Conformation-dependent O-glycosylation of threonine-containing dipeptides

'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.

Fully convergent solid phase synthesis of antifreeze glycoprotein analogues

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

Solid-phase synthesis and conformational studies of glycosylated derivatives of helper-T-cell immunogenic peptides from hen-egg lysozyme

3-Mercaptopropionic acid and N alpha-Fmoc serine, both with unprotected carboxyl groups, were stereospecifically glycosylated in 62-82% yields, using saccharide 1,2-trans peracetates and Lewis acid catalysis. The resulting glycosylated building blocks were used in the synthesis of derivatives of helper-T-cell stimulating peptides, with the carbohydrate moiety located at the amino terminus, or internally in the peptide chain. 1H NMR spectroscopy in Me2SO-d6 showed that the glycopeptides assumed random conformations, which were not influenced by the glycosylation or by single substitutions of amino acids in the peptide moiety.

Solid phase synthesis and conformation of sequential glycosylated polytripeptide sequences related to antifreeze glycoproteins

Sequential glycopeptides [Thr(beta-D-galactose)-Ala-Ala]n, with n ranging from 2 to 7, as models of natural antifreeze glycoproteins were synthesized by the continuous flow, solid phase procedure. The conformational properties of these materials in solution were investigated by c.d. and 1H-n.m.r. spectroscopy. In aqueous solution the c.d. pattern is practically independent of chain length and is very similar to that of natural antifreeze glycoproteins. The results are interpreted in terms of random coil structure. The absence of ordered structures is further confirmed by n.m.r. data. A small amount of ordered conformation can be induced either by increasing the temperature of the aqueous solution or by addition of TFE. The c.d. pattern of all glycopeptides in water at temperatures higher than 50 degrees C are compatible with the presence of a small amount of alpha-helix or 3(10) helix. Since the glyco-hexapeptide is too short to form an alpha-helix, the hypothesis is made that in the glycopeptides in water at high temperature a small amount of 3(10) helix is formed. The same is observed for the 21-residue glycopeptide in presence of 85% (v/v) TFE. In this medium, the c.d. data on the glyco-hexapeptide are more compatible with the presence of a small amount of beta-structure.

N.m.r. study on conformation of Ac-Thr(alpha-GalNAc)-Ala-Ala-OMe as a model for mucin type glycoprotein

This study report on the results of high resolution 1H n.m.r. investigations on Ac-Thr(alpha-GalNAc)-Ala-Ala-OMe 1 as a mucin type model glycopeptide of antifreeze glycoprotein (AFGP) in both dimethyl sulfoxide (DMSO) and H2O. The temperature dependence of amide proton chemical shifts strongly suggested the presence of the intramolecular hydrogen bond between the amide proton of GalNAc and the carbonyl oxygen of the Thr residues. Due to this bond, the orientation of the sugar residue of 1 appears to be fairly restricted relative to its peptide backbone. Despite the lack of the clear evidence for such intramolecular hydrogen bond in H2O, 1H coupling constant data suggested the structural similarity of 1 in DMSO and H2O, indicating the presence of the intramolecular hydrogen bond even in H2O, which may play an important role in determining the orientation of the sugar moiety with respect to the peptide backbone in glycoprotein.

Synthesis of mono- and disaccharide amino-acid derivatives for use in solid phase peptide synthesis

N-Fluorenylmethyloxycarbonyl-protected serine and threonine derivatives, carrying O-glycosidically alpha- or beta-linked peracetylated beta-D-Galp-(1-3)-D-GalNAcp carbohydrate chains, were prepared. These derivatives are intended for use in solid phase glycopeptide synthesis. Suitably protected mono- and disaccharide thioglycosides were used as carbohydrate intermediates. These were activated by treatment with bromine to give the glycosyl bromides, which were then used in silver triflate-promoted glycosidations of N-fluorenylmethyloxycarbonyl amino-acid phenacyl esters. Removal of the phenacyl esters with zinc gave the target free acids.

Synthesis of a close analog of the repeating unit of the antifreeze glycoproteins of polar fish

The protected glycopeptide N-(benzyloxycarbonyl)-L-alanyl-[O-(2,3,4,6-tetra-O-benzoyl-beta-D- galactopyranosyl)-(1----3)-O-(2,4,6-tri-O-benzyl-alpha-D-galactopyranosy l)- (1----3)]-L-threonyl-L-alanine 2,2,2-trichloroethyl ester was made by coupling the respective disaccharide and tripeptide blocks. The disaccharide block was generated by coupling tetra-O-benzoyl-alpha-D-galactopyranosyl bromide to allyl 2,4,6-tri-O-benzyl-alpha-D-galactopyranoside and converting the product into O-(2,3,4,6-tetra-O-benzoyl-beta-D-galactopyranosyl)-(1----3)-2,4,6-tri-O -benzyl - alpha-D-galactopyranosyl chloride via the 1-propenyl glycoside and the free (1-OH) sugar. Alternatively, the 1-propenyl intermediate was obtained directly by using 1-propenyl 2,4,6-tri-O-benzyl-alpha-D-galactopyranoside as the acceptor in the initial coupling reaction. An efficient 3-step synthesis of 10 was accomplished by the dibutyltin oxide-assisted, selective crotylation of allyl alpha-D-galactopyranoside at O-3, followed by benzylation and treatment of the product with potassium tert-butoxide. The N-benzyloxycarbonyl (Z) and N-tert-butoxycarbonyl (Boc) 2,2,2-trichloroethyl esters of Thr-Ala and Ala-Thr-Ala were formed by sequential coupling. The silver triflate-promoted glycosylation of the Z-protected dipeptide and tripeptide by 2,3,4,6-tetra-O-benzyl-alpha-D-galactopyranosyl chloride, and of the tripeptide by 6, proceeded with excellent alpha-stereoselectivity. From the disaccharide tripeptide 21, the carboxyl-deprotected and fully deprotected derivatives were prepared.

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.