Source of the ice-binding specificity of antifreeze protein type I

Identification of Novel Antifreeze Peptides from Takifugu obscurus Skin and Molecular Mechanism in Inhibiting Ice Crystal Growth

Foodborne hydrolyzed antifreeze peptides have been widely used in the food industry and the biomedical field. However, the components of hydrolyzed peptides are complex and the molecular mechanism remains unclear. This study focused on identification and mechanism analysis of novel antifreeze peptides from Takifugu obscurus skin by traditional methods and computer-assisted techniques. Results showed that three peptides (EGPRAGGAPG, GDAGPSGPAGPTG, and GEAGPAGPAG) possessed cryoprotection via reducing the freezing point and inhibiting ice crystal growth. Molecular docking confirmed that the cryoprotective property was related to peptide structure, especially α-helix, and hydrogen bond sites. Moreover, the antifreeze peptides were double-faces, which controlled ice crystals while affecting the arrangement of surrounding water molecules, thus exhibiting a strong antifreeze activity. This investigation deepens the comprehension of the mechanism of antifreeze peptides at molecular scale, and the novel efficient antifreeze peptides can be developed in antifreeze materials design and applied in food industry.

Structural diversity of marine anti-freezing proteins, properties and potential applications: a review

Cold-adapted organisms, such as fishes, insects, plants and bacteria produce a group of proteins known as antifreeze proteins (AFPs). The specific functions of AFPs, including thermal hysteresis (TH), ice recrystallization inhibition (IRI), dynamic ice shaping (DIS) and interaction with membranes, attracted significant interest for their incorporation into commercial products. AFPs represent their effects by lowering the water freezing point as well as preventing the growth of ice crystals and recrystallization during frozen storage. The potential of AFPs to modify ice growth results in ice crystal stabilizing over a defined temperature range and inhibiting ice recrystallization, which could minimize drip loss during thawing, improve the quality and increase the shelf-life of frozen products. Most cryopreservation studies using marine-derived AFPs have shown that the addition of AFPs can increase post-thaw viability. Nevertheless, the reduced availability of bulk proteins and the need of biotechnological techniques for industrial production, limit the possible usage in foods. Despite all these drawbacks, relatively small concentrations are enough to show activity, which suggests AFPs as potential food additives in the future. The present work aims to review the results of numerous investigations on marine-derived AFPs and discuss their structure, function, physicochemical properties, purification and potential applications.

Modelling of short synthetic antifreeze peptides: Insights into ice-pinning mechanism

Organisms living in icy environments produce antifreeze proteins to control ice growth and recrystallization. It has been proposed that these molecules pin the surface of ice crystals, thus inducing the formation of a curved surface that arrests crystal growth. Such proteins are very appealing for many potential applications in food industry, material science and cryoconservation of organs and tissues. Unfortunately, their structural complexity has seriously hampered their practical use, while efficient and accessible synthetic analogues are highly desirable. In this paper, we used molecular dynamics based techniques to model the interaction of three short antifreeze synthetic peptides with an ice surface. The employed protocols succeeded in reproducing the ice pinning action of antifreeze peptides and the consequent ice growth arrest, as well as in distinguishing between antifreeze and control peptides, for which no such effect was observed. Principal components analysis of peptides trajectories in different simulation settings permitted to highlight the main structural features associated to antifreeze activity. Modeling results are highly correlated with experimentally measured properties, and insights on ice-peptide interactions and on conformational patterns favoring antifreeze activity will prompt the design of new and improved antifreeze peptides.

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.

Identification and Characterization of an Isoform Antifreeze Protein from the Antarctic Marine Diatom, Chaetoceros neogracile and Suggestion of the Core Region

Antifreeze proteins (AFPs) protecting the cells against freezing are produced in response to extremely low temperatures in diverse psychrophilic organisms, and they are encoded by multiple gene families. The AFP of Antarctic marine diatom Chaetoceros neogracile is reported in our previous research, but like other microalgae, was considered to probably have additional genes coding AFPs. In this paper, we reported the cloning and characterization of additional AFP gene from C. neogracile (Cn-isoAFP). Cn-isoAFP protein is 74.6% identical to the previously reported Cn-AFP. The promoter sequence of Cn-isoAFP contains environmental stress responsive elements for cold, thermal, and high light conditions. Cn-isoAFP transcription levels increased dramatically when cells were exposed to freezing (−20 °C), thermal (10 °C), or high light (600 μmol photon m⁻² s⁻¹) stresses. The thermal hysteresis (TH) activity of recombinant Cn-isoAFP was 0.8 °C at a protein concentration of 5 mg/mL. Results from homology modeling and TH activity analysis of site-directed mutant proteins elucidated AFP mechanism to be a result of flatness of B-face maintained via hydrophobic interactions.

Interfacial Water Arrangement in the Ice-Bound State of an Antifreeze Protein: A Molecular Dynamics Simulation Study

Molecular dynamics (MD) simulations have been carried out to study the heterogeneous ice nucleation on modeled peptide surfaces. Simulations show that large peptide surfaces made by TxT (threonine-x-threonine) motifs with the arrangements of threonine (Thr) residues identical to the periodic arrangements of waters on either the basal or prism plane of ice are capable of ice nucleation. Nucleated ice plane is the (0001) basal plane of hexagonal ice (Ih) or (111) plane of cubic ice (Ic). However, due to predefined simulation cell dimensions, the ice growth is only observed on the surface where the Thr residues are arranged like the water arrangement on the basal plane of ice Ih. The γ-methyl and γ-hydroxyl groups of Thr residue are necessary for such ice formation. From this ice nucleation and growth simulation, the interfacial water arrangement in the ice-bound state of Tenebrio molitor antifreeze protein (TmAFP) has been determined. The interfacial water arrangement in the ice-bound state of TmAFP is characterized by five-membered hydrogen bonded rings, where each of the hydroxyl groups of the Thr residues on the ice-binding surface (IBS) of the protein is a ring member. It is found that the water arrangement at the protein-ice interface is distorted from that in bulk ice. Our analysis further reveals that the hydroxyl groups of Thr residues on the IBS of TmAFP form maximum three hydrogen bonds each with the waters in the bound state and methyl groups of Thr residues occupy wider spaces than the normal grooves on the (111) plane of ice Ic. Methyl groups are also located above and along the 3-fold rotational axes of the chair-formed hexagonal hydrogen bonded water rings on the (111) plane.

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.

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.

The biological function of an insect antifreeze protein simulated by molecular dynamics

Antifreeze proteins (AFPs) protect certain cold-adapted organisms from freezing to death by selectively adsorbing to internal ice crystals and inhibiting ice propagation. The molecular details of AFP adsorption-inhibition is uncertain but is proposed to involve the Gibbs–Thomson effect. Here we show by using unbiased molecular dynamics simulations a protein structure-function mechanism for the spruce budworm Choristoneura fumiferana AFP, including stereo-specific binding and consequential melting and freezing inhibition. The protein binds indirectly to the prism ice face through a linear array of ordered water molecules that are structurally distinct from the ice. Mutation of the ice binding surface disrupts water-ordering and abolishes activity. The adsorption is virtually irreversible, and we confirm the ice growth inhibition is consistent with the Gibbs–Thomson law.

DOI:http://dx.doi.org/10.7554/eLife.05142.001

Water expands as it freezes. If this happens to the water inside plants and animals, the resulting ice crystals can rupture cells. To prevent this, many plants and animals that live in cold climates have evolved ‘antifreeze proteins’. When a small particle of ice first starts to form, the antifreeze proteins bind to it and prevent the water around it freezing, hence preventing the growth of an ice crystal.

There are many different types of antifreeze protein, and some are more active than others. For example, some insects including the spruce budworm are exposed to extremely cold temperatures—sometimes below −30°C—and these insects have antifreeze proteins that are highly active.

It is not fully understood how different antifreeze proteins interact with ice and prevent the growth of ice crystals. This is largely because, as yet, there are no experimental techniques that make it possible to see how antifreeze proteins and water molecules arrange themselves at the surface of a growing particle of ice. Instead, scientists have developed computer simulations to investigate this process. While many of these studies have provided valuable information, the computational methods used have only recently become powerful enough to analyze how the antifreeze proteins approach the surface of the ice particle.

Kuiper et al. carried out simulations involving a highly active antifreeze protein from the spruce budworm. The results of these simulations revealed that this antifreeze protein does not bind directly to ice; instead, water molecules at the surface of the protein act as a bridge between the protein and the ice. These water molecules are highly ordered and though they have similarities with how water is structured in the ice, they are distinct from the ice lattice itself. Furthermore, this arrangement appears to be important for allowing the spruce budworm antifreeze protein to interact with the ice.

This study provides detailed insights as to how a highly active antifreeze protein helps to prevent ice crystals forming. In the future, the computational simulations used here may be extended to study the dynamics of other antifreeze proteins, and also how crystals of other materials form.

DOI:http://dx.doi.org/10.7554/eLife.05142.002

Influence of antifreeze proteins on the ice/water interface

Antifreeze proteins (AFP) are responsible for the survival of several species, ranging from bacteria to fish, that encounter subzero temperatures in their living environment. AFPs have been divided into two main families, moderately and hyperactive, depending on their thermal hysteresis activity. We have studied one protein from both families, the AFP from the snow flea (sfAFP) and from the winter flounder (wfAFP), which belong to the hyperactive and moderately active family, respectively. On the basis of molecular dynamics simulations, we have estimated the thickness of the water/ice interface for systems both with and without the AFPs attached onto the ice surface. The calculation of the diffusion profiles along the simulation box allowed us to measure the interface width for different ice planes. The obtained widths clearly show a different influence of the two AFPs on the ice/water interface. The different impact of the AFPs here studied on the interface thickness can be related to two AFPs properties: the protein hydrophobic surface and the number of hydrogen bonds that the two AFPs faces form with water molecules.

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.

Antifreeze protein in Antarctic marine diatom, Chaetoceros neogracile

The antifreeze protein gene (Cn-AFP) from the Antarctic marine diatom, Chaetoceros neogracile was cloned and characterized. The full-length Cn-AFP cDNA contained an open reading frame of 849 bp and the deduced 282 amino acid peptide chain encodes a 29.2 kDa protein, which includes a signal peptide of 30 amino acids at the N terminus. Both the Cn-AFP coding region with and without the signal sequence were cloned and expressed in Escherichia coli. Recombinant Cn-AFPs were shown to display antifreeze activities based on measuring the thermal hysteresis and modified morphology of single ice crystals. Recombinant mature Cn-AFP showed 16-fold higher thermal hysteresis activity than that of pre-mature Cn-AFP at the same concentration. The ice crystal shape changed to an elongated hexagonal shape in the presence of the recombinant mature Cn-AFP, while single ice crystal showed a circular disk shape in absence of Cn-AFP. Northern analysis demonstrated a dramatic accumulation of Cn-AFP transcripts when the cells were subjected to freezing stress. This rapid response to freeze stress, and the antifreeze activity of recombinant Cn-AFPs, indicates that Cn-AFP plays an important role in low temperature adaptation.

Purification of antifreeze protein from wheat bran (Triticum aestivum L.) based on its hydrophilicity and ice-binding capacity

Wheat-bran ( Triticum aestivum L.) antifreeze protein ( TaAFP) was purified 323-fold to electrophoretic homogeneity with an overall yield of 1.64% from wheat-bran protein by a specific three-step procedure. The three-step procedure was quicker, cheaper, and more effective than the five-step procedure we used earlier. First, TaAFP was concentrated by a phosphate buffer, on the basis of its strong hydrophilicity that was validated by thermal gravimetric analyses and a surface hydrophobicity analysis. Second, TaAFP was trapped in ice crystals for its specific ice-binding capacity, which was proved by ice-binding protocols. Remarkably, the ice-binding step was the most effective step, and the purification factor of this step was up to 270-fold. Finally, TaAFP was purified by HPLC purification, a complementary step for the specific ice-binding protocol, to electrophoretic homogeneity. Our protocols provide peers a novel and effective way for the search and purification of potential AFPs.

Antifreeze proteins at the ice/water interface: three calculated discriminating properties for orientation of type I proteins

Antifreeze proteins (AFPs) protect many plants and organisms from freezing in low temperatures. Of the different AFPs, the most studied AFP Type I from winter flounder is used in the current computational studies to gain molecular insight into its adsorption at the ice/water interface. Employing molecular dynamics simulations, we calculate the free energy difference between the hydrophilic and hydrophobic faces of the protein interacting with ice. Furthermore, we identify three properties of Type I "antifreeze" proteins that discriminate among these two orientations of the protein at the ice/water interface. The three properties are: the "surface area" of the protein; a measure of the interaction of the protein with neighboring water molecules as determined by the number of hydrogen bond count, for example; and the side-chain orientation angles of the threonine residues. All three discriminants are consistent with our free energy results, which clearly show that the hydrophilic protein face orientations toward the ice/water interface, as hypothesized from experimental and ice/vacuum simulations, are incorrect and support the hypothesis that the hydrophobic face is oriented toward the ice/water interface. The adsorption free energy is calculated to be 2-3 kJ/mol.

BIOCOMPUTATIONAL ANALYSIS AND CHARACTERIZATION OF SOME ANTIFREEZE PROTEINS

Novel bioinformatic procedures and computational methods have been used to analyze, characterize and provide more detailed description of some selected fish antifreeze proteins (AFPs) retrieved from Swiss–Prot database. Analysis shows that AFPs are rich in non-polar residues and that the AFPs Q01758 and P05140 contain SS bonds. The aliphatic index computed by ExPasy's ProtParam infers that AFPs may be stable for a wide range of temperatures and the AFP P80961 is classified as an unstable protein. The very low GRAVY index of AFP P80961 infers its higher hydrosolubility. Secondary structure analysis shows that the flounder and sculpin fish AFPs are found to be of predominant ∝–helical structures and the rest of them are with mixed secondary structures. The average molecular weight of AFPs computed is 9584 Da. SOSUI server predicts one transmembrane region in P04002 (winter flounder fish) and two regions in P09031 (yellowtail flounder fish). The predicted transmembrane regions were visualized and analyzed using helical wheel plots generated by EMBOSS pepwheel tool. The residues A, L, G and N are identified as the antigenic sites by EMBOSS antigenic program. The presence of 11 Cys residues in AFPs Q01758 (rainbow smelt fish) and P05140 (sea raven fish) indicates the presence of disulfide bridges (SS bonds) in these AFPs, and it is also recognized by CYS_REC tool and well documented from the three dimensional structure using Rasmol tool.

Cold-active winter rye glucanases with ice-binding capacity

Extracellular pathogenesis-related proteins, including glucanases, are expressed at cold temperatures in winter rye (Secale cereale) and display antifreeze activity. We have characterized recombinant cold-induced glucanases from winter rye to further examine their roles and contributions to cold tolerance. Both basic beta-1,3-glucanases and an acidic beta-1,3;1,4-glucanase were expressed in Escherichia coli, purified, and assayed for their hydrolytic and antifreeze activities in vitro. All were found to be cold active and to retain partial hydrolytic activity at subzero temperatures (e.g. 14%-35% at -4 degrees C). The two types of glucanases had antifreeze activity as measured by their ability to modify the growth of ice crystals. Structural models for the winter rye beta-1,3-glucanases were developed on which putative ice-binding surfaces (IBSs) were identified. Residues on the putative IBSs were charge conserved for each of the expressed glucanases, with the exception of one beta-1,3-glucanase recovered from nonacclimated winter rye in which a charged amino acid was present on the putative IBS. This protein also had a reduced antifreeze activity relative to the other expressed glucanases. These results support the hypothesis that winter rye glucanases have evolved to inhibit the formation of large, potentially fatal ice crystals, in addition to having enzymatic activity with a potential role in resisting infection by psychrophilic pathogens. Glucanases of winter rye provide an interesting example of protein evolution and adaptation aimed to combat cold and freezing conditions.

Theoretical study of interaction of winter flounder antifreeze protein with ice

Antifreeze proteins (AFPs) are synthesized by various organisms to enable their cells to survive subzero environment. These proteins bind to small ice crystals and inhibit their growth, which if left uncontrolled would be fatal to cells. The crystal structures of a number of AFPs have been determined; however, crystallographic analysis of AFP-ice complex is nearly impossible. Molecular modeling studies of AFPs' interaction with ice surface is therefore invaluable. Early models of AFP-ice interaction suggested H-bond as the primary driving force behind such interaction. Recent experimental evidence, however, suggested that hydrophobic interactions could be the main contributor to AFP-ice association. All computational studies published to date were carried out to verify the H-bond model, and no works attempting to verify the hydrophobic interaction model have been published. In this work, we Monte Carlo-minimized complexes of several AFPs with ice taking into account nonbonded interactions, H-bonds, and the hydration potential for proteins. Parameters of the hydration potential for ice were developed with the assumption that the free energy of the water-ice association should be close to zero at equilibrium melting temperature. Our calculations demonstrate that desolvation of hydrophobic groups in the AFPs upon their binding to the grooves at the ice surface is indeed the major stabilizing contributor to the free energy of AFP-ice binding. This study is consistent with available structural and mutation data on AFPs. In particular, it explains the paradoxical finding that substitution of Thr residues with Val does not affect the potency of winter flounder AFP whereas substitution with Ser abolished its antifreeze activity.

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.

Molecular dynamics study of the interactions of ice inhibitors on the ice {001} surface

The interactions of antifreeze protein (AFP) type I, antifreeze glycoproteins, polyvinyl pyrrolidone (PVP), and various amino acids with ice are investigated using Cerius2, a molecular modelling tool. Binding energies of these additives to a major ice crystal face {001} are computed. Binding energy comparison of threonine molecules (by themselves) and as threonine residues within AFP type I demonstrate their role in improving AFP's binding ability to the ice crystal face. The shifts in onset points of ice crystallization with AFP type I, PVP, and amino acids are measured using differential scanning calorimetry. These values when correlated with their respective binding energies reveal a direct proportionality and demonstrate AFP's effectiveness in inhibiting growth and nucleation of ice, over amino acids.

Solution structure of a hydrophobic analogue of the winter flounder antifreeze protein

The solution structure of a synthetic mutant type I antifreeze protein (AFP I) was determined in aqueous solution at pH 7.0 using nuclear magnetic resonance (NMR) spectroscopy. The mutations comprised the replacement of the four Thr residues by Val and the introduction of two additional Lys-Glu salt bridges. The antifreeze activity of this mutant peptide, VVVV2KE, has been previously shown to be similar to that of the wild type protein, HPLC6 (defined here as TTTT). The solution structure reveals an alphahelix bent in the same direction as the more bent conformer of the published crystal structure of TTTT, while the side chain chi1 rotamers of VVVV2KE are similar to those of the straighter conformer in the crystal of TTTT. The Val side chains of VVVV2KE assume the same orientations as the Thr side chains of TTTT, confirming the conservative nature of this mutation. The combined data suggest that AFP I undergoes an equilibrium between straight and bent helices in solution, combined with independent equilibria between different side chain rotamers for some of the amino acid residues. The present study presents the first complete sequence-specific resonance assignments and the first complete solution structure determination by NMR of any AFP I protein.

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.

Structure of type I antifreeze protein and mutants in supercooled water

Many organisms are able to survive subzero temperatures at which bodily fluids would normally be expected to freeze. These organisms have adapted to these lower temperatures by synthesizing antifreeze proteins (AFPs), capable of binding to ice, which make further growth of ice energetically unfavorable. To date, the structures of five AFPs have been determined, and they show considerable sequence and structural diversity. The type I AFP reveals a single 37-residue alpha-helical structure. We have studied the behavior of wild-type type I AFP and two "inactive" mutants (Ala17Leu and Thr13Ser/Thr24Ser) in normal and supercooled solutions of H(2)O and deuterium oxide (D(2)O) to see if the structure at temperatures below the equilibrium freezing point is different from the structure observed at above freezing temperatures. Analysis of 1D (1)H- and (13)C-NMR spectra illustrate that all three proteins remain folded as the temperature is lowered and even seem to become more alpha-helical as evidenced by (13)C(alpha)-NMR chemical shift changes. Furthermore, (13)C-T(2) NMR relaxation measurements demonstrate that the rotational correlation times of all three proteins behave in a predictable manner under all temperatures and conditions studied. These data have important implications for the structure of the AFP bound to ice as well as the mechanisms for ice-binding and protein oligomerization.

Hydrophobic analogues of the winter flounder 'antifreeze' protein

The synthesis, solution conformation and ice-growth inhibition properties of four new analogues of the type I 37-residue winter flounder 'antifreeze' protein are reported. All four analogues contain two extra salt bridges to facilitate comparison of results with previously published data. In two analogues, all four threonine residues in the native polypeptide were mutated to 2-amino butyric acid (an unnatural amino acid) and isoleucine, respectively. The butyric acid analogue was approximately 85% helical at 3 degrees C, modified the shape of ice growth, and exhibited reduced hysteresis compared to the native protein (9% at 4 mM). These results show that the gamma-methyl group of threonine, which is present in the sidechain of 2-amino butyric acid, is not sufficient for activity. The isoleucine analogue, in which the threonine hydroxyl group is replaced by an ethyl group, was 100% helical at 3 degrees C, showed no hysteresis but was able to modify the shape of ice crystal growth. In the third and fourth analogues, mutations of the aspartic acids 1 and 5 to alanine, and asparagines 16 and 27 to leucine in the threonine- and valine-substituted analogues did not affect the helicity of the polypeptides, but removed the ability to inhibit ice growth.