Mammalian sialyltransferases allow efficient E. coli-based production of mucin-type O-glycoproteins but can also transfer Kdo

Streamlining Sulfated Oligosaccharide and Glycan Synthesis with Engineered Mutant 6-SulfoGlcNAcases

Sulfation is a common, but poorly understood, post-glycosylational modification (PGM) used to modulate biological function. To deepen our understanding of the roles of various sulfated glycoforms and their relevant binding proteins, we must expand our enzymatic toolkit for their synthesis. Here, we bypass the need for both sulfotransferases and glycosyltransferases by engineering a series of mutants of a 6-SulfoGlcNAcase, from Streptococcus pneumoniae, to directly and efficiently synthesize not only the ubiquitous 6S-GlcNAc-β-1,3-Gal linkage prevalent within host glycans, but also the 6S-GlcNAc-β-1,6-GalNAc commonly observed within core-6 O-glycans, and the more exotic 6S-GlcNAc-β-1,4-GalNAc linkage. We further elaborate these into complex sulfated N-glycan and O-glycan structures of biological relevance. By utilizing the cost-effective activated donor pNP-6S-GlcNAc in conjunction with mutant GH185 6-SulfoGlcNAcases we demonstrate a simple yet powerful in vitro method for generating well-defined sulfated oligosaccharides and glycoforms for use in a variety of applications including glycan arrays, glycan remodeling, and specificity studies with carbohydrate binding proteins such as lectins.

Detection Strategies for Sialic Acid and Sialoglycoconjugates

Glycoconjugates are a vast class of biomolecules implicated in biological processes important for human health and disease. The structural complexity of glycoconjugates remains a challenge to deciphering their precise biological roles and for their development as biomarkers and therapeutics. Human glycoconjugates on the outside of the cell are modified with sialic (neuraminic) acid residues at their termini. The enzymes that install sialic acids are sialyltransferases (SiaTs), a family of 20 different isoenzymes. The removal and degradation of sialic acids is mediated by neuraminidase (NEU; sialidase) enzymes, of which there are four isoenzymes. In this review, we discuss chemical and biochemical approaches for the detection and analysis of sialoglycoconjugate (SGC) structures and their enzymatic products. The most common methods include affinity probes and synthetic substrates. Fluorogenic and radiolabelled substrates are also important tools for many applications, including screening for enzyme inhibitors. Strategies that give insight into the native substrate-specificity of enzymes that regulate SGCs (SiaT & NEU) are necessary to improve our understanding of the role of sialic acid metabolism in health and disease.

Carbohydrate-active enzyme (CAZyme) discovery and engineering via (Ultra)high-throughput screening

Carbohydrate-active enzymes (CAZymes) constitute a diverse set of enzymes that catalyze the assembly, degradation, and modification of carbohydrates. These enzymes have been fashioned into potent, selective catalysts by millennia of evolution, and yet are also highly adaptable and readily evolved in the laboratory. To identify and engineer CAZymes for different purposes, (ultra)high-throughput screening campaigns have been frequently utilized with great success. This review provides an overview of the different approaches taken in screening for CAZymes and how mechanistic understandings of CAZymes can enable new approaches to screening. Within, we also cover how cutting-edge techniques such as microfluidics, advances in computational approaches and synthetic biology, as well as novel assay designs are leading the field towards more informative and effective screening approaches.

Advances in the understanding and exploitation of carbohydrate-active enzymes

Carbohydrate-active enzymes (CAZymes) are responsible for the biosynthesis, modification and degradation of all glycans in Nature. Advances in genomic and metagenomic methodologies, in conjunction with lower cost gene synthesis, have provided access to a steady stream of new CAZymes with both well-established and novel mechanisms. At the same time, increasing access to cryo-EM has resulted in exciting new structures, particularly of transmembrane glycosyltransferases of various sorts. This improved understanding has resulted in widespread progress in applications of CAZymes across diverse fields, including therapeutics, organ transplantation, foods, and biofuels. Herein, we highlight a few of the many important advances that have recently been made in the understanding and applications of CAZymes.

A high-throughput screening platform for enzymes active on mucin-type O-glycoproteins

Mucin-type O-glycosylation is a post-translational modification present at the interface between cells where it has important roles in cellular communication. However, deciphering the function of O-glycoproteins and O-glycans can be challenging, especially as few enzymes are available for their assembly or selective degradation. Here, to address this deficiency, we developed a genetically encoded screening methodology for the discovery and engineering of the diverse classes of enzymes that act on O-glycoproteins. The method uses Escherichia coli that have been engineered to produce an O-glycosylated fluorescence resonance energy transfer probe that can be used to screen for O-glycopeptidase activity. Subsequent cleavage of the substrate by O-glycopeptidases provides a read-out of the glycosylation state of the probe, allowing the method to also be used to assay glycosidases and glycosyltransferases. We further show the potential of this methodology in the first ultrahigh-throughput-directed evolution of an O-glycopeptidase.