen food industry. It has long been recognized that ice recrystallization occurs through the Ostwald ripening and accretion processes. However, neither of these processes has been explored in microscopic detail by state-of-the-art experimental techniques. We carried out atomistic molecular dynamics (MD) simulations to explore ice recrystallization through the accretion process. Attempts have been made to elucidate the binding mechanism that is operating at the diffused ice-water interface. It is demonstrated that two ice crystals spontaneously recognize each other and bind together to form a large crystal in liquid water, resulting in ice recrystallization by accretion. Interestingly, the study reveals that the binding occurs due to the freezing of the interfacial water layer present between the two ice planes, even at a temperature above the melting point of the ice crystal. The synergistically enhanced ordering effect of two ice surfaces on the interfacial water leads to such freezing occurring during the binding process. However, proper crystallographic alignment is not necessarily required for the binding of the two crystals. Simulations have also been carried out to study the binding between an ice crystal and the model ice-binding surface (IBS) of an antifreeze protein above the melting point of the ice crystal. It is found that such binding at the IBS is accompanied by freezing of the interfacial water. This establishes that the synergetic ordering-driven freezing of interfacial water is a common binding mechanism at the diffused surfaces of ice crystals. We believe that this mechanism will provide a microscopic understanding of the process of recrystallization inhibition and thus help in designing suitable materials for potent applications in recrystallization inhibition.

Antifreeze Proteins: Novel Applications and Navigation towards Their Clinical Application in Cryobanking

Antifreeze proteins (AFPs) or thermal hysteresis (TH) proteins are biomolecular gifts of nature to sustain life in extremely cold environments. This family of peptides, glycopeptides and proteins produced by diverse organisms including bacteria, yeast, insects and fish act by non-colligatively depressing the freezing temperature of the water below its melting point in a process termed thermal hysteresis which is then responsible for ice crystal equilibrium and inhibition of ice recrystallisation; the major cause of cell dehydration, membrane rupture and subsequent cryodamage. Scientists on the other hand have been exploring various substances as cryoprotectants. Some of the cryoprotectants in use include trehalose, dimethyl sulfoxide (DMSO), ethylene glycol (EG), sucrose, propylene glycol (PG) and glycerol but their extensive application is limited mostly by toxicity, thus fueling the quest for better cryoprotectants. Hence, extracting or synthesizing antifreeze protein and testing their cryoprotective activity has become a popular topic among researchers. Research concerning AFPs encompasses lots of effort ranging from understanding their sources and mechanism of action, extraction and purification/synthesis to structural elucidation with the aim of achieving better outcomes in cryopreservation. This review explores the potential clinical application of AFPs in the cryopreservation of different cells, tissues and organs. Here, we discuss novel approaches, identify research gaps and propose future research directions in the application of AFPs based on recent studies with the aim of achieving successful clinical and commercial use of AFPs in the future.

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.

Operation of Kelvin Effect in the Activities of an Antifreeze Protein: A Molecular Dynamics Simulation Study

Ice growth and melting inhibition activities of antifreeze proteins (AFPs) are better explained by the adsorption-inhibition mechanism. Inhibition occurs as a result of the Kelvin effect induced by adsorbed protein molecules onto the surface of seed ice crystal. However, the Kelvin effect has not been explored by the state-of-the-art experimental techniques. In this work, atomistic molecular dynamics simulations have been carried out with Tenebrio molitor antifreeze protein ( TmAFP) placed at ice-water interface to probe the Kelvin effect in the mechanism of AFPs. Simulations show that, below equilibrium melting temperature, ice growth is inhibited through the convex ice-water interface formation toward the water phase and, above equilibrium melting temperature, ice melting is inhibited through the concave ice-water interface formation inward to ice phase. Simulations further reveal that the radius of curvature of the interface formed to stop the ice growth increases with decrease in the degree of supercooling. Our results are in qualitative agreement with the theoretical prediction of the Kelvin effect and thus reveal its operation in the activities of AFPs.

Mechanisms of antifreeze proteins investigated via the site-directed spin labeling technique

The site-directed spin labeling (SDSL) technique was used to examine the antifreeze mechanisms of type-I antifreeze proteins (AFPs). The effects on the growth of seed ice crystals by the spin-label groups attached to different side chains of the AFPs were observed, and the states of water molecules surrounding the spin-label groups were probed via analyses of variable-temperature (VT) dependent electron paramagnetic resonance (EPR) spectra. The first set of experiments revealed the antifreeze activities of the spin-labeled AFPs at the microscopic level, while the second set of experiments displayed those at the molecular level. The experimental results confirmed the putative ice-binding surface (IBS) of type-I AFPs. The VT EPR spectra indicate that type-I AFPs can inhibit the nucleation of seed ice crystals down to ~ - 20 °C in their aqueous solutions. Thus, the present authors believe that AFPs protect organisms from freezing damage in two ways: (1) inhibiting the nucleation of seed ice crystals, and (2) hindering the growth of seed ice crystals once they have formed. The first mechanism should play a more significant role in protecting against freezing damage among organisms living in cold environments. The VT EPR spectra also revealed that liquid-like water molecules existed around the spin-labeled non-ice-binding side chains of the AFPs frozen within the ice matrices, and ice surrounding the spin-label groups melted at subzero temperatures during the heating process. This manuscript concludes with the proposed model of antifreeze mechanisms of AFPs based on the experimental results.

From ice-binding proteins to bio-inspired antifreeze materials

Ice-binding proteins (IBP) facilitate survival under extreme conditions in diverse life forms. IBPs in polar fishes block further growth of internalized environmental ice and inhibit ice recrystallization of accumulated internal crystals. Algae use IBPs to structure ice, while ice adhesion is critical for the Antarctic bacterium Marinomonas primoryensis. Successful translation of this natural cryoprotective ability into man-made materials holds great promise but is still in its infancy. This review covers recent advances in the field of ice-binding proteins and their synthetic analogues, highlighting fundamental insights into IBP functioning as a foundation for the knowledge-based development of cheap, bio-inspired mimics through scalable production routes. Recent advances in the utilisation of IBPs and their analogues to e.g. improve cryopreservation, ice-templating strategies, gas hydrate inhibition and other technologies are presented.

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.

Induced ice melting by the snow flea antifreeze protein from molecular dynamics simulations

Antifreeze proteins (AFP) allow different life forms, insects as well as fish and plants, to survive in subzero environments. AFPs prevent freezing of the physiological fluids. We have studied, through molecular dynamics simulations, the behavior of the small isoform of the AFP found in the snow flea (sfAFP), both in water and at the ice/water interface, of four different ice planes. In water at room temperature, the structure of the sfAFP is found to be slightly unstable. The loop between two polyproline II helices has large fluctuations as well as the C-terminus. Torsional angle analyses show a decrease of the polyproline II helix area in the Ramachandran plots. The protein structure instability, in any case, should not affect its antifreeze activity. At the ice/water interface the sfAFP triggers local melting of the ice surface. Bipyramidal, secondary prism, and prism ice planes melt in the presence of AFP at temperatures below the melting point of ice. Only the basal plane is found to be stable at the same temperatures, indicating an adsorption of the sfAFP on this ice plane as confirmed by experimental evidence.

Hydration behavior at the ice-binding surface of the Tenebrio molitor antifreeze protein

Molecular dynamics (MD) simulations have been carried out at two different temperatures (300 and 220 K) to study the conformational rigidity of the hyperactive Tenebrio molitor antifreeze protein (TmAFP) in aqueous medium and the structural arrangements of water molecules hydrating its surface. It is found that irrespective of the temperature the ice-binding surface (IBS) of the protein is relatively more rigid than its nonice-binding surface (NIBS). The presence of a set of regularly arranged internally bound water molecules is found to play an important role in maintaining the flat rigid nature of the IBS. Importantly, the calculations reveal that the strategically located hydroxyl oxygens of the threonine (Thr) residues in the IBS influence the arrangements of five sets of ordered waters around it on two parallel planes that closely resemble the basal plane of ice. As a result, these waters can register well with the ice basal plane, thereby allowing the IBS to preferentially bind at the ice interface and inhibit its growth. This provides a possible molecular reason behind the ice-binding activity of TmAFP at the basal plane of ice.

Microfluidic experiments reveal that antifreeze proteins bound to ice crystals suffice to prevent their growth

Antifreeze proteins (AFPs) are a subset of ice-binding proteins that control ice crystal growth. They have potential for the cryopreservation of cells, tissues, and organs, as well as for production and storage of food and protection of crops from frost. However, the detailed mechanism of action of AFPs is still unclear. Specifically, there is controversy regarding reversibility of binding of AFPs to crystal surfaces. The experimentally observed dependence of activity of AFPs on their concentration in solution appears to indicate that the binding is reversible. Here, by a series of experiments in temperature-controlled microfluidic devices, where the medium surrounding ice crystals can be exchanged, we show that the binding of hyperactive Tenebrio molitor AFP to ice crystals is practically irreversible and that surface-bound AFPs are sufficient to inhibit ice crystal growth even in solutions depleted of AFPs. These findings rule out theories of AFP activity relying on the presence of unbound protein molecules.

Effects of a type I antifreeze protein (AFP) on the melting of frozen AFP and AFP+solute aqueous solutions studied by NMR microimaging experiment

The effects of a type I AFP on the bulk melting of frozen AFP solutions and frozen AFP+solute solutions were studied through an NMR microimaging experiment. The solutes studied include sodium chloride and glucose and the amino acids alanine, threonine, arginine, and aspartic acid. We found that the AFP is able to induce the bulk melting of the frozen AFP solutions at temperatures lower than 0 °C and can also keep the ice melted at higher temperatures in the AFP+solute solutions than those in the corresponding solute solutions. The latter shows that the ice phases were in super-heated states in the frozen AFP+solute solutions. We have tried to understand the first experimental phenomenon via the recent theoretical prediction that type I AFP can induce the local melting of ice upon adsorption to ice surfaces. The latter experimental phenomenon was explained with the hypothesis that the adsorption of AFP to ice surfaces introduces a less hydrophilic water-AFP-ice interfacial region, which repels the ionic/hydrophilic solutes. Thus, this interfacial region formed an intermediate chemical potential layer between the water phase and the ice phase, which prevented the transfer of water from the ice phase to the water phase. We have also attempted to understand the significance of the observed melting phenomena to the survival of organisms that express AFPs over cold winters.

Thermodynamic Analysis of Thermal Hysteresis: Mechanistic Insights into Biological Antifreezes

Antifreeze proteins (AFPs) bind to ice crystal surfaces and thus inhibit the ice growth. The mechanism for how AFPs suppress freezing is commonly modeled as an adsorption-inhibition process by the Gibbs-Thomson effect. Here we develop an improved adsorption-inhibition model for AFP action based on the thermodynamics of impurity adsorption on the crystal surfaces. We demonstrate the derivation of a realistic relationship between surface protein coverage and the protein concentration. We show that the improved model provides a quantitatively better fit to the experimental antifreeze activities of AFPs from distinct structural classes, including fish and insect AFPs, in a wide range of concentrations. Our theoretical results yielded the adsorption coefficients of the AFPs on ice, suggesting that, despite the distinct difference in their antifreeze activities and structures, the affinities of the AFPs to ice are very close and the mechanism of AFP action is a kinetically controlled, reversible process. The applications of the model to more complex systems along with its potential limitations are also discussed.

Protein-ice interaction of an antifreeze protein observed with solid-state NMR

NMR on frozen solutions is an ideal method to study fundamental questions of macromolecular hydration, because the hydration shell of many biomolecules does not freeze together with bulk solvent. In the present study, we present previously undescribed NMR methods to study the interactions of proteins with their hydration shell and the ice lattice in frozen solution. We applied these methods to compare solvent interaction of an ice-binding type III antifreeze protein (AFP III) and ubiquitin a non-ice-binding protein in frozen solution. We measured (1)H-(1)H cross-saturation and cross-relaxation to provide evidence for a molecular contact surface between ice and AFP III at moderate freezing temperatures of -35 °C. This phenomenon is potentially unique for AFPs because ubiquitin shows no such cross relaxation or cross saturation with ice. On the other hand, we detected liquid hydration water and strong water-AFP III and water-ubiquitin cross peaks in frozen solution using relaxation filtered (2)H and HETCOR spectra with additional (1)H-(1)H mixing. These results are consistent with the idea that ubiquitin is surrounded by a hydration shell, which separates it from the bulk ice. For AFP III, the water cross peaks indicate that only a portion of its hydration shell (i.e., at the ice-binding surface) is in contact with the ice lattice. The rest of AFP III's hydration shell behaves similarly to the hydration shell of non-ice-interacting proteins such as ubiquitin and does not freeze together with the bulk water.

Fluorescence microscopy evidence for quasi-permanent attachment of antifreeze proteins to ice surfaces

Many organisms are protected from freezing by the presence of extracellular antifreeze proteins (AFPs), which bind to ice, modify its morphology, and prevent its further growth. These proteins have a wide range of applications including cryopreservation, frost protection, and as models in biomineralization research. However, understanding their mechanism of action remains an outstanding challenge. While the prevailing adsorption-inhibition hypothesis argues that AFPs must bind irreversibly to ice to arrest its growth, other theories suggest that there is exchange between the bound surface proteins and the free proteins in solution. By conjugating green fluorescence protein (GFP) to a fish AFP (Type III), we observed the binding of the AFP to ice. This was accomplished by monitoring the presence of GFP-AFP on the surface of ice crystals several microns in diameter using fluorescence microscopy. The lack of recovery of fluorescence after photobleaching of the GFP component of the surface-bound GFP-AFP shows that there is no equilibrium surface-solution exchange of GFP-AFP and thus supports the adsorption-inhibition mechanism for this type of AFP. Moreover, our study establishes the utility of fluorescently labeled AFPs as a research tool for investigating the mechanisms underlying the activity of this diverse group of proteins.

Ice-surface adsorption enhanced colligative effect of antifreeze proteins in ice growth inhibition

This Communication describes a mechanism to explain antifreeze protein's function to inhibit the growth of ice crystals. We propose that the adsorption of antifreeze protein (AFP) molecules on an ice surface induces a dense AFP-water layer, which can significantly decrease the mole fraction of the interfacial water and, thus, lower the temperature for a seed ice crystal to grow in a super-cooled AFP solution. This mechanism can also explain the nearly unchanged melting point for the ice crystal due to the AFP's ice-surface adsorption. A mathematical model combining the Langmuir theory of adsorption and the colligative effect of thermodynamics has been proposed to find the equilibrium constants of the ice-surface adsorptions, and the interfacial concentrations of AFPs through fitting the theoretical curves to the experimental thermal hysteresis data. This model has been demonstrated by using the experimental data of serial size-mutated beetle Tenebrio molitor (Tm) AFPs. It was found that the AFP's ice-surface adsorptions could increase the interfacial AFP's concentrations by 3 to 4 orders compared with those in the bulk AFP solutions.

Insight into the binding of antifreeze proteins to ice surfaces via 13C spin lattice relaxation solid-state NMR

The primary sequences of type I antifreeze proteins (AFPs) are Ala rich and contain three 11-residue repeat units beginning with threonine residues. Their secondary structures consist of alpha-helices. Previous activity study of side-chain mutated AFPs suggests that the ice-binding side of type I AFPs comprises the Thr side chains and the conserved i + 4 and i + 8 Ala residues, where i indicates the positions of the Thrs. To find structural evidence for the AFP's ice-binding side, a variable-temperature dependent (13)C spin lattice relaxation solid-state NMR experiment was carried out for two Ala side chain (13)C labeled HPLC6 isoforms of the type I AFPs each frozen in H(2)O and D(2)O, respectively. The first one was labeled on the equivalent 17th and 21st Ala side chains (i + 4, 8), and the second one on the equivalent 8th, 19th, and 30th Ala side chains (i + 6). The two kinds of labels are on the opposite sides of the alpha-helical AFP. A model of Ala methyl group rotation/three-site rotational jump combined with water molecular reorientation was tested to probe the interactions of the methyl groups with the proximate water molecules. Analysis of the T(1) data shows that there could be 10 water molecules closely capping an i + 4 or an i + 8 methyl group within the range of van der Waals interaction, whereas the surrounding water molecules to the i + 6 methyl groups could be looser. This study suggests that the side of the alpha-helical AFP comprising the i + 4 and i + 8 Ala methyl groups could interact with the ice surface in the ice/water interface.

Effect of a mutation on the structure and dynamics of an α-helical antifreeze protein in water and ice

One strategy of psychrophilic organisms to survive subzero temperature is to produce antifreeze protein (AFPs), which inhibit the growth of macromolecular ice. To better understand the binding mechanism, the structure and dynamics of several AFPs have been studied by nuclear magnetic resonance (NMR) and X-ray crystallography. The results have shown that different organisms can use diverse structures (alpha-helix, beta-helix, or different globular folds) to achieve the same function. A number of studies have focused on understanding the relationship between the alpha-helical structure of fish type I AFP and its function as an inhibitor of ice growth. The results have not explained whether the 90% activity loss caused by the conservative mutation of two threonines to serines (Thr13Ser/Thr24Ser) is attributable to a change in protein structure in solution or in ice. We examine here the structure and dynamics of the winter flounder type I AFP and the mutant Thr13Ser/Thr24Ser in both solution and solid states using a wide range of NMR approaches. Both proteins remain fully alpha-helical at all temperatures and in ice, demonstrating that the activity change must therefore not be attributable to changes in the protein fold or dynamics but differences in surface properties.