Chemoenzymatic synthesis and lectin recognition of a selectively fluorinated glycoprotein

Toward Glycomaterials with Selectivity as Well as Affinity

Multivalent glycosylated materials (polymers, surfaces, and particles) often show high affinity toward carbohydrate binding proteins (e.g., lectins) due to the nonlinear enhancement from the cluster glycoside effect. This affinity gain has potential in applications from diagnostics, biosensors, and targeted delivery to anti-infectives and in an understanding of basic glycobiology. This perspective highlights the question of selectivity, which is less often addressed due to the reductionist nature of glycomaterials and the promiscuity of many lectins. The use of macromolecular features, including architecture, heterogeneous ligand display, and the installation of non-natural glycans, to address this challenge is discussed, and examples of selectivity gains are given.

Introducing affinity and selectivity into galectin-targeting nanoparticles with fluorinated glycan ligands

Galectins are potential biomarkers and therapeutic targets. However, galectins display broad affinity towards β-galactosides meaning glycan-based (nano)biosensors lack the required selectivity and affinity. Using a polymer-stabilized nanoparticle biosensing platform, we herein demonstrate that the specificity of immobilised lacto-N-biose towards galectins can be ‘turned on/off’ by using site-specific glycan fluorination and in some cases reversal of specificity can be achieved. The panel of fluoro-glycans were obtained by a chemoenzymatic approach, exploiting BiGalK and BiGalHexNAcP enzymes from Bifidobacterium infantis which are shown to tolerate fluorinated glycans, introducing structural diversity which would be very laborious by chemical methods alone. These results demonstrate that integrating non-natural, fluorinated glycans into nanomaterials can encode unprecedented selectivity with potential applications in biosensing.

A chemo-enzymatic site-specific fluorination strategy is employed to obtain glyco-nanoparticles with tuneable selectivity towards galectins.

Development of a Broadly Applicable Assay for Measurement of Glycan-Directed Enzymatic Activity

Glycosylation is a key posttranslational modification that tags protein to membranes, organelles, secretory pathways, and degradation. Aberrant protein glycosylation is present both in acquired diseases, such as cancer and neurodegeneration, and in congenital disorders of glycosylation (CDGs). Consequently, the ability to interrogate the activity of enzymes that can modify protein glycan moieties is key for drug discovery projects aimed at finding modulators of these enzymes. To date, low-throughput technologies such as SDS-PAGE and mass spectrometry have been used, which are not suitable for compound screening in drug discovery. In the present work, a broadly applicable time-resolved fluorescence resonance energy transfer (TR-FRET) assay was developed that can determine the activity of endoglycosidase enzymes in high-throughput formats. The assay was validated using PNGaseF and EndoH as tool deglycosylases. Even though the current setup is based on the recognition of glycans that bind concanavalin A (ConA), the assay concept can be adapted to glycans that bind other lectins.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

Specificity of Donor Structures for endo-β-N-Acetylglucosaminidase-Catalyzed Transglycosylation Reactions

To demonstrate the structural specificity of the glycosyl donor for the transglycosylation reaction by using endo-β-N-acetylglucosaminidase from Mucor hiemalis (endo-M), a series of tetrasaccharide oxazoline derivatives was synthesized. These derivatives correspond to the core structure of an asparagine-linked glycoprotein glycan with a β-mannose unit of a non-natural-type monosaccharide, including β-glucose, β-galactose, and β-talose in place of the β-mannose moiety. The transglycosylation activity of wildtype (WT) endo-M and two mutants, N175Q and N175A, was examined by using these tetrasaccharide donors with p-nitrophenyl N-acetylglucosaminide (GlcNAc-pNp). The essential configuration of the hydroxy group for the transglycosylation reaction was determined. On the basis of these results, the transglycosylation reaction was investigated by using chemically modified donors, and transglycosylated products were successfully obtained.

Towards the next generation of biomedicines by site-selective conjugation

Bioconjugates represent an emerging class of medicines, which offer therapeutic opportunities overtaking those of the individual components. Many novel bioconjugates have been explored in order to address various emerging medical needs. The last decade has witnessed the exponential growth of new site-selective bioconjugation techniques, however very few methods have made the way into human clinical trials. Here we discuss various applications of site-selective conjugation in biomedicines, including half-life extension, antibody-drug conjugates, conjugate vaccines, bispecific antibodies and cell therapy. The review is intended to highlight both the progress and challenges, and identify a potential roadmap to address the gap.

Recent Advances Toward Robust N-Protecting Groups for Glucosamine as Required for Glycosylation Strategies

2-Amino-2-deoxy-d-glucose (d-glucosamine) is among the most abundant monosaccharides found in natural products. This constituent, recognized for its ubiquity, is presented in most instances as its N-acetyl derivative 2-acetamido-2-deoxy-d-glucopyranose (N-acetylglucosamine, GlcNAc, NAG). It occurs as the β-linked pyranosyl group in polysaccharides and oligosaccharides, and sometimes as the monosaccharide itself, either in its native state or as a glycoconjugate. The compound's acylation profile and other aspects of its structure are important elements in determining the variety of reactivities and functions of the molecule as a whole. Methods elaborated to investigate these challenges have been intensively reviewed; however, a relatively more comprehensive reviewing of this subject is introduced here to cover some aspects that have not been sufficiently covered. This might enable those who are beginners in this field to be aware of the subject in a more comprehensive context. 2-Amino-2-deoxy-d-glucosylation strategies demand robust amino-protecting groups that survive under a variety of chemical conditions, yet provide groups that can be deprotected under relatively mild conditions. At the end of this review, a table that includes all the N-protecting groups that have been used for glucosamine is provided to introduce them at a glance to aid in constructing building blocks that will act as useful 2-amino-2-deoxy-d-glucosyl donors.

A PEGylated photocleavable auxiliary mediates the sequential enzymatic glycosylation and native chemical ligation of peptides

Research aimed at understanding the specific role of glycosylation patterns in protein function would greatly benefit from additional approaches allowing direct access to homogeneous glycoproteins. Herein the development and application of an efficient approach for the synthesis of complex homogenously glycosylated peptides based on a multifunctional photocleavable auxiliary is described. The presence of a PEG polymer within the auxiliary enables sequential enzymatic glycosylation and straightforward isolation in excellent yields. The auxiliary-modified peptides can be directly used in native chemical ligations with peptide thioesters easily obtained by direct hydrazinolysis of the respective glycosylated peptidyl resins and subsequent oxidation. The ligated glycopeptides can be smoothly deprotected by UV irradiation. We apply this approach to the preparation of variants of the epithelial tumor marker MUC1 carrying one or more Tn, T, or sialyl-T antigens.

Chemical and chemoenzymatic synthesis of glycoproteins for deciphering functions

Glycoproteins are an important class of biomolecules involved in a number of biological recognition processes. However, natural and recombinant glycoproteins are usually produced as mixtures of glycoforms that differ in the structures of the pendent glycans, which are difficult to separate in pure glycoforms. As a result, synthetic homogeneous glycopeptides and glycoproteins have become indispensable probes for detailed structural and functional studies. A number of elegant chemical and biological strategies have been developed for synthetic construction of tailor-made, full-size glycoproteins to address specific biological problems. In this review, we highlight recent advances in chemical and chemoenzymatic synthesis of homogeneous glycoproteins. Selected examples are given to demonstrate the applications of tailor-made, glycan-defined glycoproteins for deciphering glycosylation functions.