Carbohydrates in transplantation

Reverse Reaction ofAspergillus nigerAPC-9319 α-Galactosidase in a Supersaturated Substrate Solution: Production of α-Linked Galactooligosaccharide (α-GOS)

The alpha-galactosidase that effectively catalyzes a reverse reaction of galactose, Aspergillus niger APC-9319 alpha-galactosidase, was screened from industrial enzyme preparations for food processing containing alpha-galactosidase activity. Reverse reaction of A. niger APC-9319 alpha-galactosidase was performed using a supersaturated solution (90% galactose [w/v]). A. niger APC-9319 alpha-galactosidase was not inhibited even in high substrate concentration, and effectively catalyzed the reverse reaction. The yield of the reaction product, alpha-linked galactooligosaccharide (alpha-GOS), increased greatly as the initial concentration of galactose increased to 90% (w/v), and was more than 50%. Furthermore, the half life of enzyme activity was about three times as long as that using 60% galactose (w/v). alpha-GOS (1.4 g) was prepared from galactose (3.0 g) by reverse reaction of A. niger APC-9319 alpha-galactosidase. The alpha-GOS contained 58% alpha-galactobiose (alpha-Gal2), 28% alpha-galactotriose, and 14% oligosaccharides larger than alpha-galactotriose. The main component of positional isomers in alpha-Gal2 was alpha-1,6Gal2.

The sweets standing at the borderline between allo- and xenotransplantation

Animal cells are densely covered with glycoconjugates, such as N-glycan, O-glycan, and glycosphingolipids, which are important for various biological and immunological events at the cell surface and in the extracellular matrix. Endothelial α-Gal carbohydrate epitopes (Galα3Gal-R) expressed on porcine tissue or cell surfaces are such glycoconjugates and directly mediate hyperacute immunological rejection in pig-to-human xenotransplantation. Although researchers have been able to develop α1,3-galactosyltransferase (GalT) gene knockout (KO) pigs, there remain unclarified non-Gal antigens that prevent xenotransplantation. Based on our expertise in the structural analysis of xenoantigenic carbohydrates, we describe the immunologically significant non-human carbohydrate antigens, including α-Gal antigens, analyzed as part of efforts to assess the antigens responsible for hyperacute immunological rejection in pig-to-human xenotransplantation. The importance of studying human, pig, and GalT-KO pig glycoprofiles, and of developing adequate pig-to-human glycan databases, is also discussed.

An easy colorimetric assay for glycosyltransferases

Glycosyltransferases are involved in biosynthesis of both protein-bound and non-bound glycans that have multiple and important biological functions in all species. A variety of methods for assaying glycosyltransferase activity have been developed driven by the specific interests and type of information required by researchers. In this work, a novel colorimetric assay for the glycosyltransferase-catalyzed reaction was established. Compared with measuring the newly formed product, which might not exhibit visible absorption, the unreacted acceptor could be readily detected by measuring the visible absorption of the hydrolysis product. In the assay, 4-nitrophenyl-beta-D-glycoside (glycosyl-beta-pNP) is used as the glycosyl acceptor, which can be hydrolyzed by a special exoglycosidase to release the p-nitrophenol before glycosylation reactions. Absorbance change of the p-nitrophenolate corresponds to unreacted glycosyl acceptor that accompanied the glycosyl transfer. The assay is demonstrated to be useful in the initial characterization of recombinant glycosyltransferases for their kinetic parameters, optimal metal cofactor, and pH value. It provides a simple, sensitive, and quantitative method for assessing glycosyltransferase activity and is thus expected to have broad applications including automated high-throughput screening.

Enzymatic route to preparative-scale synthesis of UDP-GlcNAc/GalNAc, their analogues and GDP-fucose

Enzymatic synthesis using glycosyltransferases is a powerful approach to building polysaccharides with high efficiency and selectivity. Sugar nucleotides are fundamental donor molecules in enzymatic glycosylation reactions by Leloir-type glycosyltransferases. The applications of these donors are restricted by their limited availability. In this protocol, N-acetylglucosamine (GlcNAc)/N-acetylgalactosamine (GalNAc) are phosphorylated by N-acetylhexosamine 1-kinase (NahK) and subsequently pyrophosphorylated by N-acetylglucosamine uridyltransferase (GlmU) to give UDP-GlcNAc/GalNAc. Other UDP-GlcNAc/GalNAc analogues can also be prepared depending on the tolerance of these enzymes to the modified sugar substrates. Starting from L-fucose, GDP-fucose is constructed by one bifunctional enzyme L-fucose pyrophosphorylase (FKP) via two reactions.

Metabolic engineering of Agrobacterium sp. strain ATCC 31749 for production of an alpha-Gal epitope

Background

Oligosaccharides containing a terminal Gal-α1,3-Gal moiety are collectively known as α-Gal epitopes. α-Gal epitopes are integral components of several medical treatments under development, including flu and HIV vaccines as well as cancer treatments. The difficulty associated with synthesizing the α-Gal epitope hinders the development and application of these treatments due to the limited availability and high cost of the α-Gal epitope. This work illustrates the development of a whole-cell biocatalyst for synthesizing the α-Gal epitope, Gal-α1,3-Lac.

Results

Agrobacterium sp. ATCC 31749 was engineered to produce Gal-α1,3-Lac by the introduction of a UDP-galactose 4'-epimerase:α1,3-galactosyltransferase fusion enzyme. The engineered Agrobacterium synthesized 0.4 g/L of the α-Gal epitope. Additional metabolic engineering efforts addressed the factors limiting α-Gal epitope production, namely the availability of the two substrates, lactose and UDP-glucose. Through expression of a lactose permease, the intracellular lactose concentration increased by 60 to 110%, subsequently leading to an improvement in Gal-α1,3-Lac production. Knockout of the curdlan synthase gene increased UDP-glucose availability by eliminating the consumption of UDP-glucose for synthesis of the curdlan polysaccharide. With these additional engineering efforts, the final engineered strain synthesized approximately 1 g/L of Gal-α1,3-Lac.

Conclusions

The Agrobacterium biocatalyst developed in this work synthesizes gram-scale quantities of α-Gal epitope and does not require expensive cofactors or permeabilization, making it a useful biocatalyst for industrial production of the α-Gal epitope. Furthermore, the engineered Agrobacterium, with increased lactose uptake and improved UDP-glucose availability, is a promising host for the production of other medically-relevant oligosaccharides.

Application of carbohydrate array technology to antigen discovery and vaccine development

Carbohydrate arrays are a new technology developed for high-throughput evaluation of interactions between carbohydrates and proteins, cells or viruses. Carbohydrate arrays contain many different carbohydrate structures on a solid support. The format allows one to probe hundreds or thousands of potential receptor-ligand interactions while using only tiny amounts of material. Recently, carbohydrate arrays have been applied to vaccine development in several ways. First, carbohydrate arrays have been utilized for the discovery and characterization of carbohydrate antigens. Second, they have been used to evaluate immune responses to vaccine candidates. Third, carbohydrate arrays have been used to identify and characterize reagents necessary for vaccine development. Although still at an early stage, carbohydrate array technology has tremendous potential for accelerating vaccine development.

Immunization with an engineered mutant trans-sialidase highly protects mice from experimental Trypanosoma cruzi infection: a vaccine candidate

Chagas' disease is a major tropical disease for which a cure for chronic phase does not exist yet. Trypanosoma cruzi trans-sialidase (TS) seems to be involved in relevant processes such as infectivity, host survival and, very importantly, disease pathogenesis. In this study, we show that mice vaccinated with an engineered enzymatically deficient mutant TS containing the catalytic domain without the immunodominant SAPA (Shed Acute Phase Antigen) repeats, were highly protected against T. cruzi infection. Adult male BALB/c mice were immunized with mutant protein, purified from Pichia pastoris yeast, using three inoculations in Freund's adjuvant. All immunized mice were protected against challenge with a lethal dose of T. cruzi trypomastigotes. The protected immunized mice developed no clinical or tissue evidence of infection throughout the study. In contrast, 60-90% mortality and 100% occurrence of myocardial lesions were observed in the non-immunized counterparts. Titers of circulating antibody against TS did not correlate with protection, while anti-SAPA antibodies were coincident with disease severity. Further studies indicated that a single inoculation of mutant recombinant protein in Freund's complete adjuvant was not associated with blood or organic alterations, per se. Mutant TS vaccination seems to be a promising tool for immune intervention strategies in Chagas' disease, aimed at preventing T. cruzi-related heart tissue damage.

Capillary electrophoresis method for the enzymatic assay of galactosyltransferases with postreaction derivatization

Glycosyltransferases are key enzymes in glycoconjugate biosynthesis, which make them important targets for biomedical research. Among the different methodologies developed to analyze glycosyltransferase activities, fluorophore-assisted capillary electrophoresis (FACE) emerges as a powerful technique in carbohydrate analysis. Its application to monitor glycosyltransferase activity has been limited to reactions with derivatized sugars as acceptor substrates in which a charged fluorophore/chromophore must be introduced, thus requiring tedious preparative synthesis and purification for each single acceptor substrate. Here we describe a novel and general glycosyltransferase assay based on FACE using underivatized acceptor substrates. Enzyme activity is monitored by a discontinuous assay with postreaction derivatization by reductive amination with 8-aminonaphthalene-1,3,6-trisulfonic acid. The reaction mixture is directly analyzed by HPCE (high-performance capillary electrophoresis) under inverted electroosmotic conditions at pH 2.5 and 30 degrees C. After method validation, it was applied to the kinetic characterization of an alpha-1,3-galactosyltransferase, the enzyme responsible for the biosynthesis of alphaGal epitope involved in the hyperacute rejection in xenotransplantation. The absence of a label on the acceptor during the GT reaction avoids any interference of the label with the enzyme, and the postreaction derivatization does not require any purification step.

In vivo synthesis of mammalian-like, hybrid-type N-glycans in Pichia pastoris

The Pichia pastoris N-glycosylation pathway is only partially homologous to the pathway in human cells. In the Golgi apparatus, human cells synthesize complex oligosaccharides, whereas Pichia cells form mannose structures that can contain up to 40 mannose residues. This hypermannosylation of secreted glycoproteins hampers the downstream processing of heterologously expressed glycoproteins and leads to the production of protein-based therapeutic agents that are rapidly cleared from the blood because of the presence of terminal mannose residues. Here, we describe engineering of the P. pastoris N-glycosylation pathway to produce nonhyperglycosylated hybrid glycans. This was accomplished by inactivation of OCH1 and overexpression of an alpha-1,2-mannosidase retained in the endoplasmic reticulum and N-acetylglucosaminyltransferase I and beta-1,4-galactosyltransferase retained in the Golgi apparatus. The engineered strain synthesized a nonsialylated hybrid-type N-linked oligosaccharide structure on its glycoproteins. The procedures which we developed allow glycan engineering of any P. pastoris expression strain and can yield up to 90% homogeneous protein-linked oligosaccharides.

Design and synthesis of αGal-conjugated peptide T20 as novel antiviral agent for HIV-immunotargeting

An efficient chemo-enzymatic synthesis of alpha Gal-conjugated peptide T20 as novel HIV-immuno-targeting agent is described. The synthesis involves chemo-enzymatic preparation of maleimide-functionalized alpha Gal epitope and its chemoselective ligation with the peptide T20. The title compound contains two functional domains: the trisaccharide alpha Gal epitope that binds to human natural anti-Gal antibodies and the 36-amino acid gp41 peptide (T20) that recognizes the gp41 N-terminal ectodomain of the HIV envelope. Biological assays demonstrated that the synthetic conjugate could readily bind to natural anti-Gal antibodies (both IgG and IgM type) in normal human serum and exhibited potent anti-HIV activity even in the absence of human antibodies and complement system. The experimental data suggest that the synthetic alpha Gal-T20 might be valuable for in vivo HIV-immuno-targeting via antibody-mediated cytotoxicity and/or antibody-dependent, complement-mediated lysis of HIV particles and HIV-infected cells, thus providing an additional dimension of HIV intervention.

Solid-phase synthesis of alpha-Gal epitopes: on-resin analysis of solid-phase oligosaccharide synthesis with 19F NMR spectroscopy

A route for solid-phase synthesis of the alpha-Gal epitopes Gal(alpha1-3)Gal(beta1-4)Glc and Gal(alpha1-3)Gal(beta1-4)GlcNAc is described. These trisaccharide antigens are responsible for hyperacute rejection in xenotransplantation of porcine organs. Optimization of the solid-phase synthesis relied on use of fluorinated protective groups for the carbohydrate building blocks and use of a fluorinated linker. This allowed convenient on-resin analysis of the reactions with gel-phase (19)F NMR spectroscopy. Conditions were established which allowed reductive ring-opening of 4,6-O-benzylidene acetals to be performed on the solid phase with high regioselectivity to furnish the corresponding 6-O-benzyl ethers. It was found that glycosylations could be conveniently carried out by using thioglycosides as donors with N-iodosuccinimide and trifluoromethanesulfonic acid as the promoter system. With use of these conditions a challenging alpha-glycosidic linkage was successfully installed with complete stereoselectivity in the final glycosylation. It was also established that fluorinated benzoates, benzyl ethers, and benzylidene acetals display almost identical chemical properties as their nonfluorinated counterparts, a finding that is essential for future use of fluorinated protective groups in solid-phase oligosaccharide synthesis.

Reassembled Biosynthetic Pathway for Large-Scale Carbohydrate Synthesis:α-Gal Epitope Producing “Superbug”

A metabolic pathway engineered Escherichia coli strain (superbug) containing one plasmid harboring an artificial gene cluster encoding all the five enzymes in the biosynthetic pathway of Galalpha l,3Lac through galactose metabolism has been developed. The plasmid contains a lambda promoter, a c1857 repressor gene, an ampicillin resistance gene, and a T7 terminator. Each gene was preceded by a Shine - Dalgarno sequence for ribosome binding. In a reaction catalyzed by the recombinant E. coli strain, Galalpha 1,3Lac trisaccharide accumulated at concentrations of 14.2 mM (7.2 gL(-1)) in a reaction mixture containing galactose, glucose, lactose, and a catalytic amount of uridine 5'-diphosphoglucose. This work demonstrates that large-scale synthesis of complex oligosaccharides can be achieved economically and efficiently through a single, biosynthetic pathway engineered microorganism.

Combinatorial library of five-membered iminocyclitol and the inhibitory activities against glyco-enzymes

BACKGROUND

Oligosaccharide processing enzymes are important classes of catalysts involved in synthesizing specific oligosaccharide structures on proteins and sphingolipids. Development of specific inhibitors of such enzymes is of current interest as these inhibitors may be used to control cellular functions. Five-membered iminocyclitols have been shown to be potent inhibitors of such enzymes. Since a rational design and synthesis of inhibitors is often extremely difficult due to the limited information regarding the structure of the active site, we carried out a combinatorial library approach.

RESULTS

To create diversity, we decided to use an aldehyde group of a protected iminocyclitol for reductive amination and the Strecker reaction. After transformation of the nitrile group introduced by the Strecker reaction into an amine and amide and complete deprotection, a small library of five-membered iminocyclitols consisting of 27 compounds was synthesized. A series of compounds obtained by reductive amination was first screened as potential inhibitors of glycosidases and glycosyltransferases. Among them, compounds carrying a C(10)-alkyl group showed marked enhancement of inhibitory activity against alpha-mannosidase at 10 microM concentration when compared with its parent compound and deoxymannojirimycin. Furthermore, compounds having the phenylethyl group showed an extremely strong inhibitory effect against alpha-galactosaminidase at a K(i) value of 29.4 nM. Compounds with an aminomethyl and amide group at the C-1' position of these two molecules showed a decrease in inhibitory activities.

CONCLUSIONS

A combinatorial approach based on five-membered iminocyclitols with a galacto-configuration was exploited. The potential usefulness of the library as a source of inhibitors of glycoenzymes is clearly shown in this study.

Structure of bovine alpha-1,3-galactosyltransferase and its complexes with UDP and DPGal inferred from molecular modeling

A homology model of alpha-1,3-galactosyltransferase (alpha-1,3-GalT), the retaining enzyme responsible for the formation of alpha-galactosyl epitopes, has been developed by means of molecular modeling using the SpsA glycosyltransferase structure. A protein-ligand docking approach was used to model alpha-1,3-GalT complexed with UDP and UDP-Gal. The comparison of structural features found in the alpha-1,3-GalT homology model with available structural data on this class of enzymes revealed similarities in the UDP-binding pocket. In the predicted structure of the complexes, the pyrophosphate interacts with the DVD motif (Asp-225, Val-226, and Asp-227) of alpha-1,3-GalT through the Mn(2+) cation. The uridine part of the UDP binds into the well-defined cavity that consists of Phe-134, Tyr-139, Ile-140, Val-136, Arg-194, Arg-202, Lys-209, Asp-173, His-218, and Thr-137 in a conformation that is generally observed in the crystal structures of other glycosyltransferase complexes.

Complex carbohydrate synthesis tools for glycobiologists: enzyme-based approach and programmable one-pot strategies

The ultimate goal in complex carbohydrate synthesis is to develop synthetic tools which are simple and easily accessible to glycobiologists. This review will describe methods which have the potential to reach this goal, with particular focus on enzymatic and computer-based one-pot approaches for the preparation of complex carbohydrates and glycoconjugates.

Changing the donor cofactor of bovine alpha 1, 3-galactosyltransferase by fusion with UDP-galactose 4-epimerase. More efficient biocatalysis for synthesis of alpha-Gal epitopes

Two fusion enzymes consisting of uridine diphosphogalactose 4-epimerase (UDP-galactose 4-epimerase, EC ) and alpha1, 3-galactosyltransferase (EC ) with an N-terminal His(6) tag and an intervening three-glycine linker were constructed by in-frame fusion of the Escherichia coli galE gene either to the 3' terminus (f1) or to the 5' terminus (f2) of a truncated bovine alpha1, 3-galactosyltransferase gene, respectively. Both fusion proteins were expressed in cell lysate as active, soluble forms as well as in inclusion bodies as improperly folded proteins. Both f1 and f2 were determined to be homodimers, based on a single band observed at about 67 kDa in SDS-polyacrylamide gel electrophoresis and on a single peak with a molecular mass around 140 kDa determined by gel filtration chromatography for each of the enzymes. Without altering the acceptor specificity of the transferase, the fusion with the epimerase changed the donor requirement of alpha1, 3-galactosyltransferase from UDP-galactose to UDP-glucose and decreased the cost for the synthesis of biomedically important Galalpha1,3Gal-terminated oligosaccharides by more than 40-fold. For enzymatic synthesis of Galalpha1,3Galbeta1,4Glc from UDP-glucose and lactose, the genetically fused enzymes f1 and f2 exhibited kinetic advantages with overall reaction rates that were 300 and 50%, respectively, higher than that of the system containing equal amounts of epimerase and galactosyltransferase. These results indicated that the active sites of the epimerase and the transferase in fusion enzymes were in proximity. The kinetic parameters suggested a random mechanism for the substrate binding of the alpha1, 3-galactosyltransferase. This work demonstrated a general approach that fusion of a glycosyltransferase with an epimerase can change the required but expensive sugar nucleotide to a less expensive one.