'Antifreeze' glycoproteins from polar fish

Intramolecular hydrogen bonding and cooperative interactions in carbohydrates via the molecular tailoring approach

In spite of many theoretical and experimental attempts for understanding intramolecular hydrogen bonding (H-bonding) in carbohydrates, a direct quantification of individual intramolecular H-bond energies and the cooperativity among the H-bonded networks has not been reported in the literature. The present work attempts, for the first time, a direct estimation of individual intramolecular O-H...O interaction energies in sugar molecules using the recently developed molecular tailoring approach (MTA). The estimated H-bond energies are in the range of 1.2-4.1 kcal mol(-1). It is seen that the OH...O equatorial-equatorial interaction energies lie between 1.8 and 2.5 kcal mol(-1), with axial-equatorial ones being stronger (2.0-3.5 kcal mol(-1)). The strongest bonds are nonvicinal axial-axial H-bonds (3.0-4.1 kcal mol(-1)). This trend in H-bond energies is in agreement with the earlier reports based on the water-water H-bond angle, solvent-accessible surface area (SASA), and (1)H NMR analysis. The contribution to the H-bond energy from the cooperativity is also estimated using MTA. This contribution is seen to be typically between 0.1 and 0.6 kcal mol(-1) when H-bonds are a part of a relatively weak equatorial-equatorial H-bond network and is much higher (0.5-1.1 kcal mol(-1)) when H-bonds participate in an axial-axial H-bond network.

Pattern formation of antifreeze glycoproteins via solvent evaporation

Surface patterning of antifreeze glycoprotein fraction 8 (AFGP 8) via a solvent evaporation method is reported here. In this process, lines of AFGP 8 particles and gridlike patterns were formed as as result of the receding of the droplet contact line and the accumulation of the solute during evaporation. The solution concentration strongly affects the protein line spacing. The average height of the protein was measured to be 8.1 +/- 2.5 A, which may be attributed to the height of a single molecule.

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.

Glycal Glycosylation and 2-Nitroglycal Concatenation, a Powerful Combination for Mucin Core Structure Synthesis

A 3,4-O-unprotected galactal derivative having bulky 6-O-TIPS protection (compound 2) could be regioselectively 3-O-glycosylated with O-(galactopyranosyl) trichloroacetimidates; depending on the protecting group pattern stereoselectively alpha- and beta-linked disaccharides were obtained. With O-(2-azido-2-deoxyglucopyransyl) trichloroacetimidate as donor (compound 10A), glycosylation of 2 and of a 6-O-unprotected galactal derivative led in acetonitrile as solvent exclusively to a beta(1-3)- and a beta(1-6)-linked disaccharide, respectively. Nitration of the galactal moieties of the saccharides followed by Michael-type addition of serine and threonine derivatives (7a,b) installed the alpha-galacto-configuration, thus readily furnishing O-glycosyl amino acid building blocks for the incorporation of core 1, core 2, core 3, core 6, and core 8 structures into glycopeptides. 2-Nitrogalactal and 2-nitroglucal derivatives could be also successfully employed in glycoside bond formation via Michael-type addition in a reiterative manner, affording the corresponding core 5, core 7, and core 6 building blocks. In this approach, highly stereoselective glycoside bond formations were based exclusively on Michael-type addition to the nitro-enol ether moiety of the 2-nitroglycals. Hence, 2-nitroglycals are versatile intermediates for base-catalyzed glycoside bond formation.

In vitro studies of antifreeze glycoprotein (AFGP) and a C-linked AFGP analogue

Antifreeze glycoproteins (AFGPs) are a subclass of biological antifreezes found in deep sea Teleost fish. These compounds have the ability to depress the freezing point of the organism such that it can survive the subzero temperatures encountered in its environment. This physical property is very attractive for the cryopreservation of cells, tissues, and organs. Recently, our laboratory has designed and synthesized a functional carbon-linked (C-linked) AFGP analogue (1) that demonstrates tremendous promise as a novel cryoprotectant. Herein we describe the in vitro effects and interactions of C-linked AFGP analogue 1 and native AFGP 8. Our studies reveal that AFGP 8 is cytotoxic to human embryonic liver and human embryonic kidney cells at concentrations higher than 2 and 0.63 mg/mL, respectively, whereas lower concentrations are not toxic. The mechanism of this cytotoxicity is consistent with apoptosis because caspase-3/7 levels are significantly elevated in cell cultures treated with AFGP 8. In contrast, C-linked AFGP analogue 1 displayed no in vitro cytotoxicity even at high concentrations, and notably, caspase-3/7 activities were suppressed well below background levels in cell lines treated with 1. Although the results from these studies limit the human applications of native AFGP, they illustrate the benefits of developing functional C-linked AFGP analogues for various medical, commercial and industrial applications.

Ice Recrystallization Inhibition and Molecular Recognition of Ice Faces by Poly(vinyl alcohol)

The effects of poly(vinyl alcohol) (PVA) on the Ostwald ripening of polycrystalline ice samples are studied. At -6 degrees C, ice recrystallization in sucrose solutions is inhibited at PVA concentrations down to 0.005 mg mL(-1), with a recrystallization inhibition constant of 48.9 mL mg(-1). Ice growth-habit experiments reveal molecular recognition of the arrangement of water molecules in the ice by PVA molecules, and indicate that PVA molecules adsorb to the primary and secondary prism faces of hexagonal ice, Ih. Based on these observations, together with an analysis of the O-atom pattern in ice and the conformation of OH groups in PVA, an adsorption model is proposed. We suggest that PVA segments adsorb to the primary and secondary prism faces of ice parallel to the c axis with a linear misfit parameter of only 2.7 %, most likely via multiple hydrogen bonds. The proposed adsorption mechanism is discussed in the light of recent thermal hysteresis and scanning tunneling microscopy experiments.

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.

Scanning electron microscopy of Anisakis larvae following different treatments

Ingestion of fish parasitized with Anisakis larvae can produce infestation and/or allergy in consumers. Technological and food processing treatments have been applied to parasitized fish in order to kill the larvae and avoid the infestation; however, their influence on allergenicity has not been studied. Four lots of hake (Merluccius merluccius) steaks artificially parasitized with Anisakis larvae were subjected to two storage chilling (5 degrees C +/- 1 degrees C) and freezing (-20 degrees C +/- 1 degrees C) treatments and two food processing treatments of heat (final temperature 86.3 degrees C) and microwave (final temperature 66.9 degrees C) and studied by scanning electron microscopy, environmental scanning electron microscopy (ESEM) (acid [pH = 2] and water preparations), and emission of fluorescence. Anisakis larvae were resistant to acid conditions, remaining alive after treatment. Larvae in the heat- and microwave-treated lots presented coagulated and disrupted zones in the cuticle with release of fluids. The cylindrical shape changed to a dehydrated appearance mainly observed by ESEM. Fluorescence was only noticeable in the frozen larvae. Larvae without apparent changes, together with dehydrated ones, were observed by ESEM in the frozen lot; nevertheless, no disruptions in the cuticle were perceptible. Further studies are needed in order to elucidate if the changes observed in the cuticle reduce the resistance of the parasites to the action of gastric enzymes in the gastrointestinal tract and to determine the release of allergens to the flesh by the live larvae during chilled storage of the fish.

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.

Aggregation of antifreeze glycoprotein fraction 8 and its effect on antifreeze activity

Antifreeze glycoproteins (AFGPs) have many potential applications ranging from the cryopreservation and hypothermic storage of tissues and organs to the preservation of various frozen food products. Since supplying native AFGP for these applications is a labor-intensive and costly process, the rational design and synthesis of functional AFGP analogues is a very attractive alternative. While structure-function studies have implicated specific structural motifs as essential for antifreeze activity in AFGP, the relationship between solution conformation and antifreeze activity is poorly understood. Toward this end, we have analyzed AFGP8 in aqueous solutions using dynamic light scattering (DLS) and circular dichroism (CD). Our results indicate that AFGP8 forms discrete aggregates in solution. These aggregates are predominantly composed of dimers that form at solution concentrations greater than 20 mM. CD spectroscopy indicates that the preferred solution conformation of AFGP8 is consistent with that of random coil. However, significant beta-sheet and alpha-helix character is observed in more concentrated solutions, indicating that these glycopeptides are highly flexible in solution. Aggregation appears to have a minimal effect on the overall solution conformation. Thermal hysteresis (TH) activity of the aggregated solutions is much higher than that of less concentrated solutions that do not form aggregates. While cooperative functioning between lower and higher molecular weight AFGPs has been reported, this is the first instance where cooperative functioning in lower molecular weight AFGPs has been observed.

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.

Effect of a mutation on the structure and dynamics of an α-helical antifreeze protein in water and ice

One strategy of psychrophilic organisms to survive subzero temperature is to produce antifreeze protein (AFPs), which inhibit the growth of macromolecular ice. To better understand the binding mechanism, the structure and dynamics of several AFPs have been studied by nuclear magnetic resonance (NMR) and X-ray crystallography. The results have shown that different organisms can use diverse structures (alpha-helix, beta-helix, or different globular folds) to achieve the same function. A number of studies have focused on understanding the relationship between the alpha-helical structure of fish type I AFP and its function as an inhibitor of ice growth. The results have not explained whether the 90% activity loss caused by the conservative mutation of two threonines to serines (Thr13Ser/Thr24Ser) is attributable to a change in protein structure in solution or in ice. We examine here the structure and dynamics of the winter flounder type I AFP and the mutant Thr13Ser/Thr24Ser in both solution and solid states using a wide range of NMR approaches. Both proteins remain fully alpha-helical at all temperatures and in ice, demonstrating that the activity change must therefore not be attributable to changes in the protein fold or dynamics but differences in surface properties.

Heterologous expression of type I antifreeze peptide GS-5 in baker's yeast increases freeze tolerance and provides enhanced gas production in frozen dough

The demand for frozen-dough products has increased notably in the baking industry. Nowadays, no appropriate industrial baker's yeast with optimal gassing capacity in frozen dough is, however, available, and it is unlikely that classical breeding programs could provide significant improvements of this trait. Antifreeze proteins, found in diverse organisms, display the ability to inhibit the growth of ice, allowing them to survive at temperatures below 0 degrees C. In this study a recombinant antifreeze peptide GS-5 was expressed from the polar fish grubby sculpin (Myoxocephalus aenaeus) in laboratory and industrial baker's yeast strains of Saccharomyces cerevisiae. Production of the recombinant protein increased freezing tolerance in both strains tested. Furthermore, expression of the GS-5 encoding gene enhanced notably the gassing rate and total gas production in frozen and frozen sweet doughs. These effects are unlikely to be due to reduced osmotic damage during freezing/thawing, because recombinant cells showed growth behavior similar to that of the parent under hypermosmotic stress conditions.

Carbohydrate Intramolecular Hydrogen Bonding Cooperativity and Its Effect on Water Structure

Molecular dynamics (MD) simulations combined with water-water H-bond angle analysis and calculation of solvent accessible surface area and approximate free energy of solvation were used to determine the influence of hydroxyl orientation on solute hydration and surrounding water structure for a group of chemically identical solutes-the aldohexopyranose sugars. Intramolecular hydrogen bond cooperativity was closely associated with changes in water structure surrounding the aldohexopyranose stereoisomers. The OH-4 group played a pivotal role in hydration as it was able to participate in a number of hydrogen bond networks utilizing the OH-6 group. Networks that terminated within the molecule (OH-4 --> OH-6 --> O-5) had relatively more nonpolar-like hydration than those that ended in a free hydroxyl group (OH-6 --> OH-4 --> OH-3). The OH-2 group modulated the strength of OH-4 networks through syndiaxial OH-2/4 intramolecular hydrogen bonding, which stabilized and induced directionality in the network. Other syndiaxial interactions, such as the one between OH-1 and OH-3, only indirectly affected water structure. Water structure surrounding hydrogen bond networks is discussed in terms of water-water hydrogen bond populations. The impact of syndiaxial versus vicinal hydrogen bonds is also reviewed. The results suggest that biological events such as protein-carbohydrate recognition and cryoprotection by carbohydrates may be driven by intramolecular hydrogen bond cooperativity.

Activities of antioxidant enzymes and cytochrome c oxidase in liver of Arctic and temperate teleosts

Enhanced antioxidant status in polar fishes may occur due to high dissolved oxygen levels and membranes rich in peroxidation-sensitive polyunsaturated fatty acids. To evaluate the importance of antioxidant enzymes in polar fishes, activities of catalase (CAT), superoxide dismutase (SOD), and glutathione reductase (GR), as well as the aerobic enzyme cytochrome c oxidase (CCO), were measured at 6 degrees C and 1 degrees C in livers of confamilial Arctic and temperate teleosts: the Arctic fourhorn sculpin Triglopsis quadricornis (Cottidae) and saddled eelpout Lycodes mucosus (Zoarcidae) vs. the temperate longhorn sculpin (Myoxocephalus octodecimspinosus) (Cottidae) and ocean pout (Zoarces americanus) (Zoarcidae), respectively. At both assay temperatures, CAT activities were substantially lower in both Arctic species, SOD was similar in the cottids but lower in the Arctic zoarcid, and GR was similar in temperate and Arctic fishes. Activities at respective habitat temperatures were always significantly lower in the Arctic fishes. The lower antioxidant enzyme activities in the Arctic fishes cannot be attributed to lower aerobic status because CCO activity was similar or higher in the Arctic fishes; significant negative relationships were found between CCO and CAT and GR (but not SOD) when all species were combined, indicating that a higher apparent aerobic status does not necessarily coincide with higher antioxidant enzyme activities. Antioxidant enzyme activities may not be enhanced as part of cold adaptation in Arctic fishes, at least in the liver.

An interfacial energy mechanism for the complete inhibition of crystal growth by inhibitor adsorption

We present a unified model for complete crystal-growth inhibition based on the thermodynamics of interfaces. The premise for our model is that the adsorption of inhibitor leads to a reduction in interfacial tension or edge energy for the crystal surface or step, respectively. In our formulation, the work to add a layer or grow a step increases due to the difference in interfacial tensions or edge energies for surfaces with and without an adsorbed inhibitor. For a large enough difference in interfacial tensions or edge energies, complete inhibition of growth is realized when the total work does not decrease as more crystals are formed. We demonstrate that our model can provide a theoretical description of critical subcooling data for ice with antifreeze proteins and glycoproteins, critical subcooling data for hydrates and ionic crystals, and critical supersaturation data for various crystal systems.

Mimicking the Properties of Antifreeze Glycoproteins: Synthesis and Characterization of a Model System for Ice Nucleation and Antifreeze Studies

Synthesis of beta-D-Gal-(1 --> 3)-beta-D-GalNAc coupled to HOC2H4NHCOC15H30SH is described. This compound was coadsorbed at various proportions with C2H5OC2H4NHCOC15H30SH to form statistically mixed self-assembled monolayers (SAMs) on gold in an attempt to mimic the properties of the active domain in antifreeze glycoproteins (AFGPs). The monolayers were characterized by null ellipsometry, contact angle goniometry, X-ray photoelectron spectroscopy, and infrared reflection-absorption spectroscopy. The disaccharide compound adsorbed preferentially, and SAMs prepared at a solution molar ratio >0.3 displayed total wetting. The mixed SAMs showed well-organized alkyl chains up to a disaccharide surface fraction of 0.8. The amount of gauche conformers in the alkyls increased rapidly above this point, and the monolayers became disordered and less densely packed. Furthermore, the generated mixed SAMs were subjected to water vapor at constant relative humidity and the subsequent ice crystallization on a cooled substrate was monitored via an optical microscope. Interestingly, rapid crystallization occurred within a narrow range of temperatures on mixed SAMs with a high disaccharide content, surface fraction >0.3. The reported crystallization temperatures and the ice layer topography were compared with results obtained for a much simpler reference system composed of -OH/-CH3 terminated n-alkanethiols in order to account for changes in topography of the water/ice layer with surface energy. Although preliminary, the obtained results can be useful in the search for the molecular mechanism behind the antifreeze activity of AFGPs.

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.

Recent advances in sea-ice microbiology

Over the past 50 years there has been much effort invested in the investigation of the ecology of sea ice. Sea ice is an ephemeral feature of the Arctic and Southern Oceans and smaller water bodies such as the Baltic and Caspian Seas. The semisolid ice matrix provides a range of habitats in which a diverse range of microbial organisms thrive. In the past 5 years there has been considerable steps forward in sea-ice research, in particular regarding the analysis of sea-ice microstructure and the investigation of the diversity and adaptation of microbial communities. These studies include: (i) controlled simulated and in situ studies on a micrometer scale to unravel the dynamic of the microhabitat with consequences for the organisms; (ii) the introduction of molecular approaches to uncover the diversity of uncultured still unknown microorganisms; and (iii) studies into the molecular adaptation of selected model organisms to the extreme environment. This minireview presents some of the most recent findings from sea-ice studies within the framework of these aims.

The anti-cell death FNK protein protects cells from death induced by freezing and thawing

The FNK protein, constructed from anti-apoptotic Bcl-xL with enhanced activity, was fused with the protein transduction domain (PTD) of the HIV/Tat protein to mediate the delivery of FNK into cells. The fusion protein PTD-FNK was introduced into chondrocytes in isolated articular cartilage-bone sections, cultured neurons, and isolated bone marrow mononuclear cells to evaluate its ability to prevent cell death induced by freezing and thawing. PTD-FNK protected the cells from freeze-thaw damage in a concentration-dependent manner. Addition of PTD-FNK with conventional cryoprotectants (dimethyl sulfoxide and hydroxyethyl starch) increased surviving cell numbers around 2-fold compared with controls treated only with the cryoprotectants. Notably, PTD-FNK allowed CD34+ cells among bone marrow mononuclear cells to survive more efficiently (12-fold more than the control cells) from two successive freeze-thaw cycles. Thus, PTD-FNK prevented cell death induced by freezing and thawing, suggesting that it provides for the successful cryopreservation of biological materials.

Ice Surface Reconstruction as Antifreeze Protein-Induced Morphological Modification Mechanism

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

Cold survival in freeze-intolerant insects

Antifreeze proteins (AFPs) designate a class of proteins that are able to bind to and inhibit the growth of macromolecular ice. These proteins have been characterized from a variety of organisms. Recently, the structures of AFPs from the spruce budworm (Choristoneura fumiferana) and the yellow mealworm (Tenebrio molitor) have been determined by NMR and X-ray crystallography. Despite nonhomologous sequences, both proteins were shown to consist of beta-helices. We review the structures and dynamics data of these two insect AFPs to bring insight into the structure-function relationship and explore their beta-helical architecture. For the spruce budworm protein, the fold is a left-handed beta-helix with 15 residues per coil. The Tenebrio molitor protein consists of a right-handed beta-helix with 12 residues per coil. Mutagenesis and structural studies show that the insect AFPs present a highly rigid array of threonine residues and bound water molecules that can effectively mimic the ice lattice. Comparisons of the newly determined ryegrass and carrot AFP sequences have led to models suggesting that they might also consist of beta-helices, and indicate that the beta-helix might be used as an AFP structural motif in nonfish organisms.