Winter Flounder “Antifreeze” Proteins: Synthesis and Ice Growth Inhibition of Analogues that Probe the Relative Importance of Hydrophobic and Hydrogen-Bonding Interactions

Conjugation of type I antifreeze protein to polyallylamine increases thermal hysteresis activity

Antifreeze proteins (AFPs) are ice binding proteins found in some plants, insects, and Antarctic fish allowing them to survive at subzero temperatures by inhibiting ice crystal growth. The interaction of AFPs with ice crystals results in a difference between the freezing and melting temperatures, termed thermal hysteresis, which is the most common measure of AFP activity. Creating antifreeze protein constructs that reduce the concentration of protein needed to observe thermal hysteresis activities would be beneficial for diverse applications including cold storage of cells or tissues, ice slurries used in refrigeration systems, and food storage. We demonstrate that conjugating multiple type I AFPs to a polyallylamine chain increases thermal hysteresis activity compared to the original protein. The reaction product is approximately twice as active when compared to the same concentration of free proteins, yielding 0.5 °C thermal hysteresis activity at 0.3 mM protein concentration. More impressively, the amount of protein required to achieve a thermal hysteresis of 0.3 °C is about 100 times lower when conjugated to the polymer (3 μM) compared to free protein (300 μM). Ice crystal morphologies observed in the presence of the reaction product are comparable to those of the protein used in the conjugation reaction.

Design and synthesis of antifreeze glycoproteins and mimics

Antifreeze glycoproteins are an important class of biological antifreezes that have potential applications in many areas of medicine, agriculture and industry in which ice crystal growth is damaging. While the synthesis of antifreeze glycoproteins as pure glycoforms has recently been achieved by using ligation and polymerisation strategies, the routine production of large quantities of pure glycoforms remains challenging. A range of C-linked analogues that are readily produced by solid-phase synthesis have delivered novel compounds that are not biological antifreezes, but are potent, non-cytotoxic, ice-recrystallisation inhibitors. Structure-activity studies, the identification of cyclic antifreeze glycoproteins and conformational studies have provided further insight into the requirements for antifreeze activity. These results, coupled with significant advances in approaches to the routine synthesis of different glycoproteins and mimics, present opportunities for the design and synthesis of novel ice-growth-inhibiting and antifreeze compounds.

Antifreeze glycoprotein activity correlates with long-range protein-water dynamics

Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) enable the survival of organisms living in subfreezing habitats and serve as preservatives. Although their function is known, the underlying molecular mechanism was not understood. Mutagenesis experiments questioned the previous assumption of hydrogen bonding as the dominant mechanism. We use terahertz spectroscopy to show that antifreeze activity is directly correlated with long-range collective hydration dynamics. Our results provide evidence for a new model of how AFGPs prevent water from freezing. We suggest that antifreeze activity may be induced because the AFGP perturbs the aqueous solvent over long distances. Retarded water dynamics in the large hydration shell does not favor freezing. The complexation of the carbohydrate cis-hydroxyl groups by borate suppresses the long-range hydration shell detected by terahertz absorption. The hydration dynamics shift toward bulk water behavior strongly reduces the AFGP antifreeze activity, further supporting our model.

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.

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.

Activity of short segments of Type I antifreeze protein

In this work, we present a study on the antifreeze activity of short segments of a Type I antifreeze protein, instead of the whole protein. This approach simplifies the correlation between antifreeze protein characteristics, such as hydrophilicity/hydrophobicity, and the effect of these characteristics on the antifreeze mechanism. Three short polypeptides of Type I AFP have been synthesized. Their antifreeze activity and interactions with water and ice crystals have been analyzed by various techniques such as circular dichroism spectroscopy, X-ray diffraction, differential scanning calorimetry, and osmometry. It is shown that one short segment of Type I AFP has an antifreeze activity of about 60% of the native protein activity. In this work, we demonstrate that short segments of Type I AFPs possess nonzero thermal hysteresis and result in modifications in the growth habits and growth rates of ice. This approach enables the preparation of large quantities of short AFP segments at low cost with high antifreeze activity, and opens the possibility of developing the commercial potential of AFPs.

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.

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

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

C-linked galactosyl serine AFGP analogues as potent recrystallization inhibitors

[reaction: see text] A series of C-linked antifreeze glycoprotein analogues have been prepared to evaluate antifreeze activity as a function of distance between the carbohydrate moiety and polypeptide backbone. The building blocks for these analogues were prepared using either an olefin cross-metathesis or catalytic asymmetric hydrogenation. Analysis of antifreeze protein-specific activity revealed that only analogue 2a (n = 1) was a potent recrystallization inhibitor and thus has potential medical and industrial applications.

The mechanism by which fish antifreeze proteins cause thermal hysteresis

Antifreeze proteins are characterised by their ability to prevent ice from growing upon cooling below the bulk melting point. This displacement of the freezing temperature of ice is limited and at a sufficiently low temperature a rapid ice growth takes place. The separation of the melting and freezing temperature is usually referred to as thermal hysteresis, and the temperature of ice growth is referred to as the hysteresis freezing point. The hysteresis is supposed to be the result of an adsorption of antifreeze proteins to the crystal surface. This causes the ice to grow as convex surface regions between adjacent adsorbed antifreeze proteins, thus lowering the temperature at which the crystal can visibly expand. The model requires that the antifreeze proteins are irreversibly adsorbed onto the ice surface within the hysteresis gap. This presupposition is apparently in conflict with several characteristic features of the phenomenon; the absence of superheating of ice in the presence of antifreeze proteins, the dependence of the hysteresis activity on the concentration of antifreeze proteins and the different capacities of different types of antifreeze proteins to cause thermal hysteresis at equimolar concentrations. In addition, there are structural obstacles that apparently would preclude irreversible adsorption of the antifreeze proteins to the ice surface; the bond strength necessary for irreversible adsorption and the absence of a clearly defined surface to which the antifreeze proteins may adsorb. This article deals with these apparent conflicts between the prevailing theory and the empirical observations. We first review the mechanism of thermal hysteresis with some modifications: we explain the hysteresis as a result of vapour pressure equilibrium between the ice surface and the ambient fluid fraction within the hysteresis gap due to a pressure build-up within the convex growth zones, and the ice growth as the result of an ice surface nucleation event at the hysteresis freezing point. We then go on to summarise the empirical data to show that the dependence of the hysteresis on the concentration of antifreeze proteins arises from an equilibrium exchange of antifreeze proteins between ice and solution at the melting point. This reversible association between antifreeze proteins and the ice is followed by an irreversible adsorption of the antifreeze proteins onto a newly formed crystal plane when the temperature is lowered below the melting point. The formation of the crystal plane is due to a solidification of the interfacial region, and the necessary bond strength is provided by the protein "freezing" to the surface. In essence: the antifreeze proteins are "melted off" the ice at the bulk melting point and "freeze" to the ice as the temperature is reduced to subfreezing temperatures. We explain the different hysteresis activities caused by different types of antifreeze proteins at equimolar concentrations as a consequence of their solubility features during the phase of reversible association between the proteins and the ice, i.e., at the melting point; a low water solubility results in a large fraction of the proteins being associated with the ice at the melting point. This leads to a greater density of irreversibly adsorbed antifreeze proteins at the ice surface when the temperature drops, and thus to a greater hysteresis activity. Reference is also made to observations on insect antifreeze proteins to emphasise the general validity of this approach.

Isolation and characterization of hemolymph antifreeze proteins from larvae of the longhorn beetle Rhagium inquisitor (L.)

We describe a simple and effective procedure to isolate antifreeze proteins (AFPs) from the hemolymph of larvae of the longhorn beetle Rhagium inquisitor, and present some characteristics of their structures. Several AFPs were isolated from the hemolymph of this species by heat and acid extraction followed by cation exchange. The hemolymph contains at least six AFPs ranging in size from 12.5 to 12.8 kDa. Of these, three were separated to purity by the ion exchange step, as indicated by mass spectrometry. The remaining three forms were further separated by size exclusion chromatography, but could not be isolated to purity. All AFPs in the hemolymph of this species appears to have isoelectric points above 8.00. The dominant form, RiAFP(H4), was purified by the ion exchange step. Its amino acid composition reveals a lower level of cysteine and a higher level of threonine, arginine, alanine and glycine than seen in other insect AFPs. Its trypsin fingerprint does not match that of any known protein. It interacts with ice both in the anionic and cationic state.

Hydrophobic tendency of polar group hydration as a major force in type I antifreeze protein recognition

The random network model of water quantitatively describes the different hydration heat capacities of polar and apolar solutes in terms of distortions of the water-water hydrogen bonding angle in the first hydration shell (Gallagher and Sharp, JACS 2003;125:9853). The distribution of this angle in pure water is bimodal, with a low-angle population and high-angle population. Polar solutes increase the high-angle population while apolar solutes increase the low-angle population. The ratio of the two populations quantifies the hydrophobicity of the solute and provides a sensitive measure of water structural distortions. This method of analysis is applied to study hydration of type I thermal hysteresis protein (THP) from winter flounder and three quadruple mutants of four threonine residues at positions 2, 13, 24, and 35. Wild-type and two mutants (VVVV and AAAA) have antifreeze (thermal hysteresis) activity, while the other mutant (SSSS) has no activity. The analysis reveals significant differences in the hydration structure of the ice-binding site. For the SSSS mutant, polar groups have a typical polar-like hydration, that is, more high-angle H-bonds than bulk water. For the wild-type and active mutants, polar groups have unusual, very apolar-like hydration, that is, more low-angle H-bonds than bulk water. This pattern of hydration was seen previously in the structurally distinct type III THPs (Yang & Sharp Biophys Chem 2004;109:137), suggesting for the first time a general mechanism for different THP classes. The specific shape, residue size, and clustering of both polar and apoler groups are essential for an active ice binding surface.

The effect of recombinant mouse amelogenins on the formation and organization of hydroxyapatite crystals in vitro

Amelogenin is the most abundant protein in developing dental enamel. It is believed to play an important role in the regulation of the growth and organization of enamel crystals. Amelogenin, unlike many other proteins found in biominerals, is mostly hydrophobic except for a 13 amino acid hydrophilic C-terminal domain. To clarify the role of amelogenin in enamel mineralization, we designed calcium phosphate crystal growth experiments in the presence of recombinant amelogenins with or without the charged C-terminal domain. The shape and organization of the crystals were examined by TEM in bright field and diffraction modes. It was found that both full-length and truncated amelogenin inhibit crystal growth in directions normal to the c-axis. At the same time, crystallites organized into parallel arrays only in the presence of the full-length amelogenin in monomeric form. Pre-assembled amelogenins had no effect on crystals organization. These results imply that the hydrophobic portion of amelogenin plays a role in an inhibition of crystal growth, whereas the C-terminal domain is essential for the alignment of crystals into parallel arrays. Our data also suggest that nascent enamel structure emerges as a result of cooperative interactions between forming crystals and assembling proteins.

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.

The mechanism of the type III antifreeze protein action: a computational study

The random network model of water quantitatively describes the different hydration heat capacities of polar and apolar solutes in terms of differential distortions of the water-water hydrogen bonding angle in the first hydration shell. This method of hydration analysis is applied here to study the hydration of the wild type III thermal hysteresis protein from eel pout and three mutations at residue 16. Wild type and one mutant have full activity, the other two mutants have little or no anti-freeze (thermal hysteresis) activity. The analysis reveals significant differences in the hydration structure of the ice-binding site (centered on residue 16) among four proteins. For the A16T and A16Y mutants with reduced activity, polar groups have a typical polar-like hydration. For the wild type and mutant A16C with 100% of the wild type activity, polar groups have unusual, very apolar-like hydration. In the latter case, hydrating water molecules form a more ice-like pattern of hydrogen bonding on the ice-binding face, while in the former case water-water H-bonds are more distorted and more heterogenous. Overall, the binding surface of active protein strongly enhances the water tetrahedral structure, i.e. promotes ice-like hydration. It is concluded that the specific shape, residue size and clustering of both polar/apolar groups are essential for the binding surface to recognize, and preferentially interact with nascent ice crystals forming in liquid water.

Intermolecular interaction studies of winter flounder antifreeze protein reveal the existence of thermally accessible binding state

The physical nature underlying intermolecular interactions between two rod-like winter flounder antifreeze protein (AFP) molecules and their implication for the mechanism of antifreeze function are examined in this work using molecular dynamics simulations, augmented with free energy calculations employing a continuum solvation model. The energetics for different modes of interactions of two AFP molecules is examined in both vacuum and aqueous phases along with the water distribution in the region encapsulated by two antiparallel AFP backbones. The results show that in a vacuum two AFP molecules intrinsically attract each other in the antiparallel fashion, where their complementary charge side chains face each other directly. In the aqueous environment, this attraction is counteracted by both screening and entropic effects. Therefore, two nearly energetically degenerate states, an aggregated state and a dissociated state, result as a new aspect of intermolecular interaction in the paradigm for the mechanism of action of AFP. The relevance of these findings to the mechanism of function of freezing inhibition in the context of our work on Antarctic cod antifreeze glycoprotein (Nguyen et al., Biophysical Journal, 2002, Vol. 82, pp. 2892-2905) is discussed.

The effect of hydrophobic analogues of the type I winter flounder antifreeze protein on lipid bilayers

The effect of four synthetic analogues of the 37-residue winter flounder type I antifreeze protein (AFP), which contain four Val, Ala or Ile residues in place of Thr residues at positions 2, 13, 24 and 37 and two additional salt bridges, on the binary lipid system prepared from a 1:1 mixture of the highly unsaturated DGDG and saturated DMPC has been determined using FTIR spectroscopy. In contrast to the natural protein, which increases the thermotropic phase transition, the Thr, Val and Ala analogues decreased the thermotropic phase transitions of the liposomes by 2.2 degrees Celsius, 3.4 degrees Celsius and 2.4 degrees Celsius, while the Ile analogue had no effect on the transition. Experiments performed using perdeuterated DMPC showed that the Ala and Thr peptides interacted preferentially with the DGDG in the lipid mixture, while the Val peptide showed no preference for either lipid. The results are consistent with interactions involving the hydrophobic face of type I AFPs and model bilayers, i.e. the same face of the protein that is responsible for antifreeze properties. The different effects correlate with the helicity of the peptides and suggest that the solution conformation of the peptides has a significant role in determining the effects of the peptides on thermotropic membrane phase transitions.

The role of side chain conformational flexibility in surface recognition by Tenebrio molitor antifreeze protein

Two-dimensional nuclear magnetic resonance spectroscopy was used to investigate the flexibility of the threonine side chains in the beta-helical Tenebrio molitor antifreeze protein (TmAFP) at low temperatures. From measurement of the (3)J(alphabeta) (1)H-(1)H scalar coupling constants, the chi(1) angles and preferred rotamer populations can be calculated. It was determined that the threonines on the ice-binding face of the protein adopt a preferred rotameric conformation at near freezing temperatures, whereas the threonines not on the ice-binding face sample many rotameric states. This suggests that TmAFP maintains a preformed ice-binding conformation in solution, wherein the rigid array of threonines that form the AFP-ice interface matches the ice crystal lattice. A key factor in binding to the ice surface and inhibition of ice crystal growth appears to be the close surface-to-surface complementarity between the AFP and crystalline ice, and the lack of an entropic penalty associated with freezing out motions in a flexible ligand.

'Antifreeze' glycoproteins from polar fish

Antifreeze glycoproteins (AFGPs) constitute the major fraction of protein in the blood serum of Antarctic notothenioids and Arctic cod. Each AFGP consists of a varying number of repeating units of (Ala-Ala-Thr)n, with minor sequence variations, and the disaccharide beta-D-galactosyl-(1-->3)-alpha-N-acetyl-D-galactosamine joined as a glycoside to the hydroxyl oxygen of the Thr residues. These compounds allow the fish to survive in subzero ice-laden polar oceans by kinetically depressing the temperature at which ice grows in a noncolligative manner. In contrast to the more widely studied antifreeze proteins, little is known about the mechanism of ice growth inhibition by AFGPs, and there is no definitive model that explains their properties. This review summarizes the structural and physical properties of AFGPs and advances in the last decade that now provide opportunities for further research in this field. High field NMR spectroscopy and molecular dynamics studies have shown that AFGPs are largely unstructured in aqueous solution. While standard carbohydrate degradation studies confirm the requirement of some of the sugar hydroxyls for antifreeze activity, the importance of following structural elements has not been established: (a) the number of hydroxyls required, (b) the stereochemistry of the sugar hydroxyls (i.e. the requirement of galactose as the sugar), (c) the acetamido group on the first galactose sugar, (d) the stereochemistry of the beta-glycosidic linkage between the two sugars and the alpha-glycosidic linkage to Thr, (e) the requirement of a disaccharide for activity, and (f) the Ala and Thr residues in the polypeptide backbone. The recent successful synthesis of small AFGPs using solution methods and solid-phase chemistry provides the opportunity to perform key structure-activity studies that would clarify the important residues and functional groups required for activity. Genetic studies have shown that the AFGPs present in the two geographically and phylogenetically distinct Antarctic notothenioids and Arctic cod have evolved independently, in a rare example of convergent molecular evolution. The AFGPs exhibit concentration dependent thermal hysteresis with maximum hysteresis (1.2 degrees C at 40 mg x mL-1) observed with the higher molecular mass glycoproteins. The ability to modify the rate and shape of crystal growth and protect cellular membranes during lipid-phase transitions have resulted in identification of a number of potential applications of AFGPs as food additives, and in the cryopreservation and hypothermal storage of cells and tissues.

The role of N and C termini in the antifreeze activity of winter flounder (Pleuronectes americanus) antifreeze proteins

Antifreeze proteins (AFPs) are found in many marine fish and have been classified into five biochemical classes: AFP types I-IV and the antifreeze glycoproteins. Type I AFPs are alpha-helical, partially amphipathic, Ala-rich polypeptides. The winter flounder (Pleuronectes americanus) produces two type I AFP subclasses, the liver-type AFPs (wflAFPs) and the skin-type AFPs (wfsAFPs), that are encoded by distinct gene families with different tissue-specific expression. wfsAFPs and wflAFPs share a high level of identity even though the wfsAFPs have approximately half the activity of the wflAFPs. Synthetic polypeptides based on two representative wflAFPs and wfsAFPs were generated to examine the role of the termini in antifreeze activity. Through systematic exchange of N and C termini between wflAFP-6 and wfsAFP-2, the termini were determined to be the major causative agents for the variation in activity levels between the two AFPs. Furthermore, the termini of wflAFP-6 possessed greater helix-stabilizing ability compared with their wfsAFP-2 counterparts. The observed 50% difference in activity between wflAFP-6 and wfsAFP-2 can be divided into approximately 20% for differences at each termini and approximately 10% for differences in the core. Furthermore, the N terminus was determined to be the most critical component for antifreeze activity.

The physico-chemical characterization of a boiling stable antifreeze protein from a perennial grass (Lolium perenne)

We have characterized a cold-induced, boiling stable antifreeze protein. This highly active ice recrystallization inhibition protein shows a much lower thermal hysteresis effect and displays binding behavior that is uncharacteristic of any AFP from fish or insects. Ice-binding studies show it binds to the (1 0 1 0) plane of ice and FTIR studies reveal that it has an unusual type of highly beta-sheeted secondary structure. Ice-binding studies of both glycosylated and nonglycosylated expressed forms indicate that it adsorbs to ice through the protein backbone. These results are discussed in light of the currently proposed mechanisms of AFP action.

Fully convergent solid phase synthesis of antifreeze glycoprotein analogues

The convergent solid phase synthesis of C-linked analogues of antifreeze glycoprotein (AFGP) has been achieved. In this approach, three to six carbohydrate residues are simultaneously coupled to a resin-bound polypeptide. Glycopeptides ranging from 1.6 to 3.0 kDa are easily prepared in 26-44% yield demonstrating the utility of this approach.

Contribution of hydrophobic residues to ice binding by fish type III antifreeze protein

Type III antifreeze protein (AFP) is a 7-kDa globular protein with a flat ice-binding face centered on Ala 16. Neighboring hydrophilic residues Gln 9, Asn 14, Thr 15, Thr 18 and Gln 44 have been implicated by site-directed mutagenesis in binding to ice. These residues have the potential to form hydrogen bonds with ice, but the tight packing of side chains on the ice-binding face limits the number and strength of possible hydrogen bond interactions. Recent work with alpha-helical AFPs has emphasized the hydrophobicity of their ice-binding sites and suggests that hydrophobic interactions are important for antifreeze activity. To investigate the contribution of hydrophobic interactions between type III AFP and ice, Leu, Ile and Val residues on the rim of the ice-binding face were changed to alanine. Mutant AFPs with single alanine substitutions, L19A, V20A, and V41A, showed a 20% loss in activity. Doubly substituted mutants, L19A/V41A and L10A/I13A, had less than 50% of the activity of the wild type. Thus, side chain substitutions that leave a cavity or undercut the contact surface are almost as deleterious to antifreeze activity as those that lengthen the side chain. These mutations emphasize the importance of maintaining a specific surface contour on the ice-binding face for docking to ice.

Type I shorthorn sculpin antifreeze protein: recombinant synthesis, solution conformation, and ice growth inhibition studies

A number of structurally diverse classes of "antifreeze" proteins that allow fish to survive in sub-zero ice-laden waters have been isolated from the blood plasma of cold water teleosts. However, despite receiving a great deal of attention, the one or more mechanisms through which these proteins act are not fully understood. In this report we have synthesized a type I antifreeze polypeptide (AFP) from the shorthorn sculpin Myoxocephalus scorpius using recombinant methods. Construction of a synthetic gene with optimized codon usage and expression as a glutathione S-transferase fusion protein followed by purification yielded milligram amounts of polypeptide with two extra residues appended to the N terminus. Circular dichroism and NMR experiments, including residual dipolar coupling measurements on a 15N-labeled recombinant polypeptide, show that the polypeptides are alpha-helical with the first four residues being more flexible than the remainder of the sequence. Both the recombinant and synthetic polypeptides modify ice growth, forming facetted crystals just below the freezing point, but display negligible thermal hysteresis. Acetylation of Lys-10, Lys-20, and Lys-21 as well as the N terminus of the recombinant polypeptide gave a derivative that displays both thermal hysteresis (0.4 degrees C at 15 mg/ml) and ice crystal faceting. These results confirm that the N terminus of wild-type polypeptide is functionally important and support our previously proposed mechanism for all type I proteins, in which the hydrophobic face is oriented toward the ice at the ice/water interface.

The dynamics, structure, and conformational free energy of proline-containing antifreeze glycoprotein

Recent NMR studies of the solution structure of the 14-amino acid antifreeze glycoprotein AFGP-8 have concluded that the molecule lacks long-range order. The implication that an apparently unstructured molecule can still have a very precise function as a freezing inhibitor seems startling at first consideration. To gain insight into the nature of conformations and motions in AFGP-8, we have undertaken molecular dynamics simulations augmented with free energy calculations using a continuum solvation model. Starting from 10 different NMR structures, 20 ns of dynamics of AFGP were explored. The dynamics show that AFGP structure is composed of four segments, joined by very flexible pivots positioned at alanine 5, 8, and 11. The dynamics also show that the presence of prolines in this small AFGP structure facilitates the adoption of the poly-proline II structure as its overall conformation, although AFGP does adopt other conformations during the course of dynamics as well. The free energies calculated using a continuum solvation model show that the lowest free energy conformations, while being energetically equal, are drastically different in conformations. In other words, this AFGP molecule has many structurally distinct and energetically equal minima in its energy landscape. In addition, conformational, energetic, and hydrogen bond analyses suggest that the intramolecular hydrogen bonds between the N-acetyl group and the protein backbone are an important integral part of the overall stability of the AFGP molecule. The relevance of these findings to the mechanism of freezing inhibition is discussed.

Crystal structure of beta-helical antifreeze protein points to a general ice binding model

Reported here is the 2.3 A resolution crystal structure of spruce budworm (Choristoneura fumiferana) antifreeze protein (CfAFP), solved by single anomalous scattering. The structure reveals an extremely regular left-handed beta-helical platform consisting of 15-amino acid loops with a repetitive Thr-X-Thr motif displayed on one of the helix's three faces. This motif results in a two-dimensional array of threonine residues in an identical orientation to those in the nonhomologous, right-handed beta-helical beetle AFP from Tenebrio molitor (TmAFP). The CfAFP structure led us to reevaluate our ice binding model, and the analysis of three possible modes of docking gives rise to a binding mechanism based on surface complementarity. This general mechanism is applicable to both fish and insect AFPs.

A mechanism for stabilization of membranes at low temperatures by an antifreeze protein

Polar fish, cold hardy plants, and overwintering insects produce antifreeze proteins (AFPs), which lower the freezing point of solutions noncolligatively and inhibit ice crystal growth. Fish AFPs have been shown to stabilize membranes and cells in vitro during hypothermic storage, probably by interacting with the plasma membrane, but the mechanism of this stabilization has not been clear. We show here that during chilling to nonfreezing temperatures the alpha-helical AFP type I from polar fish inhibits leakage across model membranes containing an unsaturated chloroplast galactolipid. The mechanism involves binding of the AFP to the bilayer, which increases the phase transition temperature of the membranes and alters the molecular packing of the acyl chains. We suggest that this change in acyl chain packing results in the reduced membrane permeability. The data suggest a hydrophobic interaction between the peptide and the bilayer. Further, we suggest that the expression of AFP type I in transgenic plants may be significant for thermal adaptation of chilling-sensitive plants.