Antifreeze glycoproteins from Polar fish. Structural requirements for function of glycopeptide 8

The Modification of Polyvinyl Alcohol for Ice Nucleation Based upon the Structures of Antifreeze Glycoproteins Found in Antarctic Fish

Various alternative compounds have been investigated to prevent icing, one of which includes poly(vinyl) alcohol (PVA), which has shown promising anti-freeze effects. However, determining the optimal structures and formulations of PVA for anti-icing applications has remained a challenge. Building upon our previous work, which used molecular dynamics simulations to assess the effects of hydroxyl group separation distance on ice nucleation, in this work, PVA was modified based upon the structures of antifreeze glycoproteins (AFGPs) found in Antarctic fish, and examined as a potential antifreeze compound. Four different PVA samples with different degrees of hydrolysis were fabricated and subsequently examined for their effects on ice crystallization. The results showed that the modified PVA samples with degrees of hydrolysis of 76% and 66% had an effect on ice crystallization, delaying ice crystallization by an average of approximately 20 min, and even preventing ice crystallization altogether in a small portion of the sample. Meanwhile, other samples with degrees of hydrolysis of 100% and 34% either showed no effect on ice crystallization, shortened the ice crystallization time, and appeared to promote ice nucleation.

Computational Assessment of Modified Antifreeze Glycoproteins on Ice Nucleation

Antifreeze glycoproteins (AFGPs) found in various fish are used by the organisms to prevent freezing. While these compounds have been studied for their ability to bind to, and prevent the complete crystallization of water, the exact mechanisms by which AFGPs prevent freezing are still undetermined. Therefore, building upon our previous work, this study uses molecular dynamics simulations to assess the effects of hydroxyl group separation distance on AFGP ice nucleation activity. Water droplet crystallization simulations showed that modified AFGP structures containing hydroxyl distances smaller than ~3.0 Å lost their ability to prevent ice crystallization. Furthermore, modified AFGP containing hydroxyl distances of 7.327 Å and 6.160 Å was correlated with a promotion in ice nucleation, as demonstrated by the changes in the energy of the system. This supports the notion that the distance, and therefore, geometry characteristics between the hydroxyl groups located on the saccharide structures play a key role in the ice crystallization inhibition properties of AFGP compounds.

Antifreeze glycopeptides: from structure and activity studies to current approaches in chemical synthesis

Antifreeze glycopeptides (AFGPs) are a class of biological antifreeze agents found predominantly in Arctic and Antarctic species of fish. They possess the ability to regulate ice nucleation and ice crystal growth, thus creating viable life conditions at temperatures below the freezing point of body fluids. AFGPs usually consist of 4–55 repetitions of the tripeptide unit Ala–Ala–Thr that is O-glycosylated at the threonine side chains with β-d-galactosyl-(1 → 3)-α-N-acetyl-d-galactosamine. Due to their interesting properties and high antifreeze activity, they have many potential applications, e.g., in food industry and medicine. Current research is focused towards understanding the relationship between the structural preferences and the activity of the AFGPs, as well as developing time and cost efficient ways of synthesis of this class of molecules. Recent computational studies in conjunction with experimental results from NMR and THz spectroscopies were a possible breakthrough in understanding the mechanism of action of AFGPs. At the moment, as a result of these findings, the focus of research is shifted towards the analysis of behaviour of the hydration shell around AFGPs and the impact of water-dynamics retardation caused by AFGPs on ice crystal growth. In the field of organic synthesis of AFGP analogues, most of the novel protocols are centered around solid-phase peptide synthesis and multiple efforts are made to optimize this approach. In this review, we present the current state of knowledge regarding the structure and activity of AFGPs, as well as approaches to organic synthesis of these molecules with focus on the most recent developments.

Perturbation of long-range water dynamics as the mechanism for the antifreeze activity of antifreeze glycoprotein

Very little is known about the mechanism of antifreeze action of antifreeze glycoproteins (AFGPs) present in Antarctic teleost fish. Recent NMR and CD studies assisted with total synthesis of synthetic AFGP variants have provided insight into the structure of short AFGP glycopeptides, though the observations did not yield information on the antifreeze mechanism of action. In this study, we use Hamiltonian replica exchange (HREX) molecular dynamics simulations to probe the structure and surrounding aqueous environments of both the natural (AFGP8) and synthetic (s-AFGP4) AFGPs. AFGPs can adopt both amphiphilic and pseudoamphiphilic conformations, the preference of which is related to the proline content of the peptide. The arrangement of carbohydrates allows the hydroxyl groups on terminal galactose units to form stable water bridges which in turn influence the hydrogen-bond network, structure, and dynamics of the surrounding solvent. Interestingly, these local effects lead to the perturbation of the tetrahedral environment for water molecules in hydration layers far (10.0-12.0 Å) from the AFGPs. This structure-induced alteration of long-range hydration dynamics is proposed to be the major contributor to antifreeze activity, a conclusion that is in line with terahertz spectroscopy experiments. The detailed structure-mechanism correlation provided in this study could lead to the design of better synthetic AFGP variants.

Designing ice recrystallization inhibitors: from antifreeze (glyco)proteins to small molecules

Ice recrystallization occurs during cryopreservation and is correlated with reduced cell viability after thawing.

Expression of Ixodes scapularis antifreeze glycoprotein enhances cold tolerance in Drosophila melanogaster

Drosophila melanogaster experience cold shock injury and die when exposed to low non-freezing temperatures. In this study, we generated transgenic D. melanogaster that express putative Ixodes scapularis antifreeze glycoprotein (IAFGP) and show that the presence of IAFGP increases the ability of flies to survive in the cold. Male and female adult iafgp-expressing D. melanogaster exhibited higher survival rates compared with controls when placed at non-freezing temperatures. Increased hatching rates were evident in embryos expressing IAFGP when exposed to the cold. The TUNEL assay showed that flight muscles from iafgp-expressing female adult flies exhibited less apoptotic damage upon exposure to non-freezing temperatures in comparison to control flies. Collectively, these data suggest that expression of iafgp increases cold tolerance in flies by preventing apoptosis. This study defines a molecular basis for the role of an antifreeze protein in cryoprotection of flies.

Synthesis of an Isotopically-labelled Antarctic Fish Antifreeze Glycoprotein Probe

Antifreeze glycoproteins (AFGPs) are glycosylated polypeptides produced by Antarctic and Arctic fishes, which allow them to survive in seawater at sub-zero temperatures. An investigation into the postulated enteric uptake of AFGP synthesized in the exocrine pancreas of Antarctic fishes required a custom-prepared AFGP probe that incorporated seven isotopically-labelled Ala residues for detection by mass spectrometry. The AFGPs are composed of a repetitive three amino acid unit (Ala-Ala-Thr), in which the threonine residue is glycosylated with the disaccharide β-d-Gal-(1→3)-α-d-GalNAc. The synthesis of isotopically-labelled AFGP8 (1), as well as the optimized synthesis of the protected glycosylated amino acid building block 2, is reported.

Anaplasma phagocytophilum induces Ixodes scapularis ticks to express an antifreeze glycoprotein gene that enhances their survival in the cold

In the United States, Ixodes scapularis ticks overwinter in the Northeast and Upper Midwest and transmit the agent of human granulocytic anaplasmosis, Anaplasma phagocytophilum, among other pathogens. We now show that the presence of A. phagocytophilum in I. scapularis ticks increases their ability to survive in the cold. We identified an I. scapularis antifreeze glycoprotein, designated IAFGP, and demonstrated via RNAi knockdown studies the importance of IAFGP for the survival of I. scapularis ticks in a cold environment. Transfection studies also show that IAFGP increased the viability of yeast cells subjected to cold temperature. Remarkably, A. phagocytophilum induced the expression of iafgp, thereby increasing the cold tolerance and survival of I. scapularis. These data define a molecular basis for symbiosis between a human pathogenic bacterium and its arthropod vector and delineate what we believe to be a new pathway that may be targeted to alter the life cycle of this microbe and its invertebrate host.

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.

Characterization of the restricted rotation of the dimethyl groups in chemically N-terminal 13C-labeled antifreeze glycoproteins: a temperature-dependent study in water to ice through the supercooled state

Site-specific chemical modification, especially with isotopically enriched groups, allows one to study the structure and dynamics of proteins for which uniform enrichment is difficult. When the N-terminal alanine in antifreeze glycoprotein (AFGP) is replaced with an N,N-dimethyl alanine the methyl groups show signatures of slow rotation about the C-N bond. In order to separate the local dynamics of the N-terminus from the overall protein dynamics, we present a complete characterization of this dynamics. Temperature-dependent nuclear magnetic-resonance experiments from room temperature to subzero temperatures, including the supercooled state and in the presence of ice, are presented. Quantum chemical calculations are also performed on a localized N-terminus of the AFGP. Our results show that in the solution state at room temperature and in the super cooled regime, the dimethyl groups undergo a slow, restricted rotation with an unequal distribution of population between two major conformations. At lower temperatures in the presence of ice, the dynamics become much more complex due to freezing out of several conformational states. Based on these results, we conclude that the segmental dynamics of the N-terminus are local to the first residue and do not affect the overall dynamics of the protein.

'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.

Dynamics of antifreeze glycoproteins in the presence of ice

Antifreeze glycoproteins from the Greenland cod Boreogadus saida were dimethylated at the N-terminus (m*AFGP) and their dynamics and conformational properties were studied in the presence of ice using (13)C-NMR and FTIR spectroscopy. (13)C-NMR experiments of m*AFGP in D(2)O, in H(2)O, and of freeze-dried m*AFGP were performed as a function of temperature. Dynamic parameters ((1)H T(1 rho) and T(CH)) obtained by varying the contact time revealed notable differences in the motional properties of AFGP between the different states. AFGP/ice dynamics was dominated by fast-scale motions (nanosecond to picosecond time scale), suggesting that the relaxation is markedly affected by the protein hydration. The data suggest that AFGP adopts a similar type of three-dimensional fold both in the presence of ice and in the freeze-dried state. FTIR studies of the amide I band did not show a single prevailing secondary structure in the frozen state. The high number of conformers suggests a high flexibility, and possibly reflects the necessity to expose more ice-binding groups. The data suggest that the effect of hydration on the local mobility of AFGP and the lack of significant change in the backbone conformation in the frozen state may play a role in inhibiting the ice crystal growth.

Antifreeze glycoproteins: relationship between molecular weight, thermal hysteresis and the inhibition of leakage from liposomes during thermotropic phase transition

Antifreeze glycoproteins (AFGP) were isolated and purified from the blood plasma of rock cod (Gadus ogac), using DEAE-Bio-gel ion exchange chromatography, followed by high performance liquid chromatography (HPLC). The purified proteins were analyzed using polyacrylamide gel electrophoresis (PAGE), and electrospray mass spectrometry. The results indicated that rock cod synthesize seven size classes of glycoproteins, ranging from 2.6 to 24 kDa, with each size class containing multiple isoforms. Antifreeze activity, as determined by thermal hysteresis, indicated that the AFGP could be separated into two groups, with the larger size classes (molecular mass>13 kDa) having approximately 3-4 times the activity of the smaller, proline containing, size classes (molecular mass<10 kDa). All of the AFGP size classes prevented leakage from dielaidoylphosphatidylcholine (DEPC) liposomes as they were cooled through their phase transition temperature, with the larger size classes being approximately 4 times as effective as the smaller ones. It is hypothesized that AFGP prevent liposomes from leaking as they pass through the phase transition temperature by binding to the phospholipid membrane.

Egg-white and blood-serum proteins functioning by noncovalent interactions: studies by chemical modification and comparative biochemistry

Some of the more interesting and important proteins are those that function by forming associations or complexes with other substances. The structure-function relationships of three of these with very different substances are transferrins, which chelate metal ions; avian ovomucoids, which form complexes with proteolytic enzymes; and antifreeze glycoproteins, which interact at the ice-solution interface. Interrelating studies on the comparative biochemistry with studies using chemical modification have helped identify the side-chain groups of the proteins involved in function as well as to be useful for studies on general protein chemistry. The most strongly associated interaction is the chelation of iron by transferrin, with an association constant of approximately 10(21); tyrosines, histidines, and sometimes aspartate are involved. For ovomucoids, individual substratelike residues such as lysine are involved in a Michaelis-like complex, and association constants are as high as 10(10). By contrast, the antifreeze glycoproteins appear to function by a polymeric interaction at the surface of ice, with a much weaker association.

Protection against heat-induced cell killing by alanine

When L-alanine was added either to full growth medium or to Hanks' balanced salt solution (HBSS) prior to hyperthermia, survival of heated cells was significantly increased in a concentration-dependent manner. Maximal heat protection was not immediate, but required at least 1 h at 37 degrees C incubation prior to heating. Heat protection was principally reflected in an increased Dq on the 45 degrees C survival curve; for example, with 100 mM L-alanine, the Dq increased from approximately equal to 20 (control) to 30 min at 45 degrees C. Hyperthermia of 1 h at temperatures between 42 degrees C and 45 degrees C indicated that 100 mM alanine had shifted the isotoxic temperature by 0.5 degrees C. Comparable heat protection was also observed with D-alanine and amino acid dimers, such as alanyl-alanine or alanyl-leucine. Leucine at similar concentrations by itself, without alanine, did not protect cells against heat killing, but increased cellular heat sensitivity. The data suggest that heat protection by alanine does not require incorporation of alanine into cellular protein, but is mediated by the free amino acid.

Relationship of amino acid composition and molecular weight of antifreeze glycopeptides to non-colligative freezing point depression

Many polar fishes synthesize a group of eight glycopeptides that exhibit a non-colligative lowering of the freezing point of water. These glycopeptides range in molecular weight between 2600 and 33 700. The largest glycopeptides [1-5] lower the freezing point more than the small ones on a weight basis and contain only two amino acids, alanine and threonine, with the disaccharide galactose-N-acetyl-galactosamine attached to threonine. The small glycopeptides, 6, 7, and 8, also lower the freezing point and contain proline, which periodically substitutes for alanine. Glycopeptides with similar antifreeze properties isolated from the saffron cod and the Atlantic tomcod contain an additional amino acid, arginine, which substitutes for threonine in glycopeptide 6. In this study we address the question of whether differences in amino acid composition or molecular weight between large and small glycopeptides are responsible for the reduced freezing point depressing capability of the low molecular weight glycopeptides. The results indicate that the degree of amino acid substitutions that occur in glycopeptides 6-8 do not have a significant effect on the unusual freezing point lowering and that the observed decrease in freezing point depression with smaller glycopeptides can be accounted for on the basis of molecular weight.

Penguin egg-white and polar fish blood-serum proteins

The development of, and findings in, a long-term research program on penguin proteins and polar fish blood proteins are described. Two of the egg-white proteins from the Adelie penguin (Pygoscelis adeliae) have unique properties: a glycoprotein named penalbumin that is a major constituent with some characteristics similar to ovalbumin, and an ovomucoid with strong inhibitory capacity for subtilisin as well as for bovine trypsin and alpha-chymotrypsin. The antifreeze glycoproteins from Antarctic fish (Trematomus borchgrevinki and Dissostichus mawsoni) and an Arctic fish (Boreogadus saida) appear to function noncolligatively by lowering the freezing temperature without affecting the melting point. Current evidence indicates that the antifreeze glycoprotein functions at the ice-solution interface, either on the ice surface or in a transition layer between the solution and the ice.