A kinetic description of antifreeze glycoprotein activity

Diffusion Attachment Model for Long Helical Antifreeze Proteins to Ice

Some of the most potent antifreeze proteins (AFPs) are approximately rigid helical structures that bind with one side in contact with the ice surface at specific orientations. These AFPs take random orientations in solution; however, most orientations become sterically inaccessible as the AFP approaches the ice surface. The effect of these inaccessible orientations on the rate of adsorption of AFP to ice has never been explored. Here, we present a diffusion-controlled theory of adsorption kinetics that accounts for these orientational restrictions to predict a rate constant for adsorption (k_on, in m/s) as a function of the length and width of the AFP molecules. We find that k_on decreases with length and diameter of the AFP and is almost proportional to the inverse of the area of the binding surface. We demonstrate that the restricted orientations create an entropic barrier to AFP adsorption, which we compute to be approximately 7 k_BT for most AFPs and up to 9 k_BT for Maxi, the largest known AFP. We compare the entropic resistance 1/k_on to resistances for diffusion through boundary layers and across typical distances in the extracellular matrix and find that these entropic and diffusion resistances could become comparable in the small confined spaces of biological environments.

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

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

The Ensemble of Conformations of Antifreeze Glycoproteins (AFGP8): A Study Using Nuclear Magnetic Resonance Spectroscopy

The primary sequence of antifreeze glycoproteins (AFGPs) is highly degenerate, consisting of multiple repeats of the same tripeptide, Ala–Ala–Thr*, in which Thr* is a glycosylated threonine with the disaccharide beta-d-galactosyl-(1,3)-alpha-N-acetyl-d-galactosamine. AFGPs seem to function as intrinsically disordered proteins, presenting challenges in determining their native structure. In this work, a different approach was used to elucidate the three-dimensional structure of AFGP8 from the Arctic cod Boreogadussaida and the Antarctic notothenioid Trematomusborchgrevinki. Dimethyl sulfoxide (DMSO), a non-native solvent, was used to make AFGP8 less dynamic in solution. Interestingly, DMSO induced a non-native structure, which could be determined via nuclear magnetic resonance (NMR) spectroscopy. The overall three-dimensional structures of the two AFGP8s from two different natural sources were different from a random coil ensemble, but their “compactness” was very similar, as deduced from NMR measurements. In addition to their similar compactness, the conserved motifs, Ala–Thr*–Pro–Ala and Ala–Thr*–Ala–Ala, present in both AFGP8s, seemed to have very similar three-dimensional structures, leading to a refined definition of local structural motifs. These local structural motifs allowed AFGPs to be considered functioning as effectors, making a transition from disordered to ordered upon binding to the ice surface. In addition, AFGPs could act as dynamic linkers, whereby a short segment folds into a structural motif, while the rest of the AFGPs could still be disordered, thus simultaneously interacting with bulk water molecules and the ice surface, preventing ice crystal growth.

Antifreeze Glycoproteins Bind Irreversibly to Ice

Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) inhibit ice growth via an adsorption-inhibition mechanism that assumes irreversible binding of AF(G)Ps to embryonic ice crystals and the inhibition of further growth. The irreversible binding of antifreeze glycoproteins (AFGPs) to ice has been questioned and remains poorly understood. Here, we used microfluidics and fluorescence microscopy to investigate the nature of the binding of small and large AFGP isoforms. We found that both AFGP isoforms bind irreversibly to ice, as evidenced by microfluidic solution exchange experiments. We measured the adsorption rate of the large AFGP isoform and found it to be 50% faster than that of AFP type III. We also found that the AFGP adsorption rate decreased by 65% in the presence of borate, a well-known inhibitor of AFGP activity. Our results demonstrate that the adsorption rate of AFGPs to ice is crucial for their ice growth inhibition capability.

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

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

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.

Interaction of ice binding proteins with ice, water and ions

Ice binding proteins (IBPs) are produced by various cold-adapted organisms to protect their body tissues against freeze damage. First discovered in Antarctic fish living in shallow waters, IBPs were later found in insects, microorganisms, and plants. Despite great structural diversity, all IBPs adhere to growing ice crystals, which is essential for their extensive repertoire of biological functions. Some IBPs maintain liquid inclusions within ice or inhibit recrystallization of ice, while other types suppress freezing by blocking further ice growth. In contrast, ice nucleating proteins stimulate ice nucleation just below 0 °C. Despite huge commercial interest and major scientific breakthroughs, the precise working mechanism of IBPs has not yet been unraveled. In this review, the authors outline the state-of-the-art in experimental and theoretical IBP research and discuss future scientific challenges. The interaction of IBPs with ice, water and ions is examined, focusing in particular on ice growth inhibition mechanisms.

Dynamical mechanism of antifreeze proteins to prevent ice growth

The fascinating ability of algae, insects, and fishes to survive at temperatures below normal freezing is realized by antifreeze proteins (AFPs). These are surface-active molecules and interact with the diffusive water-ice interface thus preventing complete solidification. We propose a dynamical mechanism on how these proteins inhibit the freezing of water. We apply a Ginzburg-Landau-type approach to describe the phase separation in the two-component system (ice, AFP). The free-energy density involves two fields: one for the ice phase with a low AFP concentration and one for liquid water with a high AFP concentration. The time evolution of the ice reveals microstructures resulting from phase separation in the presence of AFPs. We observed a faster clustering of pre-ice structure connected to a locking of grain size by the action of AFP, which is an essentially dynamical process. The adsorption of additional water molecules is inhibited and the further growth of ice grains stopped. The interfacial energy between ice and water is lowered allowing the AFPs to form smaller critical ice nuclei. Similar to a hysteresis in magnetic materials we observe a thermodynamic hysteresis leading to a nonlinear density dependence of the freezing point depression in agreement with the experiments.

Utilizing avidity to improve antifreeze protein activity: a type III antifreeze protein trimer exhibits increased thermal hysteresis activity

Antifreeze proteins (AFPs) are ice growth inhibitors that allow the survival of several species living at temperatures colder than the freezing point of their bodily fluids. AFP activity is commonly defined in terms of thermal hysteresis, which is the difference observed for the solution freezing and melting temperatures. Increasing the thermal hysteresis activity of these proteins, particularly at low concentrations, is of great interest because of their wide range of potential applications. In this study, we have designed and expressed one-, two-, and three-domain antifreeze proteins to improve thermal hysteresis activity through increased binding avidity. The three-domain type III AFP yielded significantly greater activity than the one- and two-domain proteins, reaching a thermal hysteresis of >1.6 °C at a concentration of <1 mM. To elucidate the basis of this increase, the data were fit to a multidomain protein adsorption model based on the classical Langmuir isotherm. Fits of the data to the modified isotherms yield values for the equilibrium binding constants for the adsorption of AFP to ice and indicate that protein surface coverage is proportional to thermal hysteresis activity.

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

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

Thermodynamic Analysis of Thermal Hysteresis: Mechanistic Insights into Biological Antifreezes

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

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.

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

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

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

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

A two-dimensional adsorption kinetic model for thermal hysteresis activity in antifreeze proteins

Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs), collectively abbreviated as AF(G)Ps, are synthesized by various organisms to enable their cells to survive in subzero environments. Although the AF(G)Ps are markedly diverse in structure, they all function by adsorbing to the surface of embryonic ice crystals to inhibit their growth. This adsorption results in a freezing temperature depression without an appreciable change in the melting temperature. The difference between the melting and freezing temperatures, termed thermal hysteresis (TH), is used to detect and quantify the antifreeze activity. Insights from crystallographic structures of a number of AFPs have led to a good understanding of the ice-protein interaction features. Computational studies have focused either on verifying a specific model of AFP-ice interaction or on understanding the protein-induced changes in the ice crystal morphology. In order to explain the origin of TH, we propose a novel two-dimensional adsorption kinetic model between AFPs and ice crystal surfaces. The validity of the model has been demonstrated by reproducing the TH curve on two different beta-helical AFPs upon increasing the protein concentration. In particular, this model is able to accommodate the change in the TH behavior observed experimentally when the size of the AFPs is increased systematically. Our results suggest that in addition to the specificity of the AFPs for the ice, the coverage of the AFPs on the ice surface is an equally necessary condition for their TH activity.

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.

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.

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

Antifreeze proteins (AFPs) protect organisms from freezing damage by inhibiting the growth of seed-ice crystals. It has long been hypothesized that irreversible binding of AFPs to ice surfaces is responsible for inhibiting the growth of seed-ice crystals as such a mechanism supports the popularly accepted Kelvin effect for the explanation of local freezing-point depression. However, whether the binding is reversible or irreversible is still under debate due to the lack of direct experimental evidence. Here, we report the first direct experimental result, by using the newly developed multiple quantum (MQ) filtering-spin exchange NMR experiment, that shows that the binding of HPLC6 peptides to ice surfaces is reversible. It was found that the reversible process can be explained by the model of monolayer adsorption. These results suggest that the Kelvin effect is not suitable for explaining the antifreeze mechanism, and direct interactions between the peptides and the ice-surface binding sites are the driving forces for the binding of AFPs to ice surfaces. We propose that there exists a concentration gradient of AFP from an ice-binding surface to the solution due to the affinity of ice surfaces to AFPs. This concentration gradient creates a dense layer of AFP in contact with the ice-binding surface, which depresses the local freezing point because of the colligative property, but not the Kelvin effect.

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.

Comparison of the solution conformation and dynamics of antifreeze glycoproteins from Antarctic fish

The (1)H- and (13)C-NMR spectra of antifreeze glycoprotein fractions 1-5 from Antarctic cod have been assigned, and the dynamics have been measured using (13)C relaxation at two temperatures. The chemical shifts and absence of non-sequential (1)H-(1)H NOEs are inconsistent with a folded, compact structure. (13)C relaxation measurements show that the protein has no significant long-range order, and that the local correlation times are adequately described by a random coil model. Hydroxyl protons of the sugar residues were observed at low temperature, and the presence of exchange-mediated ROEs to the sugar indicate extensive hydration. The conformational properties of AFGP1-5 are compared with those of the previously examined 14-mer analog AFGP8, which contains proline residues in place of some alanine residues (Lane, A. N., L. M. Hays, R. E. Feeney, L. M. Crowe, and J. H. Crowe. 1998. Protein Sci. 7:1555-1563). The infrared (IR) spectra of AFGP8 and AFGP1-5 in the amide I region are quite different. The presence of a wide distribution of backbone torsion angles in AFGP1-5 leads to a rich spectrum of frequencies in the IR spectrum, as interconversion among conformational states is slow on the IR frequency time scale. However, these transitions are fast on the NMR chemical shift time scales. The restricted motions for AFGP8 may imply a narrower distribution of possible o, psi angles, as is observed in the IR spectrum. This has significance for attempts to quantify secondary structures of proteins by IR in the presence of extensive loops.

Conformational and dynamic properties of a 14 residue antifreeze glycopeptide from Antarctic cod

The 1H and 13C NMR spectra of a 14-residue antifreeze glycopeptide from Antarctic cod (Tetramatomnus borchgrevinki) containing two proline residues have been assigned. 13C NMR relaxation experiments indicate motional anisotropy of the peptide, with a tumbling time in water at 5 degrees C of 3-4 ns. The relaxation data and lack of long-range NOEs are consistent with a linear peptide undergoing significant segmental motion. However, extreme values of some coupling constants and strong sequential NOEs indicate regions of local order, which are most evident at the two ATPA subsequences. Similar spectroscopic properties were observed in the 16-residue analogue containing an Arg-Ala dipeptide added to the C-terminus. Molecular modeling also showed no evidence of long-range order, but the two ATPA subsequences were relatively well determined by the experimental data. These motifs were quite distinct from helical structures or beta turns commonly found in proteins, but rather resemble sections of an extended polyproline helix. Thus, the NMR data provide a description of the local order, which is of relevance to the mechanism of action of the antifreeze activity of the antifreeze glycopeptides as well as their ability to protect cells during hypothermic storage.

Protein interaction with ice

Many organisms have evolved novel mechanisms to minimize freezing injury due to extracellular ice formation. This article reviews our present knowledge on the structure and mode of action of two types of proteins capable of ice interaction. The antifreeze proteins inhibit ice crystal formation and alter ice growth habits. The ice nucleation proteins, on the other hand, provide a proper template to stimulate ice growth. The potential applications of these proteins in different industries are discussed.

Modification of galactose and N-acetylgalactosamine residues by oxidation of C-6 hydroxyls to the aldehydes followed by reductive amination: model systems and antifreeze glycoproteins

Amino acids and peptides have been attached to the C-6 hydroxyls of the galactose and the N-acetylgalactosamine by first oxidizing the C-6 hydroxyls to the aldehydes by galactose oxidase in the presence of small amounts of catalase, followed by reductive amination (alpha-amino group) in the presence of cyanoborohydride. The activity of oxidized antifreeze glycoprotein was greater than 70% of the original, and considerable activity has been retained with some substitutions on reductive amination using cyanoborohydride. The following were some activities retained (as compared with the oxidized antifreeze glycoprotein): Gly, 64; (Gly)2, 88; (Gly)3, 82; (Gly)4, 70; Gly-Gly-NH2, 44; Gly-Glu, 13; Gly-Leu, 40; Gly-Tyr, 57; Gly-Gly-Leu, 50; Gly-Gly-Phe, 30; and Gly-Gly-Val, 35. On amino acid analysis of acid hydrolysates, some release of the amino acid attached by amination occurred; e.g., Gly-Tyr gave 0.26 Gly and 0.49 Tyr per disaccharide.

Molecular aspects of adaptation to extreme cold environments

Here are reviewed and summarized the strategies adopted by living organisms to survive low temperatures, from a molecular and membrane point of view. The presentation is aimed at a wide variety of readers. Two prime examples of connections between biological cold adaptation and the molecular level are (1) antifreeze proteins in fish from cold sea water, (the DNA sequence of the protein gene is now known) (2) the fluidity characteristics of cell membranes in a wide variety of organisms. In model membranes of phospholipids, stabler "s-phases" have recently been found to form at low temperatures. Antarctic endolithic organisms, living just under the surface of rocks, are exposed to long periods of low temperatures, and may develop such phases in their membranes. In the saturated phosphatidyl cholines, only lipids with a restricted range of acyl chain lengths show simultaneously s-phases and a main transition : This restricted range is about the restricted range found in natural membranes. The s-phases also form in the presence of natural cryoprotectants, and may be connected with botanical vernalization.