Enzyme-Catalyzed Synthesis of Oligosaccharides That Contain Functionalized Sialic Acids

Synthesis of N-Glycolylneuraminic Acid (Neu5Gc) and Its Glycosides

Sialic acids constitute a family of negatively charged structurally diverse monosaccharides that are commonly presented on the termini of glycans in higher animals and some microorganisms. In addition to N-acetylneuraminic acid (Neu5Ac), N-glycolyl neuraminic acid (Neu5Gc) is among the most common sialic acid forms in nature. Nevertheless, unlike most animals, human cells loss the ability to synthesize Neu5Gc although Neu5Gc-containing glycoconjugates have been found on human cancer cells and in various human tissues due to dietary incorporation of Neu5Gc. Some pathogenic bacteria also produce Neu5Ac and the corresponding glycoconjugates but Neu5Gc-producing bacteria have yet to be found. In addition to Neu5Gc, more than 20 Neu5Gc derivatives have been found in non-human vertebrates. To explore the biological roles of Neu5Gc and its naturally occurring derivatives as well as the corresponding glycans and glycoconjugates, various chemical and enzymatic synthetic methods have been developed to obtain a vast array of glycosides containing Neu5Gc and/or its derivatives. Here we provide an overview on various synthetic methods that have been developed. Among these, the application of highly efficient one-pot multienzyme (OPME) sialylation systems in synthesizing compounds containing Neu5Gc and derivatives has been proven as a powerful strategy.

Unique self-anhydride formation in the degradation of cytidine-5'-monophosphosialic acid (CMP-Neu5Ac) and cytidine-5'-diphosphosialic acid (CDP-Neu5Ac) and its application in CMP-sialic acid analogue synthesis

Sialyloligosaccharides are synthesised by various glycosyltransferases and sugar nucleotides. All of these nucleotides are diphosphate compounds except for cytidine-5'-monophosphosialic acid (CMP-Neu5Ac). To obtain an insight into why cytidine-5'-diphosphosialic acid (CDP-Neu5Ac) has not been used for the sialyltransferase reaction and why it is not found in biological organisms, the compound was synthesised. This synthesis provided the interesting finding that the carboxylic acid moiety of the sialic acid attacks the attached phosphate group. This interaction yields an activated anhydride between carboxylic acid and the phosphate group and leads to hydrolysis of the pyrophosphate linkage. The mechanism was demonstrated by stable isotope-labelling experiments. This finding suggested that CMP-Neu5Ac might also form the corresponding anhydride structure between carboxylic acid and phosphate, and this seems to be the reason why CMP-Neu5Ac is acid labile in relation to other sugar nucleotides. To confirm the role of the carboxylic acid, CMP-Neu5Ac derivatives in which the carboxylic acid moiety in the sialic acid was substituted with amide or ester groups were synthesised. These analogues clearly exhibited resistance to acid hydrolysis. This result indicated that the carboxylic acid of Neu5Ac is associated with its stability in solution. This finding also enabled the development of a novel chemical synthetic method for CMP-Neu5Ac and CMP-sialic acid derivatives.

Combinatorial chemoenzymatic synthesis and high-throughput screening of sialosides

Although the vital roles of structures containing sialic acid in biomolecular recognition are well documented, limited information is available on how sialic acid structural modifications, sialyl linkages, and the underlying glycan structures affect the binding or the activity of sialic acid-recognizing proteins and related downstream biological processes. A novel combinatorial chemoenzymatic method has been developed for the highly efficient synthesis of biotinylated sialosides containing different sialic acid structures and different underlying glycans in 96-well plates from biotinylated sialyltransferase acceptors and sialic acid precursors. By transferring the reaction mixtures to NeutrAvidin-coated plates and assaying for the yields of enzymatic reactions using lectins recognizing sialyltransferase acceptors but not the sialylated products, the biotinylated sialoside products can be directly used, without purification, for high-throughput screening to quickly identify the ligand specificity of sialic acid-binding proteins. For a proof-of-principle experiment, 72 biotinylated alpha2,6-linked sialosides were synthesized in 96-well plates from 4 biotinylated sialyltransferase acceptors and 18 sialic acid precursors using a one-pot three-enzyme system. High-throughput screening assays performed in NeutrAvidin-coated microtiter plates show that whereas Sambucus nigra Lectin binds to alpha2,6-linked sialosides with high promiscuity, human Siglec-2 (CD22) is highly selective for a number of sialic acid structures and the underlying glycans in its sialoside ligands.

Synthesis and properties of O-carboxymethyl chitosan/methoxy poly(ethylene glycol) graft copolymers

O-carboxymethyl chitosan/methoxy poly(ethylene glycol) graft copolymers (OCMCS-g-MPEGs) with different degrees of substitution (DS) were synthesized by reductive N-alkylation of chitosan with poly(ethylene glycol) aldehyde. The properties of OCMCS-g-MPEGs, including the solubility, structure, hydrodynamic behaviors, isoelectric point (IEP) and interaction with water-soluble chitosan, were investigated. As a PEGylated polyampholyte, OCMCS-g-MPEGs can resolve in water over all pH range and the pH value at IEP (pH(IEP)) decreases when DS increases. The hydrodynamic behaviors of OCMCS-g-MPEGs in deionized H(2)O are markedly affected by DS and pH(IEP) in the experiment concentration range. The particle size of the complexes of OCMCS-g-MPEGs with water-soluble chitosan is strongly affected by the concentration of water-soluble chitosan and the pH value.

One-pot three-enzyme chemoenzymatic approach to the synthesis of sialosides containing natural and non-natural functionalities

Chemoenzymatic synthesis, which combines the flexibility of chemical synthesis and the high selectivity of enzymatic synthesis, is a powerful approach to obtain complex carbohydrates. It is a preferred method for synthesizing sialic acid-containing structures, including those with diverse naturally occurring and non-natural sialic acid forms, different sialyl linkages and different glycans that link to the sialic acid. Starting from N-acetylmannosamine, mannose or their chemically or enzymatically modified derivatives, sialic acid aldolase-catalyzed condensation reaction leads to the formation of sialic acids and their derivatives. These compounds are subsequently activated by a CMP-sialic acid synthetase and transferred to a wide range of suitable acceptors by a suitable sialyltransferase for the formation of sialosides containing natural and non-natural functionalities. The three-enzyme coupled synthesis of sialosides can be carried out in one pot without the isolation of intermediates. The time for synthesis is 4-18 h. Purification and characterization of the product can be completed within 2-3 d.

Chemoenzymatic synthesis of CMP-sialic acid derivatives by a one-pot two-enzyme system: comparison of substrate flexibility of three microbial CMP-sialic acid synthetases

Three C terminal His6-tagged recombinant microbial CMP-sialic acid synthetases [EC 2.7.7.43] cloned from Neisseria meningitidis group B, Streptococcus agalactiae serotype V, and Escherichia coli K1, respectively, were evaluated for their ability in the synthesis of CMP-sialic acid derivatives in a one-pot two-enzyme system. In this system, N-acetylmannosamine or mannose analogs were condensed with pyruvate, catalyzed by a recombinant sialic acid aldolase [EC 4.1.3.3] cloned from E. coli K12 to provide sialic acid analogs as substrates for the CMP-sialic acid synthetases. The substrate flexibility and the reaction efficiency of the three recombinant CMP-sialic acid synthetases were compared, first by qualitative screening using thin layer chromatography, and then by quantitative analysis using high performance liquid chromatography. The N. meningitidis synthetase was shown to have the highest expression level, the most flexible substrate specificity, and the highest catalytic efficiency among the three synthetases. Finally, eight sugar nucleotides, including cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-Neu5Ac) and its derivatives with substitutions at carbon-5, carbon-8, or carbon-9 of Neu5Ac, were synthesized in a preparative (100-200 mg) scale from their 5- or 6-carbon sugar precursors using the N. meningitidis synthetase and the aldolase.

The total synthesis of ganglioside GM3

Previous syntheses of ganglioside GM3 (NeuAc alpha3Gal beta4Glc beta1Cer) are reviewed, and both chemoenzymatic and chemical total synthetic approaches were investigated. In a chemoenzymatic approach, (2S,3R,4E)-5'''-acetyl-alpha-neuraminyl-(2''' --> 3'')-beta-galactopyranosyl-(1'' --> 4')-beta-glucopyranosyl-(1' <--> 1)-2-azido-4-octadecene-1,3-diol (azidoGM3) was readily prepared utilizing recombinant beta-Gal-(1'' --> 3'/4')-GlcNAc alpha-(2''' --> 3'')-sialyltransferase enzyme, and was evaluated as a synthetic intermediate to ganglioside GM3. The chemical total synthesis of ganglioside GM3 was performed on one of the largest scales yet reported. The highlights of this synthesis include minimizing the steps necessary to prepare the lactosyl acceptor as a useful anomeric mixture, which was present in excess for the highly regioselective and fairly stereoselective sialylation with a known neuraminyl donor to give the protected GM3 trisaccharide. The synthetic methodology maximized convergence by a subsequent glycosidic coupling of the well-characterized GM3 trisaccharide trichloroacetimidate derivative with protected ceramide. The ganglioside GM3 was nearly homogeneous as the two glycosidic couplings utilized preparative HLPC purifications, and variations in the sphingosine base and fatty acyl group were under 0.1 and 0.2%, respectively.

Synthesis and characterization of an anomeric sulfur analogue of CMP-sialic acid

alpha-2,3-Sialyltransferase catalyzes the transfer of sialic acid from CMP-sialic acid (1) to a lactose acceptor. An analogue of 1 was synthesized in which the anomeric oxygen atom was replaced with a sulfur atom (1S). The key step in the synthesis of 1S was a tetrazole-promoted coupling of a cytidine-5'-phosphoramidite with a glycosyl thiol of a protected sialic acid. Compounds 1 and 1S were characterized for their activity in a sialyl transfer assay. The rate of solvolysis in aqueous buffer of analogue 1S was 50-fold slower than that of 1. Analogue 1S was found to be substrate for alpha-2,3-sialyltransferase. The K(m) of 1S was just 3-fold higher than that of 1, while the k(cat) of 1S was 2 orders of magnitude lower compared to 1.

Design of fluorogenic substrates for continuous assay of sialyltransferase by resonance energy transfer

Glycosyltransferases are important synthetic enzymes for the construction of naturally occurring glycoconjugates as well as for the design of neoglycoconjugates. The assay methods currently available for these enzymes require tedious and time-consuming procedures for separation of products and do not permit continual assay of enzyme activities. As a set of convenient fluorogenic substrates for continuous monitoring of sialyltransferase activities, we designed and synthesized a novel CMP-Neu5Ac derivative with a naphthylmethyl group at the C-9 position and N-acetyllactosamine derivative containing a dansyl group at the terminal position of aglycon. In such substrates, the emission peak of the naphthylmethyl group (lambdaem = 340 nm) of the glycosyl donor is successfully overlapped with the excitation peak due to the dansyl group (lambdaex = 335 nm) of the glycosyl acceptor. A coupling reaction of these two substrates catalyzed by rat liver 2,6-sialyltransferase caused an increase of dansyl fluorescence (lambdaem = 525 nm) and a decrease of naphthylmethyl fluorescence on the basis of resonance energy transfer between two fluorescence probes. The substrates presented here permit continuous fluorescent monitoring of enzymatic sugar combining reactions. Actually, using this time course of enzymatic reactions, kinetic constants of rat liver 2,6-sialyltransferase against glycosyl donor substrates were estimated to be Km = 4.85 microM and Vmax. = 0.119 micromol/min, respectively. This strategy allows precise and efficient analyses of enzyme kinetics not possible with the conventional assay methods for the glycosyltransferases that usually require separation of products from the reaction mixture.

Synthesis of a six-carbon sialic acid using an indium-mediated coupling

[reaction: see text] The synthesis of a six-carbon truncated sialic acid is described. A key step in the synthesis was an indium-mediated allyl addition to a serine-derived aldehyde. Careful choice of protecting groups was found to be necessary in order to prevent unwanted side reactions throughout the sequence. The truncated sialic acid was obtained in a form suitable for activation as a glycosyl donor.

Transglycosylation reactions of Bacillus stearothermophilus maltogenic amylase with acarbose and various acceptors

It was observed that Bacillus stearothermophilus maltogenic amylase cleaved the first glycosidic bond of acarbose to produce glucose and a pseudotrisaccharide (PTS) that was transferred to C-6 of the glucose to give an alpha-(1-->6) glycosidic linkage and the formation of isoacarbose. The addition of a number of different carbohydrates to the digest gave transfer products in which PTS was primarily attached alpha-(1-->6) to D-glucose, D-mannose, D-galactose, and methyl alpha-D-glucopyranoside. With D-fructopyranose and D-xylopyranose, PTS was linked alpha-(1-->5) and alpha-(1-->4), respectively. PTS was primarily transferred to C-6 of the nonreducing residue of maltose, cellobiose, lactose, and gentiobiose. Lesser amounts of alpha-(1-->3) and/or alpha-(1-->4) transfer products were also observed for these carbohydrate acceptors. The major transfer product to sucrose gave PTS linked alpha-(1-->4) to the glucose residue. alpha,alpha-Trehalose gave two major products with PTS linked alpha-(1-->6) and alpha-(1-->4). Maltitol gave two major products with PTS linked alpha-(1-->6) and alpha-(1-->4) to the glucopyranose residue. Raffinose gave two major products with PTS linked alpha-(1-->6) and alpha-(1-->4) to the D-galactopyranose residue. Maltotriose gave two major products with PTS linked alpha-(1-->6) and alpha-(1-->4) to the nonreducing end glucopyranose residue. Xylitol gave PTS linked alpha-(1-->5) as the major product and D-glucitol gave PTS linked alpha-(1-->6) as the only product. The structures of the transfer products were determined using thin-layer chromatography, high-performance ion chromatography, enzyme hydrolysis, methylation analysis and 13C NMR spectroscopy. The best acceptor was gentiobiose, followed closely by maltose and cellobiose, and the weakest acceptor was D-glucitol.

Enzymes and protecting group chemistry

Enzymatic protecting group techniques are increasingly finding their use in almost all areas of synthetic organic chemistry. Some of the recent papers have dealt with the use of such protecting groups in combination with classical methods. The modification of known protecting groups to increase the efficiency and selectivity of deprotection is on the rise. The methodology needs to be explored more intensively and systematically to realise its full potential.