Solution structure of antifreeze glycopeptides

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.

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.

Preferred conformations and dynamics of five core structures of mucin type O-glycans determined by NMR spectroscopy and force field calculations

Glycosyltransferases acting on O-glycans have been shown to exhibit distinct specificity for the carbohydrate and the peptide moiety of their substrates. As an approach to study the 3-dimensional interactions between enzymes and O-glycan substrates, we determined the preferred conformations of five oligosaccharide-core structures of mucin type glycoproteins by NMR spectroscopy and by static and dynamic force field calculations. Seven oligosaccharides, representing five basic core structures, were investigated: Gal beta (1-3)GalNAc alpha Bzl (1, core 1), GlcNAc beta (1-6)[Gal beta (1-3)]GalNAc alpha Bzl (2, core 2), GlcNAc beta (1-3)GalNAc alpha Bzl (3, core 3), GlcNAc beta (1-6)[GlcNAc beta (1-3)]GalNAc alpha Bzl (4, core 4), GlcNAc beta (1-6)GalNAc alpha Bzl (5, core 6), the elongated core 2, Gal beta (1-4)GlcNAc beta (1-6)[Gal beta (1-3)]GalNAc alpha pNp (6) and GalNAc alpha-Bzl (7). The dynamic behaviour of the molecules was studied by Metropolis Monte Carlo (MMC) simulations. Experimental coupling constants, chemical shift changes, and NOEs were compared with results from static energy minimizations and dynamic MMC simulations and show a good agreement. MMC simulations show that the (1-6) linkage is much more flexible than the (1-3) or the (1-4) linkages. The preferred conformations of the disaccharides (1) and (3) show only slight differences due to the additional N-acetyl group in (3). The conformational equilibrium of beta (1-3) glycosidic bonds of 1 and 3 was not affected by attaching a beta (1-6) linked GlcNAc unit to the GalNAc residue in 2 and 4. However, experimental and theoretical data show that the beta (1-6) linkages of the trisaccharides 2 and 4, which carry an additional beta (1-3) linked glycosyl residue, change their preferred conformations when compared with (5). The 6-branch also shows significant interactions with the benzyl aglycon altering the preferred conformation of the hydroxymethyl group of the GalNAc to a higher proportion of the gt conformer. The (1-6) linkage of 2, 4, and 6 can have two different families of conformations of which the lower energy state is populated only to about 20% of the time whereas the other state with a relative enthalpy of approximately 4 kcal mol-1 is populated to 80%. This fact demonstrates that the two conformational states have different entropy contents. Entropy is implicitly included in MMC simulations but cannot be derived from energy minimizations.

Cryotoxicity of antifreeze proteins and glycoproteins to spinach thylakoid membranes--comparison with cryotoxic sugar acids

We have used thylakoids from spinach (Spinacia oleracea L.) chloroplasts to test the effects of antifreeze proteins (AFP) from the starry flounder (Platichthys stellatus; AFP-SF) and from the antarctic eel pout (Austrolycichthys brachycephalus; AFP-AB), and antifreeze glycoproteins (AFGP) from the antarctic fish Dissostichus mawsoni on biological membranes during freezing. Freeze-thaw damage, measured as the release of the lumenal protein plastocyanin from the thylakoid vesicles, was strongly increased in the presence of all proteins tested. Measurements of the time dependence of plastocyanin release in a simplified artificial chloroplast stroma medium showed that all the fish proteins increased damage during the initial rapid phase while only AFGP increased plastocyanin release during the linearly time dependent slow phase. A slow plastocyanin release is also seen in the absence of freezing. It is increased by the presence of AFGP and AFP-AB, but not by AFP-SF. In order to distinguish between the contribution of the polypeptide and the carbohydrate part of AFGP on freeze-thaw damage we investigated the effects of galactose and N-acetylgalactosamine. While galactose was protective, N-acetylgalactosamine increased the rate of plastocyanin release in an artificial stroma medium at -20 degrees C. It had no effect on the rapid phase of damage and was also ineffective at 0 degree C. The same was found for several other sugar derivatives (N-acetylglucosamine, gluconic acid, glucuronic acid, galacturonic acid). From these data we conclude that the increased plastocyanin release during the rapid phase of freeze-thaw damage is a function of the polypeptide part of AFGP. The increased rate of plastocyanin loss at longer incubation times both at 0 degree C and at -20 degrees C may be mediated by the N-acetylgalactosamine moiety of the AFGP, but is strongly amplified by the polypeptide.

Carbohydrate analysis of glycoproteins. A review

Many of the products prepared by biotechnological approaches, including recombinant genetic engineering, cell tissue culture, and monoclonal technologies, are glycoproteins. As little as five years ago, glycosylation was believed to play no significant role in the function of glycoproteins. Recent large scale testing of glycoprotein-based pharmaceuticals has indicated that both the extent and type of glycosylation can play a central role in glycoprotein activity. Although methods for compositional and sequence analysis of proteins and nucleic acids are generally available, similar methods have yet to be developed for carbohydrate oligomers and polymers. This review focuses on new, developing methods for the analysis and sequencing of the carbohydrate portion of glycoproteins. Included are: (1) the release of oligosaccharides and hydrolysis of carbohydrate chains using enzymatic and chemical methods; (2) fractionation by LPLC, electrophoresis, HPLC, and lectin affinity chromatography; (3) detection through the preparation of derivatives or by new electrochemical methods; (4) analysis by spectroscopic methods, including MS and high-field NMR; and (5) their sequencing through the use of multiple, well-integrated techniques. The ultimate goal of the analytical approaches discussed is to firmly establish structure and, thus, permit the study of structure-function relationships and eventually to allow the intelligent application of carbohydrate remodeling techniques in the preparation of new glycoproteins.

Conformational study of O-glycosylated threonine containing peptide models

1H n.m.r. studies of Z-Thr-OMe, Z-Thr-Ala-OMe, Z-Ala-Thr-OMe and their glycosylated derivatives indicate the possibility of an intramolecular hydrogen bond between Thr N alpha H and the N-acetyl carbonyl of the carbohydrate moiety, 3,4,6-tri-O-acetyl-2-acetamido-2-deoxy-alpha-D-galactopyranose (AcGalNAc). This is especially true in the case of Z-Thr(AcGalNAc)-Ala-OMe, suggesting that the strength of this hydrogen bond is dependent on the neighboring amino acids on the carbonyl terminal side of Thr. The existence of such a hydrogen bond implies a conformation in which the carbohydrate moiety is restricted to an orientation with its plane roughly perpendicular to the peptide backbone. In such an orientation, steric problems will be minimized in the case of clustered O-glycosidically linked Thr(Ser) residues as found in human erythrocyte glycophorin. A locked orientation of the carbohydrate moiety with respect to the peptide backbone may also play a conformational role in antifreeze glycoproteins.