Recent advances on glycosyltransferases involved in the biosynthesis of the proteoglycan linkage region

Proteoglycans (PGs) are an essential family of glycoproteins, which can play roles in many important biological events including cell proliferation, cancer development, and pathogen infections. Proteoglycans consist of a core protein with one or multiple glycosaminoglycan (GAG) chains, which are covalently attached to serine residues of serine-glycine dipeptide within the core protein through a common tetrasaccharide linkage. In the past three decades, four key glycosyl transferases involved in the biosynthesis of PG linkage have been discovered and investigated. This review aims to provide an overview on progress made on these four enzymes, with foci on enzyme expression/purification, substrate specificity, activity determination, product characterization, and structure-activity relationship analysis.

Heparin Binding Proteins as Therapeutic Target: An Historical Account and Current Trends

The polyanionic nature and the ability to interact with proteins with different affinities are properties of sulfated glycosaminoglycans (GAGs) that determine their biological function. In designing drugs affecting the interaction of proteins with GAGs the challenge has been to generate agents with high binding specificity. The example to emulated has been a heparin-derived pentasaccharide that binds to antithrombin-III with high affinity. However, the portability of this model to other biological situations is questioned on several accounts. Because of their structural flexibility, oligosaccharides with different sulfation and uronic acid conformation can display the same binding proficiency to different proteins and produce comparable biological effects. This circumstance represents a formidable obstacle to the design of drugs based on the heparin scaffold. The conceptual framework discussed in this article is that through a direct intervention on the heparin-binding functionality of proteins is possible to achieve a high degree of action specificity. This objective is currently pursued through two strategies. The first makes use of small molecules for which in the text we provide examples from past and present literature concerning angiogenic factors and enzymes. The second approach entails the mutagenesis of the GAG-binding site of proteins as a means to generate a new class of biologics of therapeutic interest.

UDP-Glucose 4-Epimerase and β-1,4-Galactosyltransferase from the Oyster Magallana gigas as Valuable Biocatalysts for the Production of Galactosylated Products

Uridine diphosphate galactose (UDP-galactose) is a valuable building block in the enzymatic synthesis of galactose-containing glycoconjugates. UDP-glucose 4-epimerase (UGE) is an enzyme which catalyzes the reversible conversion of abundantly available UDP-glucose to UDP-galactose. Herein, we described the cloning, expression, purification, and biochemical characterization of an unstudied UGE from the oyster Magallana gigas (MgUGE). Activity tests of recombinantly expressed MgUGE, using HPLC (high-performance liquid chromatography), mass spectrometry, and photometric assays, showed an optimal temperature of 16 °C, and reasonable thermal stability up to 37 °C. No metal ions were required for enzymatic activity. The simple nickel-affinity-purification procedure makes MgUGE a valuable biocatalyst for the synthesis of UDP-galactose from UDP-glucose. The biosynthetic potential of MgUGE was further exemplified in a coupled enzymatic reaction with an oyster-derived β-1,4-galactosyltransferase (MgGalT7), allowing the galactosylation of the model substrate para-nitrophenol xylose (pNP-xylose) using UDP-glucose as the starting material.

Synthesis of a library of variously modified 4-methylumbelliferyl xylosides and a structure-activity study of human β4GalT7

Proteoglycans (PGs) are complex macromolecules that are composed of glycosaminoglycan (GAG) chains covalently attached to a core protein through a tetrasaccharide linker. The biosynthesis of PGs is complex and involves a large number of glycosyltranferases. Here we present a structure-activity study of human β4GalT7, which transfers the first Gal residue onto a xyloside moiety of the linkage region. An efficient and regiocontrolled synthesis of a library of modified analogs of 4-methylumbelliferyl xyloside (XylMU) is reported herein. Hydroxyl groups at the position C-2, C-3 or C-4 have been epimerized and/or replaced by a hydrogen or a fluorine, while the anomeric oxygen was replaced by either a sulfur or a sulfone. The effect of these compounds on human β4GalT7 activity in vitro and on GAG biosynthesis in cellulo was then evaluated.

'Click'-xylosides as initiators of the biosynthesis of glycosaminoglycans: Comparison of mono-xylosides with xylobiosides

Different mono-xylosides and their corresponding xylobiosides obtained by a chemo-enzymatic approach featuring various substituents attached to a triazole ring were probed as priming agents for glycosaminoglycan (GAG) biosynthesis in the xylosyltransferase-deficient pgsA-745 Chinese hamster ovary cell line. Xylosides containing a hydrophobic aglycone moiety were the most efficient priming agents. Mono-xylosides induced higher GAG biosynthesis in comparison with their corresponding xylobiosides. The influence of the degree of polymerization of the carbohydrate part on the priming activity was investigated through different experiments. We demonstrated that in case of mono-xylosides, the cellular uptake as well as the affinity and the catalytic efficiency of β-1,4-galactosyltransferase 7 were higher than for xylobiosides. Altogether, these results indicate that hydrophobicity of the aglycone and degree of polymerization of glycone moiety were critical factors for an optimal priming activity for GAG biosynthesis.

β-Xylopyranosides: synthesis and applications

In recent years, β-xylopyranosides have attracted interest due to the development of biomass-derived molecules. This review focuses on general routes for the preparation of β-xylopyranosides by chemical and enzymatic pathways and their main uses.

Rules for priming and inhibition of glycosaminoglycan biosynthesis; probing the β4GalT7 active site

Xylose is the optimal substrate for β4GalT7, an essential enzyme in GAG biosynthesis, but analogs act as effective inhibitors.

Crystal structures of β-1,4-galactosyltransferase 7 enzyme reveal conformational changes and substrate binding

The β-1,4-galactosyltransferase 7 (β4GalT7) enzyme is involved in proteoglycan synthesis. In the presence of a manganese ion, it transfers galactose from UDP-galactose to xylose on a proteoglycan acceptor substrate. We present here the crystal structures of human β4GalT7 in open and closed conformations. A comparison of these crystal structures shows that, upon manganese and UDP or UDP-Gal binding, the enzyme undergoes conformational changes involving a small and a long loop. We also present the crystal structures of Drosophila wild-type β4GalT7 and D211N β4GalT7 mutant enzymes in the closed conformation in the presence of the acceptor substrate xylobiose and the donor substrate UDP-Gal, respectively. To understand the catalytic mechanism, we have crystallized the ternary complex of D211N β4GalT7 mutant enzyme in the presence of manganese with the donor and the acceptor substrates together in the same crystal structure. The galactose moiety of the bound UDP-Gal molecule forms seven hydrogen bonds with the protein molecule. The nonreducing end of the xylose moiety of xylobiose binds to the hydrophobic acceptor sugar binding pocket created by the conformational changes, whereas its extended xylose moiety forms hydrophobic interactions with a Tyr residue. In the ternary complex crystal structure, the nucleophile O4 oxygen atom of the xylose molecule is found in close proximity to the C1 and O5 atoms of the galactose moiety. This is the first time that a Michaelis complex of a glycosyltransferase has been described, and it clearly suggests an SN2 type catalytic mechanism for the β4GalT7 enzyme.

Recent structures, evolution and mechanisms of glycosyltransferases

Cellular glycome assembly requires the coordinated action of a large number of glycosyltransferases that catalyse the transfer of a sugar residue from a donor to specific acceptor molecules. This enzyme family is very ancient, encompassing all three domains of life. There has been considerable recent progress in structural glycobiology with the determination of crystal structures of several important glycosyltransferase members, showing novel folds and variations around a common α/β scaffold. Structural, kinetic and inhibitor data have led to the emergence of various scenarios with respect to their evolutionary history and reaction mechanisms thus highlighting the different solutions that nature has selected to catalyse glycosyl transfer.