Concentration-dependent oligomerization of an alpha-helical antifreeze polypeptide makes it hyperactive

Expanding the cryoprotectant toolbox in biomedicine by multifunctional antifreeze peptides

The global cryopreservation market size rises exponentially due to increased demand for cell therapy-based products, assisted reproductive technology, and organ transplantation. Cryoprotectants (CPAs) are required to reduce ice-related damage, osmotic cell injury, and protein denaturation. Antioxidants are needed to hamper membrane lipid peroxidation under freezing stress, and antibiotics are added to the cryo-solutions to prevent contamination. The vitrification process for sized organs requires a high concentration of CPA, which is hardly achievable using conventional penetrating toxic CPAs like DMSO. Antifreeze peptides (AFpeps) are biocompatible CPAs leveraging inspiration from nature, such as freeze-tolerant and freeze-avoidant organisms, to circumvent logistic limitations in cryogenic conditions. This study aims to introduce the advances of AFpeps with cell-penetrating, antioxidant, and antimicrobial characteristics. We herein revisit the placement of AFpeps in the biobanking of cancer cells, immune cells, stem cells, blood cells, germ cells (sperms and oocytes), and probiotics. Implementing low-immunogenic AFpeps for allograft cryopreservation minimizes HLA mismatching risk after organ transplantation. Applying AFpeps to formulate bioinks with optimal rheology in extrusion-based 3D cryobiopriners expedites the bench-to-beside transition of bioprinted scaffolds. This study advocates that the fine-tuned synthetic or insect-derived AFpeps, forming round blunt-shape crystals, are biomedically broad-spectrum, and cell-permeable AFpeps from marine and plant sources, which result in sharp ice crystals, are appropriate for cryosurgery. Perspectives of the available room for developing peptide mimetics in favor of higher activity and stability and peptide-functionalized nanoparticles for enhanced delivery are delineated. Finally, antitumor immune activation by cryoimmunotherapy as an autologous in-vivo tumor lysate vaccine has been illustrated.

BERT-DomainAFP: Antifreeze protein recognition and classification model based on BERT and structural domain annotation

Antifreeze proteins (AFPs) are crucial for organisms to adapt to low temperatures, with applications in medicine, food storage, aquaculture, and agriculture. Accurate AFP identification is challenging due to structural and sequence diversity. To improve prediction and classification, we propose BERT-DomainAFP, a deep learning model trained on the AntiFreezeDomains dataset created with a novel annotation strategy. The model uses pre-trained ProteinBERT and incorporates oversampling and undersampling techniques to handle unbalanced data, ensuring high predictive ability. BERT-DomainAFP achieves 98.48% accuracy, the highest among existing models, and can classify different AFP types based on structural domain features. This model outperforms current tools, offering a promising solution for AFP recognition and classification in research and applications.

Convergent evolution of type I antifreeze proteins from four different progenitors in response to global cooling

Alanine-rich, alpha-helical type I antifreeze proteins (AFPs) in fishes are thought to have arisen independently in the last 30 Ma on at least four occasions. This hypothesis has recently been proven for flounder and sculpin AFPs, which both originated by gene duplication and divergence followed by substantial gene copy number expansion. Here, we examined the origins of the cunner (wrasse) and snailfish (liparid) AFPs. The cunner AFP has arisen by a similar route from the duplication and divergence of a GIMAP gene. The coding region for this AFP stems from an alanine-rich region flanking the GTPase domain of GIMAPa. The AFP gene has remained in the GIMAP gene locus and has undergone amplification there along with the GIMAPa gene. The AFP gene originated after the cunner diverged from its common ancestor with the closely related spotty and ballan wrasses, which exhibit similar gene synteny but lack AFP genes. Snailfish AFPs have also recently evolved because they are confined to a single genus of this family. In these AFP-producing species, the AFP locus does not share any similarity to functional genes. Instead, it is replete with repetitive DNAs and transposons, several stretches of which could encode alanine tracts with a dominant codon (GCC) that matches the bias observed in the AFP genes. All four known instances of type I AFPs occurring in fishes are independent evolutionary events that occurred soon after the onset of Northern Hemisphere Cenozoic glaciation events. Collectively, these results provide a remarkable example of convergent evolution to one AFP type.

Design and Mechanistic Insights into α-Helical p-Terphenyl Guanidines as Potent Small-Molecule Antifreeze Agents

Ice formation is a critical challenge across multiple fields, from industrial applications to biological preservation. Inspired by natural antifreeze proteins, we designed and synthesized a new class of small-molecule antifreezes based on α-helical p-terphenyl scaffolds with guanidine side chains. These p-terphenyl guanidines 1, among the smallest molecules that mimic α-helical structures, exhibit potent ice recrystallization inhibition (IRI) activity, similar to that of existing large α-helical antifreeze compounds. The most effective compound, 1a, with four C1-carbon guanidine moieties, demonstrated a superior IRI activity of 0.46 (1 mg/mL). Using molecular dynamics simulations with density-functional theory and separate pKa calculations, we elucidated the mechanisms underlying their antifreeze properties.

Paradoxical effects on ice nucleation are intrinsic to a small winter flounder antifreeze protein

Antifreeze proteins (AFPs) bind to ice in solutions, resulting in non-colligative freezing point depression; however, their effects on ice nucleation are not well understood. The predominant plasma AFP of winter flounder (Pseudopleuronectes americanus) is AFP6, which is an amphiphilic alpha helix. In this study, AFP6 and modified constructs were produced as fusion proteins in Escherichia coli, subjected to proteolysis when required and purified prior to use. AFP6 and its recombinant fusion precursor generated similar thermal hysteresis and bipyramidal ice crystals, whereas an inactive mutant AFP6 produced hexagonal crystals and no hysteresis. Circular dichroism spectra of the wild-type and mutant AFP6 were consistent with an alpha helix. The effects of these proteins on ice nucleation were investigated alongside non-AFP proteins using a standard droplet freezing assay. In the presence of nucleating AgI, modest reductions in the nucleation temperature occurred with the addition of mutant AFP6, and several non-AFPs, suggesting non-specific inhibition of AgI-induced ice nucleation. In these experiments, both AFP6 and its recombinant precursor resulted in lower nucleation temperatures, consistent with an additional inhibitory effect. Conversely, in the absence of AgI, AFP6 induced ice nucleation, with no other proteins showing this effect. Nucleation by AFP6 was dose-dependent, reaching a maximum at 1.5 mM protein. Nucleation by AFP6 also required an ice-binding site, as the inactive mutant had no effect. Furthermore, the absence of nucleation by the recombinant precursor protein suggested that the fusion moiety was interfering with the formation of a surface capable of nucleating ice.

Evaluation of Ice Recrystallization Inhibition of Ice-Binding Proteins by Monitoring Specific Ice Crystals

Ice recrystallization is a phenomenon in which large ice crystals are formed at the expense of smaller ones. The resultant large ice crystals degrade the quality of frozen foods and cryopreserved biomaterials. To minimize freeze damage by controlling the ice recrystallization process, various compounds have been developed, including biological antifreezes, synthetic peptides, glycopeptides, polymers, and small molecules. To compare their efficiency, evaluation methods of ice recrystallization inhibition are important. This chapter describes a practical protocol to quantify the inhibition efficiency by observing specific ice crystals exhibiting uniform growth.

Genetic and Structural Diversity of Prokaryotic Ice-Binding Proteins from the Central Arctic Ocean

Ice-binding proteins (IBPs) are a group of ecologically and biotechnologically relevant enzymes produced by psychrophilic organisms. Although putative IBPs containing the domain of unknown function (DUF) 3494 have been identified in many taxa of polar microbes, our knowledge of their genetic and structural diversity in natural microbial communities is limited. Here, we used samples from sea ice and sea water collected in the central Arctic Ocean as part of the MOSAiC expedition for metagenome sequencing and the subsequent analyses of metagenome-assembled genomes (MAGs). By linking structurally diverse IBPs to particular environments and potential functions, we reveal that IBP sequences are enriched in interior ice, have diverse genomic contexts and cluster taxonomically. Their diverse protein structures may be a consequence of domain shuffling, leading to variable combinations of protein domains in IBPs and probably reflecting the functional versatility required to thrive in the extreme and variable environment of the central Arctic Ocean.

Adsorption of ice-binding proteins onto whole ice crystal surfaces does not necessarily confer a high thermal hysteresis activity

Many psychrophilic microorganisms synthesize ice-binding proteins (IBPs) to survive the cold. The functions of IBPs are evaluated by the effect of the proteins on the nonequilibrium water freezing-point depression, which is called “thermal hysteresis (TH)”, and the inhibitory effect of the proteins on the growth of larger ice crystals, which is called “ice recrystallization inhibition (IRI)”. To obtain mechanical insight into the two activities, we developed a modified method of ice affinity purification and extracted two new IBP isoforms from Psychromyces glacialis, an Arctic glacier fungus. One isoform was found to be an approximately 25 kDa protein (PsgIBP_S), while the other is a 28 kDa larger protein (PsgIBP_L) that forms an intermolecular dimer. Their TH activities were less than 1 °C at millimolar concentrations, implying that both isoforms are moderately active but not hyperactive IBP species. It further appeared that both isoforms exhibit high IRI activity even at submicromolar concentrations. Furthermore, the isoforms can bind to the whole surface of a hemispherical single ice crystal, although such ice-binding was generally observed for hyperactive IBP species. These results suggest that the binding ability of IBPs to whole ice crystal surfaces is deficient for hyperactivity but is crucial for significant IRI activity.

Synthesis of poly(vinyl alcohol) by blue light bismuth oxide photocatalysed RAFT. Evaluation of the impact of freeze/thaw cycling on ice recrystallisation inhibition

Poly(vinyl alcohol), PVA, is the most potent polymeric ice recrystallisation inhibitor (IRI), mimicking a complex function of ice binding proteins. The IRI activity of PVA scales with its molecular weight and hence broad molecular weight distributions in free radical-derived PVAs lead to activity measurements dominated by small amounts of heavier fractions. Well-defined PVA can be prepared by thermally initiated RAFT/MADIX polymerization using xanthates by the polymerization of the less activated monomer vinyl acetate. The low conversions and molecular weights obtained during this approach, often requires feeding of additional initiator and bulk polymerization. Here we employ bismuth oxide photo-RAFT in solution, using blue light (450 nm), rather than previously reported white light, to obtain a library of PVA's. The use of blue light enabled quantitative conversion and acceptable dispersities. Purple light (380 nm) was also used, but asymmetric molecular weight distributions were obtained in some cases. High concentrations of high molecular weight PVA is known to form cryogels during freeze/thaw which has led to speculation this might limit the use of PVA in environments where the temperature cycles e.g. the construction industry. After 4 freeze/thaw cycles there was only small changes in observable IRI for all synthesised PVAs and two commercial standards. In an extended test, activity was retained after 100 freeze/thaw cycles, mitigating concerns that PVA could not be used in situations where freeze/thaw cycles occur. This work presents a convenient method to obtain well-defined PVAs for cryoscience studies compared to conventional thermal-RAFT and indicates that cryogelation concerns may not prevent their use.

Identification of antifreeze peptides in shrimp byproducts autolysate using peptidomics and bioinformatics

In the present study, a novel method based on peptidomics and bioinformatic was applied to identification and characterization of antifreeze peptides (AFPs) from shrimp byproducts autolysate (SBPA). According to the results of in silico prediction and high peptide structural inflexibility, DEYEESGPGIVH and EQICINFCNEK were picked as potential AFP-1 and AFP-2, respectively. The outcomes of DSC determination indicated that TH of synthesized AFP-1 and AFP-2 (10 mg/mL) were 1.37 °C and 1.57 °C, respectively. Besides, 0.1 %-3 % AFPs showed significant cryoprotection in shrimp muscle after 3 and 6 freeze-thaw cycles, evidenced by higher SSP content, Ca²⁺-ATPase activity, sulfhydryl content and lower surface hydrophobicity than control; while the higher concentration resulted in better protection against freeze induced denaturation. Both AFP-1&2 showed favorable hydrogen bonding affinity which facilitated ice binding and ice crystal growth inhibition. This work could provide new ideals for identification and characterization of AFPs.

Origin of an antifreeze protein gene in response to Cenozoic climate change

Antifreeze proteins (AFPs) inhibit ice growth within fish and protect them from freezing in icy seawater. Alanine-rich, alpha-helical AFPs (type I) have independently (convergently) evolved in four branches of fishes, one of which is a subsection of the righteye flounders. The origin of this gene family has been elucidated by sequencing two loci from a starry flounder, Platichthys stellatus, collected off Vancouver Island, British Columbia. The first locus had two alleles that demonstrated the plasticity of the AFP gene family, one encoding 33 AFPs and the other allele only four. In the closely related Pacific halibut, this locus encodes multiple Gig2 (antiviral) proteins, but in the starry flounder, the Gig2 genes were found at a second locus due to a lineage-specific duplication event. An ancestral Gig2 gave rise to a 3-kDa "skin" AFP isoform, encoding three Ala-rich 11-a.a. repeats, that is expressed in skin and other peripheral tissues. Subsequent gene duplications, followed by internal duplications of the 11 a.a. repeat and the gain of a signal sequence, gave rise to circulating AFP isoforms. One of these, the "hyperactive" 32-kDa Maxi likely underwent a contraction to a shorter 3.3-kDa "liver" isoform. Present day starry flounders found in Pacific Rim coastal waters from California to Alaska show a positive correlation between latitude and AFP gene dosage, with the shorter allele being more prevalent at lower latitudes. This study conclusively demonstrates that the flounder AFP arose from the Gig2 gene, so it is evolutionarily unrelated to the three other classes of type I AFPs from non-flounders. Additionally, this gene arose and underwent amplification coincident with the onset of ocean cooling during the Cenozoic ice ages.

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.

Subzero Nonfreezing Hypothermia with Insect Antifreeze Protein Dramatically Improves Survival Rate of Mammalian Cells

Cells for therapeutic use are often preserved at +4 °C, and the storage period is generally limited to 2–3 days. Here, we report that the survival rate (%) of mammalian cells is improved to 10–20 days when they are preserved with a subzero supercooled solution containing the antifreeze protein (AFP), for which an ability to stabilize both supercooled water and cell membrane integrity has been postulated. We chose adherent rat insulinoma (RIN-5F) cells as the preservation target, which were immersed into −5 °C-, −2 °C-, or +4 °C-chilled “unfrozen” solution of Euro-Collins or University of Washington (UW) containing the AFP sample obtained from insect or fish. Our results show that the survival rate of the cells preserved with the solution containing insect AFP was always higher than that of the fish AFP solution. A combination of the −5 °C-supercooling and insect AFP gave the best preservation result, namely, UW solution containing insect AFP kept 53% of the cells alive, even after 20 days of preservation at −5 °C. The insect AFP locates highly organized ice-like waters on its molecular surface. Such waters may bind to semiclathrate waters constructing both embryonic ice crystals and a membrane–water interface in the supercooled solution, thereby protecting the cells from damage due to chilling.

The role of trifunctional cryoprotectants in the frozen storage of aquatic foods: Recent developments and future recommendations

Freeze-induced changes including protein denaturation, ice crystals formation and lipid oxidation are mainly responsible for the quality deterioration persistent in aquatic foods. Here, for the first time, the cryoprotectants with trifunctional properties have been suggested for aquatic food cryopreservation and have exhibited exceptional cryoprotective abilities. In this study, in-depth discussion of protein denaturation, ice crystal formation and lipid oxidation is added in order to understand their mechanism, emphasizing on the necessity and use of trifunctional cryoprotectants in aquatic foods during frozen storage. Trifunctional cryoprotectants have strong abilities to prevent the formation of malondihaldehyde and aldehydes resulting from lipid oxidation, which further interact with proteins, subsequently lead to protein denaturation. Besides these all cryoprotective properties, ice crystal binding abilities distinguish trifunctional cryoprotectants from conventional cryoprotectants. Moreover, this study added with recent advances in cryoprotectants including antifreeze proteins and protein hydrolysates with their role in retarded freeze-induced changes. This study concluded that trifunctional cryoprotectants are effective owing to their hydrophilic amino acid chains, radical scavenging, water entrapping abilities, as well as the hydroxyl groups, which interact at the functional sites of protein molecules. Furthermore, polysaccharides and protein hydrolysates are the potential ingredients with trifunctional cryoproperties. However, more scientific research is required for material optimization to attain the desired level of cryoprotection.

Characterization of microbial antifreeze protein with intermediate activity suggests that a bound-water network is essential for hyperactivity

Antifreeze proteins (AFPs) inhibit ice growth by adsorbing onto specific ice planes. Microbial AFPs show diverse antifreeze activity and ice plane specificity, while sharing a common molecular scaffold. To probe the molecular mechanisms responsible for AFP activity, we here characterized the antifreeze activity and crystal structure of TisAFP7 from the snow mold fungus Typhula ishikariensis. TisAFP7 exhibited intermediate activity, with the ability to bind the basal plane, compared with a hyperactive isoform TisAFP8 and a moderately active isoform TisAFP6. Analysis of the TisAFP7 crystal structure revealed a bound-water network arranged in a zigzag pattern on the surface of the protein's ice-binding site (IBS). While the three AFP isoforms shared the water network pattern, the network on TisAFP7 IBS was not extensive, which was likely related to its intermediate activity. Analysis of the TisAFP7 crystal structure also revealed the presence of additional water molecules that form a ring-like network surrounding the hydrophobic side chain of a crucial IBS phenylalanine, which might be responsible for the increased adsorption of AFP molecule onto the basal plane. Based on these observations, we propose that the extended water network and hydrophobic hydration at IBS together determine the TisAFP activity.

Antifreeze Proteins and Their Practical Utilization in Industry, Medicine, and Agriculture

Antifreeze proteins (AFPs) are specific proteins, glycopeptides, and peptides made by different organisms to allow cells to survive in sub-zero conditions. AFPs function by reducing the water’s freezing point and avoiding ice crystals’ growth in the frozen stage. Their capability in modifying ice growth leads to the stabilization of ice crystals within a given temperature range and the inhibition of ice recrystallization that decreases the drip loss during thawing. This review presents the potential applications of AFPs from different sources and types. AFPs can be found in diverse sources such as fish, yeast, plants, bacteria, and insects. Various sources reveal different α-helices and β-sheets structures. Recently, analysis of AFPs has been conducted through bioinformatics tools to analyze their functions within proper time. AFPs can be used widely in various aspects of application and have significant industrial functions, encompassing the enhancement of foods’ freezing and liquefying properties, protection of frost plants, enhancement of ice cream’s texture, cryosurgery, and cryopreservation of cells and tissues. In conclusion, these applications and physical properties of AFPs can be further explored to meet other industrial players. Designing the peptide-based AFP can also be done to subsequently improve its function.

Ice recrystallisation inhibiting polymer nano-objects via saline-tolerant polymerisation-induced self-assembly

Chemical tools to modulate ice formation/growth have great (bio)-technological value, with ice binding/antifreeze proteins being exciting targets for biomimetic materials. Here we introduce polymer nanomaterials that are potent inhibitors of ice recrystallisation using polymerisation-induced self-assembly (PISA), employing a poly(vinyl alcohol) graft macromolecular chain transfer agent (macro-CTA). Crucially, engineering the core-forming block with diacetone acrylamide enabled PISA to be conducted in saline, whereas poly(2-hydroxypropyl methacrylate) cores led to coagulation. The most active particles inhibited ice growth as low as 0.5 mg mL-1, and were more active than the PVA stabiliser block alone, showing that the dense packing of this nanoparticle format enhanced activity. This provides a unique route towards colloids capable of modulating ice growth.

An Ice-Binding Protein from an Antarctic Ascomycete Is Fine-Tuned to Bind to Specific Water Molecules Located in the Ice Prism Planes

Many microbes that survive in cold environments are known to secrete ice-binding proteins (IBPs). The structure–function relationship of these proteins remains unclear. A microbial IBP denoted AnpIBP was recently isolated from a cold-adapted fungus, Antarctomyces psychrotrophicus. The present study identified an orbital illumination (prism ring) on a globular single ice crystal when soaked in a solution of fluorescent AnpIBP, suggesting that AnpIBP binds to specific water molecules located in the ice prism planes. In order to examine this unique ice-binding mechanism, we carried out X-ray structural analysis and mutational experiments. It appeared that AnpIBP is made of 6-ladder β-helices with a triangular cross section that accompanies an “ice-like” water network on the ice-binding site. The network, however, does not exist in a defective mutant. AnpIBP has a row of four unique hollows on the IBS, where the distance between the hollows (14.7 Å) is complementary to the oxygen atom spacing of the prism ring. These results suggest the structure of AnpIBP is fine-tuned to merge with the ice–water interface of an ice crystal through its polygonal water network and is then bound to a specific set of water molecules constructing the prism ring to effectively halt the growth of ice.

Fish-Derived Antifreeze Proteins and Antifreeze Glycoprotein Exhibit a Different Ice-Binding Property with Increasing Concentration

The concentration of a protein is highly related to its biochemical properties, and is a key determinant for its biotechnological applications. Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) are structurally diverse macromolecules that are capable of binding to embryonic ice crystals below 0 °C, making them useful as protectants of ice-block formation. In this study, we examined the maximal solubility of native AFP I–III and AFGP with distilled water, and evaluated concentration dependence of their ice-binding property. Approximately 400 mg/mL (AFP I), 200 mg/mL (AFP II), 100 mg/mL (AFP III), and >1800 mg/mL (AFGP) of the maximal solubility were estimated, and among them AFGP’s solubility is much higher compared with that of ordinary proteins, such as serum albumin (~500 mg/mL). The samples also exhibited unexpectedly high thermal hysteresis values (2–3 °C) at 50–200 mg/mL. Furthermore, the analysis of fluorescence-based ice plane affinity showed that AFP II binds to multiple ice planes in a concentration-dependent manner, for which an oligomerization mechanism was hypothesized. The difference of concentration dependence between AFPs and AFGPs may provide a new clue to help us understand the ice-binding function of these proteins.

The properties, biotechnologies, and applications of antifreeze proteins

By natural selection, organisms evolve different solutions to cope with extremely cold weather. The emergence of an antifreeze protein gene is one of the most momentous solutions. Antifreeze proteins possess an importantly functional ability for organisms to survive in cold environments and are widely found in various cold-tolerant species. In this review, we summarize the origin of antifreeze proteins, describe the diversity of their species-specific properties and functions, and highlight the related biotechnology on the basis of both laboratory tests and bioinformatics analysis. The most recent advances in the applications of antifreeze proteins are also discussed. We expect that this systematic review will contribute to the comprehensive knowledge of antifreeze proteins to readers.

Synthesis of Anthracene Conjugates of Truncated Antifreeze Protein Sequences: Effect of the End Group and Photocontrolled Dimerization on Ice Recrystallization Inhibition Activity

Biomacromolecular antifreezes distinguish ice from water, function by binding to specific planes of ice, and could have many applications from cryobiology to aerospace where ice is a problem. In biology, antifreeze protein (AFP) activity is regulated by protein expression levels via temperature and light-regulated expression systems, but in the laboratory (or applications), the antifreeze activity is "always on" without any spatial or temporal control, and hence methods to enable this switching represent an exciting synthetic challenge. Introduction of an abiotic functionality into short peptides (e.g., from solid-phase synthesis) to enable switching is also desirable rather than on full-length recombinant proteins. Here, truncated peptide sequences based on the consensus repeat sequence from type-I AFPs (TAANAAAAAAA) were conjugated to an anthracene unit to explore their photocontrolled dimerization. Optimization of the synthesis to ensure solubility of the hydrophobic peptide included the addition of a dilysine solubilizing linker. It was shown that UV-light exposure triggered reversible dimerization of the AFP sequence, leading to an increase in molecular weight. Assessment of the ice recrystallization inhibition activity of the peptides before and after dimerization revealed only small effects on activity. However, it is reported here for the first time that addition of the anthracene unit to a 22-amino-acid truncated peptide significantly enhanced ice recrystallization inhibition compared to the free peptide, suggesting an accessible synthetic route to allow AFP activity using shorter, synthetically accessible peptides with a photoreactive functionality.

Site-specific conjugation of antifreeze proteins onto polymer-stabilized nanoparticles

Antifreeze proteins (AFPs) have many potential applications, ranging from cryobiology to aerospace, if they can be incorporated into materials. Here, a range of engineered AFP mutants were prepared and site-specifically conjugated onto RAFT polymer-stabilized gold nanoparticles to generate new hybrid multivalent ice growth inhibitors. Only the SNAP-tagged AFPs lead to potent 'antifreeze' active nanomaterials with His-Tag capture resulting in no activity, showing the mode of conjugation is essential. This versatile strategy will enable the development of multivalent AFPs for translational and fundamental studies.

Multivalent Presentation of Ice Recrystallization Inhibiting Polymers on Nanoparticles Retains Activity

Poly(vinyl alcohol) (PVA) has emerged as the most potent mimic of antifreeze (glyco)proteins ice recrystallization inhibition (IRI) activity, despite its lack of structural similarities and flexible, rather than rigid, backbone. The precise spacing of hydroxyl groups is hypothesized to enable PVA to recognize the prism planes of ice but not the basal plane, due to hydroxyl pattern matching of the ice surface giving rise to the macroscopic activity. Here, well-defined PVA derived from reversible addition-fragmentation chain-transfer (RAFT) polymerization is immobilized onto gold nanoparticles to enable the impact of nanoscale assembly and confinement on the observed IRI activity. Unlike previous reports using star-branched or bottle-brush PVAs, the nanoparticle-PVA retains all IRI activity compared to polymers in solution. Evidence is presented to show that this is due to the low grafting densities on the particle surface meaning the chains are free to explore the ice faces, rather than being constrained as in star-branched polymers. These results demonstrate a route to develop more functional IRI's and inclusion of metallic particle cores for imaging and associated applications in cryobiology.

Saturn-Shaped Ice Burst Pattern and Fast Basal Binding of an Ice-Binding Protein from an Antarctic Bacterial Consortium

Ice-binding proteins (IBPs) bind to ice crystals and control their growth, enabling host organisms to adapt to subzero temperatures. By binding to ice, IBPs can affect the shape and recrystallization of ice crystals. The shapes of ice crystals produced by IBPs vary and are partially due to which ice planes the IBPs are bound to. Previously, we have described a bacterial IBP found in the metagenome of the symbionts of Euplotes focardii ( EfcIBP). EfcIBP shows remarkable ice recrystallization inhibition activity. As recrystallization inhibition of IBPs and other materials are important to the cryopreservation of cells and tissues, we speculate that the EfcIBP can play a future role as an ice recrystallization inhibitor in cryopreservation applications. Here we show that EfcIBP results in a Saturn-shaped ice burst pattern, which may be due to the unique ice-plane affinity of the protein that we elucidated using the fluorescent-based ice-plane affinity analysis. EfcIBP binds to ice at a speed similar to that of other moderate IBPs (5 ± 2 mM^-1 s^-1); however, it is unique in that it binds to the basal and previously unobserved pyramidal near-basal planes, while other moderate IBPs typically bind to the prism and pyramidal planes and not basal or near-basal planes. These insights into EfcIBP allow a better understanding of the recrystallization inhibition for this unique protein.

Freeze Tolerance in Sculpins (Pisces; Cottoidea) Inhabiting North Pacific and Arctic Oceans: Antifreeze Activity and Gene Sequences of the Antifreeze Protein

Many marine species inhabiting icy seawater produce antifreeze proteins (AFPs) to prevent their body fluids from freezing. The sculpin species of the superfamily Cottoidea are widely found from the Arctic to southern hemisphere, some of which are known to express AFP. Here we clarified DNA sequence encoding type I AFP for 3 species of 2 families (Cottidae and Agonidae) belonging to Cottoidea. We also examined antifreeze activity for 3 families and 32 species of Cottoidea (Cottidae, Agonidae, and Rhamphocottidae). These fishes were collected in 2013–2015 from the Arctic Ocean, Alaska, Japan. We could identify 8 distinct DNA sequences exhibiting a high similarity to those reported for Myoxocephalus species, suggesting that Cottidae and Agonidae share the same DNA sequence encoding type I AFP. Among the 3 families, Rhamphocottidae that experience a warm current did not show antifreeze activity. The species inhabiting the Arctic Ocean and Northern Japan that often covered with ice floe showed high activity, while those inhabiting Alaska, Southern Japan with a warm current showed low/no activity. These results suggest that Cottoidea acquires type I AFP gene before dividing into Cottidae and Agonidae, and have adapted to each location with optimal antifreeze activity level.

Ice recrystallization is strongly inhibited when antifreeze proteins bind to multiple ice planes

Ice recrystallization is a phenomenon observed as the increase in ice crystal size within an already frozen material. Antifreeze proteins (AFPs), a class of proteins capable of arresting ice crystal growth, are known to inhibit this phenomenon even at sub milli-molar concentrations. A tremendous range in the possible applications of AFPs is hence expected in both medical and industrial fields, while a key determinant of the ice recrystallization inhibition (IRI) is hardly understood. Here, IRI efficiency and ice plane affinity were examined for the wild-type AFPI–III, a defective AFPIII isoform, and a fungal AFP isoform. To simplify the IRI analysis using the formal representation of Ostwald-ripening (r³ = r₀³ + kt), we monitored specific ice grains exhibiting only uniform growth, for which maximum Feret diameter was measured. The cube of an ice grain’s radius (r³) increased proportionately with time (t), and its slope gave the recrystallization rate (k). There was a significant difference in the IRI efficiency between the samples, and the fungal AFP possessing the activity with the smallest amount (0.27 μM) exhibited an affinity to multiple ice planes. These results suggest that the IRI efficiency is maximized when AFPs bind to a whole set of ice planes.

Kinetic inhibition effect of Type I and III antifreeze proteins on unidirectional tetrahydrofuran hydrate crystal growth

Experiments were performed to evaluate the kinetic inhibition effect of Type I and Type III antifreeze proteins (AFPs) on the growth of tetrahydrofuran (THF) clathrate hydrate crystals using the unidirectional growth technique.

Unexpected Rise of Glass Transition Temperature of Ice Crystallized from Antifreeze Protein Solution

Antifreeze protein (AFP) is known to bind to a single ice crystal composed of hexagonally arranged waters, hexagonal ice. To investigate the effect of the AFP binding to a general ice block that is an assembly of numerous hexagonal ice crystals, thermodynamic properties, dynamics, and the crystal structure of the ice block were examined in the presence of type I AFP (AFP-I). Previously, it was found that hexagonal ice has a glass transition based on the proton ordering in the ice lattice at low temperature. Measurements of heat capacity under adiabatic conditions, dielectric permittivity, and powder X-ray diffraction revealed that the glass transition occurs around 140 K in the ice containing 0.01-1% (w/w) of the AFP-I, which is greater than the value for the pure hexagonal ice (ca. 110 K). These data imply that AFP affects the glass transition kinetics, i.e., the slowness of the proton migration in the ice block. Hence, adsorption of AFP molecules to each hexagonal ice is thought to change the physicochemical properties of the bulk ice.

Applications of Antifreeze Proteins: Practical Use of the Quality Products from Japanese Fishes

Numerous embryonic ice crystals are generated in water at the moment of freezing. These crystals grow and merge together to form an ice block that can be generally observed. Antifreeze protein (AFP) is capable of binding to the embryonic ice crystals, inhibiting such an ice block formation. Fish-derived AFP additionally binds to membrane lipid bilayers to prolong the lifetime of cells. These unique abilities of AFP have been studied extensively for the development of advanced techniques, such as ice recrystallization inhibitors, freeze-tolerant gels, cell preservation fluids, and high-porosity ceramics, for which mass-preparation method of the quality product of AFP utilizing fish muscle homogenates made a significant contribution. In this chapter, we present both fundamental and advanced information of fish AFPs that have been especially discovered from mid-latitude sea area, which will provide a hint to develop more advanced techniques applicable in both medical and industrial fields.

Why do antifreeze proteins require a solenoid?

Proteins whose presence prevents water from freezing in living organisms at temperatures below 0 °C are referred to as antifreeze proteins. This group includes molecules of varying size (from 30 to over 300 aa) and variable secondary/supersecondary conformation. Some of these proteins also contain peculiar structural motifs called solenoids. We have applied the fuzzy oil drop model in the analysis of four categories of antifreeze proteins: 1 - very small proteins, i.e. helical peptides (below 40 aa); 2 - small globular proteins (40-100 aa); 3 - large globular proteins (>100 aa) and 4 - proteins containing solenoids. The FOD model suggests a mechanism by which antifreeze proteins prevent freezing. In accordance with this theory, the presence of the protein itself produces an ordering of water molecules which counteracts the formation of ice crystals. This conclusion is supported by analysis of the ordering of hydrophobic and hydrophilic residues in antifreeze proteins, revealing significant variability - from perfect adherence to the fuzzy oil drop model through structures which lack a clearly defined hydrophobic core, all the way to linear arrangement of alternating local minima and maxima propagating along the principal axis of the solenoid (much like in amyloids). The presented model - alternative with respect to the ice docking model - explains the antifreeze properties of compounds such as saccharides and fatty acids. The fuzzy oil drop model also enables differentiation between amyloids and antifreeze proteins.