Keratanase II-Catalyzed Synthesis of Keratan Sulfate Oligomers by Using Sugar Oxazolines as Transition-State Analogue Substrate Monomers: A Novel Insight into the Enzymatic Catalysis Mechanism

A Biomimetic Synthetic Strategy Can Provide Keratan Sulfate I and II Oligosaccharides with Diverse Fucosylation and Sulfation Patterns

Keratan sulfate (KS) is a proteoglycan that is widely expressed in the extracellular matrix of various tissue types, where it performs multiple biological functions. KS is the least understood proteoglycan, which in part is due to a lack of panels of well-defined KS oligosaccharides that are needed for structure-binding studies, as analytical standards, to examine substrate specificities of keratinases, and for drug development. Here, we report a biomimetic approach that makes it possible to install, in a regioselective manner, sulfates and fucosides on oligo-N-acetyllactosamine (LacNAc) chains to provide any structural element of KS by using specific enzyme modules. It is based on the observation that α1,3-fucosides, α2,6-sialosides and C-6 sulfation of galactose (Gal6S) are mutually exclusive and cannot occur on the same LacNAc moiety. As a result, the pattern of sulfation on galactosides can be controlled by installing α1,3-fucosides or α2,6-sialosides to temporarily block certain LacNAc moieties from sulfation by keratan sulfate galactose 6-sulfotransferase (CHST1). The patterns of α1,3-fucosylation and α2,6-sialylation can be controlled by exploiting the mutual exclusivity of these modifications, which in turn controls the sites of sulfation by CHST1. Late-stage treatment with a fucosidase or sialidase to remove blocking fucosides or sialosides provides selectively sulfated KS oligosaccharides. These treatments also unmasked specific galactosides for further modification by CHST1. To showcase the potential of the enzymatic strategy, we have prepared a range of poly-LacNAc derivatives having different patterns of fucosylation and sulfation and several N-glycans decorated by specific arrangements of sulfates.

Exploiting Substrate Specificities of 6-O-Sulfotransferases to Enzymatically Synthesize Keratan Sulfate Oligosaccharides

Keratan sulfate (KS) is a glycosaminoglycan that is widely expressed in the extracellular matrix of various tissue types, where it is involved in many biological processes. Herein, we describe a chemo-enzymatic approach to preparing well-defined KS oligosaccharides by exploiting the known and newly discovered substrate specificities of relevant sulfotransferases. The premise of the approach is that recombinant GlcNAc-6-O-sulfotransferases (CHST2) only sulfate terminal GlcNAc moieties to give GlcNAc6S that can be galactosylated by B4GalT4. Furthermore, CHST1 can modify the internal galactosides of a poly-LacNAc chain; however, it was found that a GlcNAc6S residue greatly increases the reactivity of CHST1 of a neighboring and internal galactoside. The presence of a 2,3-linked sialoside further modulates the site of modification by CHST1, and a galactoside flanked by 2,3-Neu5Ac and GlcNAc6S is preferentially sulfated over the other Gal residues. The substrate specificities of CHST1 and 2 were exploited to prepare a panel of KS oligosaccharides, including selectively sulfated N-glycans. The compounds and several other reference derivatives were used to construct a microarray that was probed for binding by several plant lectins, Siglec proteins, and hemagglutinins of influenza viruses. It was found that not only the sulfation pattern but also the presentation of epitopes as part of an O- or N-glycan determines binding properties.

Chemoenzymatic Synthesis of Sialyl Sulfo-Oligosaccharides as Potent Siglec-8 Ligands via Transglycosylation Catalyzed by Keratanase II

Sialyl type-II sulfo-oligosaccharides are gaining much attention as bioactive ligands for Siglecs. In this study, we have achieved the first synthesis of sialyl type-II sulfo-oligosaccharides chemoenzymatically by utilizing the transglycosylation activity of keratanase II. The oxazoline derivative of α(2→3)-sialylated 6,6'-di-sulfo-LacNAc (3) was newly designed as the glycosyl donor for enzymatic transglycosylation. Keratanase II efficiently catalyzed the transglycosylation of 3 with two kinds of glycosyl acceptors, 6-sulfo-Lewis X and 6,6'-di-sulfo-LacNAc derivatives, providing sialyl sulfo-hexasaccharide (1) and sialyl sulfo-pentasaccharide (2) with 86 and 95% yields, respectively. The products 1 and 2 showed higher affinity to Siglec-8 with K_D 70 and 25 μmol·L^-1, respectively, compared to the known ligand of the α(2→3)-sialylated 6,6'-di-sulfo-Lewis X with K_D 185 μmol·L^-1. Thus, this study will advance not only the study of Siglec-8 biology but also the exploration of functions of sialyl sulfo-oligosaccharides having various microstructures.

Progress and challenges in the synthesis of sequence controlled polysaccharides

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure-properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

Current state on the enzymatic synthesis of glycosaminoglycans

Glycosaminoglycans (GAGs) are linear anionic polysaccharides, and most of them show a specific sulfation pattern. GAGs have been studied for decades, and still, new biological functions are discovered. Hyaluronic acid and heparin are sold for medical or cosmetic applications. With increased market and applications, the production of GAGs stays in the focus of research groups and the industry. Common industrial GAG production relies on the extraction of animal tissue. Contamination, high dispersity, and uncontrolled sulfation pattern are still obstacles to this process. Tailored production strategies for the chemoenzymatic synthesis have been developed to address these obstacles. In recent years, enzyme cascades, including uridine-5'-diphosphate sugar syntheses, were established to obtain defined polymer size and dispersity, as well as defined sulfation patterns. Nevertheless, the complex synthesis of GAGs is still a challenging research field.

Proteoglycan (Keratan Sulfate) Barrier in Developing Human Forebrain Isolates Cortical Epileptic Networks From Deep Heterotopia, Insulates Axonal Fascicles, and Explains Why Axosomatic Synapses Are Inhibitory

Axons from deep heterotopia do not extend through U-fibers, except transmantle dysplasias. Keratan sulfate (KS) in fetal spinal cord/brainstem median septum selectively repels glutamatergic axons while enabling GABAergic commissural axons. Immunocytochemical demonstration of KS in neocortical resections and forebrain at autopsy was studied in 12 fetuses and neonates 9-41 weeks gestational age (GA), 9 infants, children, and adolescents and 5 patients with focal cortical dysplasias (FCD1a). From 9 to 15 weeks GA, no KS is seen in the cortical plate; 19-week GA reactivity is detected in the molecular zone. By 28 weeks GA, patchy granulofilamentous reactivity appears in extracellular matrix and adheres to neuronal somata with increasing intensity in deep cortex and U-fibers at term. Perifascicular KS surrounds axonal bundles of both limbs of the internal capsule and within basal ganglia from 9 weeks GA. Thalamus and globus pallidus exhibit intense astrocytic reactivity from 9 weeks GA. In FCD1a, U-fiber reactivity is normal, discontinuous or radial. Ultrastructural correlates were not demonstrated; KS is not electron-dense. Proteoglycan barrier of the U-fiber layer impedes participation of deep heterotopia in cortical epileptic networks. Perifascicular KS prevents aberrant axonal exit from or entry into long and short tracts. KS adhesion to neuronal somatic membranes may explain inhibitory axosomatic synapses.

Oligosaccharide Synthesis and Translational Innovation

The translation of biological glycosylation in humans to the clinical applications involves systematic studies using homogeneous samples of oligosaccharides and glycoconjugates, which could be accessed by chemical, enzymatic or other biological methods. However, the structural complexity and wide-range variations of glycans and their conjugates represent a major challenge in the synthesis of this class of biomolecules. To help navigate within many methods of oligosaccharide synthesis, this Perspective offers a critical assessment of the most promising synthetic strategies with an eye on the therapeutically relevant targets.

Synthesis of 13 C-labelled sulfated N-acetyl-d-lactosamines to aid in the diagnosis of mucopolysaccharidosis diseases

Morquio A syndrome is an autosomal mucopolysaccharide storage disorder that leads to accumulation of keratan sulfate. Diagnosis of this disease can be aided by measuring the levels of keratan sulfate in the urine. This requires the liquid chromatography tandem mass spectrometry (LCMS/MS) measurement of sulfated N-acetyl-d-lactosamines in the urine after cleavage of the keratan sulfate with keratanase II. Quantification requires isotopically-labelled internal standards. The synthesis of these ¹³ C₆ -labelled standards from ¹³ C₆ -galactose and N-acetylglucosamine is described. The required protected disaccharide is prepared utilising a regioselective, high yielding β-galactosylation of a partially protected glucosamine acceptor and an inverse addition protocol. Subsequent synthesis of the ¹³ C₆ -labelled mono and disulfated N-acetyllactosamines was achieved in five and eight steps, respectively, from this intermediate to provide internal standards for the LCMS/MS quantification of keratan sulfate in urine.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.