Artificial antifreeze polypeptides: alpha-helical peptides with KAAK motifs have antifreeze and ice crystal morphology modifying properties

Versatility in Self-assembly and Morphology of Non-Coded Anthranilic acid and Phenylglycine based Dipeptide Stereoisomers

Beauty in the self-assembly patterns of isomeric dipeptides of Boc-Ant-L-Phg-OMe (1) bearing two rigid, unnatural amino acids (Ant: Anthranilic acid, Phg: Phenylglycine) is demonstrated. Additionally, self-assembly and morphological variation by...

Protecting Group-Directed Diversity in the Morphology of Self-Assembled Ant-Aib Dipeptides: Garland-Like Architecture and Nanovesicle Formation

The morphology and molecular organization of a set of different N-terminal protecting groups containing dipeptides were investigated. The dipeptides consisted of two rigid noncanonical amino acids, Ant and Aib (X-Ant-Aib-OMe; Ant: anthranilic acid and 2-aminobenzoic acid, Aib: 2-aminoisobutyric acid). The change of the N-terminal protecting groups (X = Boc (peptide 1), N^α-fluorenylmethoxycarbonyl (Fmoc) (peptide 2), o-NBS (peptide 3), and p-NBS (peptide 4); NBS = nitrobenzyl sulfonyl group) displayed a characteristic morphological variety. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) experiments suggested that while t-butyloxycarbonyl (Boc) and p-NBS containing peptides exhibited distinct rod-like fiber structures, Fmoc and o-NBS containing peptides displayed remarkable vesicular structures. FE-SEM and thermogravimetric analysis (TGA) suggested that peptide nanostructures demonstrated excellent thermal stability in dry conditions. Interestingly, peptides 2 and 4 exhibited a type-III N₂ gas adsorption isotherm. Fluorescence microscopy analysis revealed that nanovesicles formed by peptides 2 and 3 have drug encapsulation properties exemplified by curcumin, rhodamine B, and carboxyfluorescein. These results will help in designing peptide-based nanomaterials for diverse applications.

Peptidic Antifreeze Materials: Prospects and Challenges

Necessitated by the subzero temperatures and seasonal exposure to ice, various organisms have developed a remarkably effective means to survive the harsh climate of their natural habitats. Their ice-binding (glyco)proteins keep the nucleation and growth of ice crystals in check by recognizing and binding to specific ice crystal faces, which arrests further ice growth and inhibits ice recrystallization (IRI). Inspired by the success of this adaptive strategy, various approaches have been proposed over the past decades to engineer materials that harness these cryoprotective features. In this review we discuss the prospects and challenges associated with these advances focusing in particular on peptidic antifreeze materials both identical and akin to natural ice-binding proteins (IBPs). We address the latest advances in their design, synthesis, characterization and application in preservation of biologics and foods. Particular attention is devoted to insights in structure-activity relations culminating in the synthesis of de novo peptide analogues. These are sequences that resemble but are not identical to naturally occurring IBPs. We also draw attention to impactful developments in solid-phase peptide synthesis and 'greener' synthesis routes, which may aid to overcome one of the major bottlenecks in the translation of this technology: unavailability of large quantities of low-cost antifreeze materials with excellent IRI activity at (sub)micromolar concentrations.

Effects of hydrophobic and hydrogen-bond interactions on the binding affinity of antifreeze proteins to specific ice planes

Tenebrio molitor antifreeze protein (TmAFP) was simulated with growing ice surfaces such as primary prism, secondary prism, basal, and pyramidal planes. The ice-binding site of TmAFP, which is full of threonine (Thr), binds to the primary-prism plane but does not bind to other ice planes, in agreement with experiments showing the fast adsorption of TmAFP to the primary-prism plane. To mimic the ice-binding site of shorthorn sculpin AFP (ssAFP; type I) that predominantly consists of alanine (Ala) and has the binding affinity to the secondary-prism plane, the ice-binding site of TmAFP was mutated by replacing a few Thr residues with Ala residues, showing that mutated TmAFP binds to the secondary-prism plane, similar to the ice-binding affinity of ssAFP. Ala residues are located at the cavity of ice, while Thr residues form hydrogen bonds with water molecules. When the mutated TmAFP is further modified by removing Thr, it does not bind to the secondary-prism plane. These findings indicate that simulations can successfully capture the experimentally observed binding affinity of AFP to specific ice planes, to an extent dependent on hydrophobicity of the ice-binding site. In particular, the addition of hydrophobic residues influences the ice-binding affinity of TmAFP, while a certain amount of hydrophilic residue is still required for hydrogen-bond interactions, which supports experimental observations regarding the key roles of hydrophobic and hydrophilic interactions on the AFP-ice binding.

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.

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.

Benchmark experimental data set and assessment of adsorption free energy for peptide-surface interactions

With the increasing interest in protein adsorption in fields ranging from bionanotechnology to biomedical engineering, there is a growing need to understand protein-surface interactions at a fundamental level, such as the interaction between individual amino acid residues of a protein and functional groups presented by a surface. However, relatively little data are available that experimentally provide a quantitative, comparative measure of these types of interactions. To address this deficiency, the objective of this study was to generate a database of experimentally measured standard state adsorption free energy (DeltaGoads) values for a wide variety of amino acid residue-surface interactions using a host-guest peptide and alkanethiol self-assembled monolayers (SAMs) with polymer-like functionality as the model system. The host-guest amino acid sequence was synthesized in the form of TGTG-X-GTGT, where G and T are glycine and threonine amino acid residues and X represents a variable residue. In this paper, we report DeltaGoads values for the adsorption of 12 different types of the host-guest peptides on a set of nine different SAM surfaces, for a total of 108 peptide-surface systems. The DeltaGoads values for these 108 peptide-surface combinations show clear trends in adsorption behavior that are dependent on both peptide composition and surface chemistry. These data provide a benchmark experimental data set from which fundamental interactions that govern peptide and protein adsorption behavior can be better understood and compared.

Design and synthesis of alpha-helical peptides and mimetics

The alpha-helix is the most abundant secondary structural element in proteins and is an important structural domain for mediating protein-protein and protein-nucleic acid interactions. Strategies for the rational design and synthesis of alpha-helix mimetics have not matured as well as other secondary structure mimetics such as strands and turns. This perspective will focus on developments in the design, synthesis and applications of alpha-helices and mimetics, particularly in the last 5 years. Examples where synthetic compounds have delivered promising biological results will be highlighted as well as opportunities for the design of mimetics of the type I alpha-helical antifreeze proteins.

Diffusion NMR studies on fish antifreeze proteins and synthetic analogues

Pulsed field gradient spin echo NMR spectroscopy was used to measure diffusion coefficients of the alpha-helical type I antifreeze protein from the winter flounder, two synthetic derivatives in which the four Thr residues were replaced with Val and Ala, respectively, and the low molecular weight fraction antifreeze glycoprotein. Under the conditions studied, the natural type I antifreeze protein and low molecular weight glycoprotein gave diffusion values that were consistent with the presence of monomeric protein in solution. While significant aggregation of the Ala analogue was observed (2-10 mM), there was no evidence for aggregation in the Val analogue (1-3 mM). These results are compared with previously reported solubility and thermal hysteresis data and the implications for the design of synthetic antifreeze proteins are discussed.

Hyperactive Antifreeze Protein from Winter Flounder Is a Very Long Rod-like Dimer of α-Helices

The winter flounder (Pseudopleuronectes americanus) produces short, monomeric alpha-helical antifreeze proteins (type I AFP), which adsorb to and inhibit the growth of ice crystals. These proteins alone are not sufficiently active to protect this fish against freezing at -1.9 degrees C, the freezing point of seawater. We have recently isolated a hyperactive antifreeze protein from the plasma of the flounder with activity 10-100-fold higher than type I AFP. It is comparable in activity to the AFPs produced by insects, and is capable of conferring freeze resistance to the flounder. This novel AFP has a molecular mass of 16,683 Da and a remarkable amino acid composition that is >60% alanine. CD spectra indicate that the protein is almost entirely alpha-helical at 4 degrees C but partially denatures at 20 degrees C, resulting in a species with a moderately reduced helix content that is stable at up to 50 degrees C. This transformation correlates with irreversible loss of activity. Analytical ultracentrifugation (sedimentation velocity and equilibrium) indicates that the predominant species in solution is dimeric (molecular weight, 32,275). Size-exclusion chromatography reveals a 2-fold higher apparent molecular weight suggesting that this molecule has an unusually large Stokes radius. The axial ratio of the dimer calculated from the sedimentation velocity data is 18:1, confirming that this protein has an extraordinarily long, rod-like structure, consistent with a novel dimeric alpha-helical arrangement. The structural model that best fits these data is one in which the approximately 195 amino acids of each monomer form one approximately 290-A long alpha-helix and associate via a unique dimerization motif that is distinct from that of the leucine zipper and any other coiled-coil.

Ice Surface Reconstruction as Antifreeze Protein-Induced Morphological Modification Mechanism

The crystal growth process by which fish antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) modify the ice morphology is analyzed in the AFP-ice system. A newly identified AFP-induced surface reconstruction mechanism enables one-dimensional helical and irregular globular ice binding surfaces to stabilize secondary, kinetically less stable ice surfaces with variable face indices. Not only are the relative growth rates controlled by the IBS engagement but also the secondary face indices themselves become adjusted in the process of maximizing the AFP-substrate interaction, through attaining the best structural match. The theoretical formulation leads to comprehensive agreement with experiment.

Antifreeze Protein-induced Morphological Modification Mechanisms Linked to Ice Binding Surface

The mechanisms by which the antifreeze protein (AFP) modifies the ice morphology are identified precisely as surface poisoning by the ice binding surface (IBS) of insect AFPs and as bridge-induced surface reconstruction by the IBS of fish AFPs and antifreeze glycoproteins. The primary surfaces of hexagonal ice have predetermined face indices. The "two-dimensional" insect type IBS has regularly spaced binding intervals in two directions. It causes surface poisoning by matching and reinforcing simultaneously intersecting strong bonding directions on the primary ice surfaces. The secondary ice surfaces have variable face indices. The "one-dimensional" and "irregular" IBS variants of fish AFPs and antifreeze glycoproteins are either linearly extended with regular ice binding intervals or have ice binding sites lacking spacing regularity. These variants can bridge transversely lattice periods or shorter oxygen-oxygen distances between parallel adjacent strong bonding directions that do not intersect. Thus, one-dimensional and irregular IBS variants induce supplementary bridges cross-wise on selected secondary surfaces by mimicking strong bonding directions that are not present in the ice structure. These proteins cause surfaces with variable face indices, which in the absence of the AFPs would not grow flat, to appear in the morphology. Whereas for the primary ice surfaces it is only the morphological importance that is determined by the experimental conditions, for the secondary ice surfaces it is the face indices themselves that become adjusted in the process of maximizing the AFP-substrate interaction through attainment of the best structural match. The growth morphology of the AFP-ice system is derived from various factors, including the face indices, surface molecular compositions, relative growth rates, and the mechanisms responsible for that morphology. The theoretical formulation agrees with experiments over a wide range and resolves these, to date, unexplained phenomena.

Helix formation and the unfolded state of a 52-residue helical protein

A growing class of proteins in biological processes has been found to be unfolded on isolation under normal solution conditions. We have used NMR spectroscopy to characterize the structural and dynamic properties of the unfolded and partially folded states of a 52-residue alanine-rich protein (Ala-14) at temperatures from -5 degrees C to 40 degrees C. At 40 degrees C, alanine residues in Ala-14 adopt phi and psi angles, consistent with a significant ensemble population of polyproline II conformation. Analysis of relaxation rates in the protein reveals that a series of residues, Gln 35-Ala 36-Ala 37-Lys 38-Asp 39-Asp 40-Ala 41-Ala 42, displays slow motional dynamics at both -5 degrees C and 40 degrees C. Temperature-dependent chemical shift changes indicate that this region is the site of helix initiation. The remaining N-terminal residues become increasingly dynamic as they extend from the nucleation site. The C terminus remains dynamic and changes less with temperature, indicating it is relatively unstructured. Ala-14 provides a high-resolution portrait of the unfolded state and the process of helix nucleation and propagation in the absence of tertiary contacts, information that bears on early events in protein folding.

Plants in a cold climate

Plants are able to survive prolonged exposure to sub-zero temperatures; this ability is enhanced by pre-exposure to low, but above-zero temperatures. This process, known as cold acclimation, is briefly reviewed from the perception of cold, through transduction of the low-temperature signal to functional analysis of cold-induced gene products. The stresses that freezing of apoplastic water imposes on plant cells is considered and what is understood about the mechanisms that plants use to combat those stresses discussed, with particular emphasis on the role of the extracellular matrix.

Structure and function of antifreeze proteins

High-resolution three-dimensional structures are now available for four of seven non-homologous fish and insect antifreeze proteins (AFPs). For each of these structures, the ice-binding site of the AFP has been defined by site-directed mutagenesis, and ice etching has indicated that the ice surface is bound by the AFP. A comparison of these extremely diverse ice-binding proteins shows that they have the following attributes in common. The binding sites are relatively flat and engage a substantial proportion of the protein's surface area in ice binding. They are also somewhat hydrophobic -- more so than that portion of the protein exposed to the solvent. Surface-surface complementarity appears to be the key to tight binding in which the contribution of hydrogen bonding seems to be secondary to van der Waals contacts.

Structure-function relationship in the antifreeze activity of synthetic alanine-lysine antifreeze polypeptides

Recently antifreeze proteins (AFP) have been the subject of many structure-function relationship studies regarding their antifreeze activity. Attempts have been made to elucidate the structure-function relationship by various amino acid substitutions, but to our knowledge there has been no successful from first principles design of a polypeptide that would bind to designated ice planes along a specific direction. In this paper we show the results of our first attempt on an entirely de novo design of an alanine-lysine-rich antifreeze polypeptide. This 43 residue alanine-lysine peptide exhibits characteristic nonequilibrium freezing point depression and binds to the designated (210) planes of ice along the [122] vector. The structural and thermodynamic properties of this polypeptide were determined using circular dichroism spectroscopy and its nonequilibrium antifreeze properties were investigated using an ice-etching method and nanoliter osmometry.

Antifreeze protein from shorthorn sculpin: identification of the ice-binding surface

Shorthorn sculpins, Myoxocephalus scorpius, are protected from freezing in icy seawater by alanine-rich, alpha-helical antifreeze proteins (AFPs). The major serum isoform (SS-8) has been reisolated and analyzed to establish its correct sequence. Over most of its length, this 42 amino acid protein is predicted to be an amphipathic alpha-helix with one face entirely composed of Ala residues. The other side of the helix, which is more heterogeneous and hydrophilic, contains several Lys. Computer simulations had suggested previously that these Lys residues were involved in binding of the peptide to the [11-20] plane of ice in the <-1102> direction. To test this hypothesis, a series of SS-8 variants were generated with single Ala to Lys substitutions at various points around the helix. All of the peptides retained significant alpha-helicity and remained as monomers in solution. Substitutions on the hydrophilic helix face at position 16, 19, or 22 had no obvious effect, but those on the adjacent Ala-rich surface at positions 17, 21, and 25 abolished antifreeze activity. These results, with support from our own modeling and docking studies, show that the helix interacts with the ice surface via the conserved alanine face, and lend support to the emerging idea that the interaction of fish AFPs with ice involves appreciable hydrophobic interactions. Furthermore, our modeling suggests a new N terminus cap structure, which helps to stabilize the helix, whereas the role of the lysines on the hydrophilic face may be to enhance solubility of the protein.

New ice-binding face for type I antifreeze protein

Type I antifreeze protein (AFP) from winter flounder is an alanine-rich, 37 amino acid, single alpha-helix that contains three 11 amino acid repeats (Thr-X(2)-Asx-X(7)), where X is generally Ala. The regularly spaced Thr, Asx and Leu residues lie on one face of the helix and have traditionally been thought to form hydrogen bonds and van der Waals interactions with the ice surface. Recently, substitution experiments have called into question the importance of Leu and Asn for ice-binding. Sequence alignments of five type I AFP isoforms show that Leu and Asn are not well conserved, whereas Ala residues adjacent to the Thr, at right angles to the Leu/Asn-rich face, are completely conserved. To investigate the role of these Ala residues, a series of Ala to Leu steric mutations was made at various points around the helix. All the substituted peptides were fully alpha-helical and remained as monomers in solution. Wild-type activity was retained in A19L and A20L. A17L, where the substitution lies adjacent to the Thr-rich face, had no detectable antifreeze activity. The nearby A21L substitution had 10% wild-type activity and demonstrated weak interactions with the ice surface. We propose a new ice-binding face for type I AFP that encompasses the conserved Ala-rich surface and adjacent Thr.