Prominent members of the human gut microbiota express endo-acting O-glycanases to initiate mucin breakdown

Abstract

33The human gut microbiota (HGM) are closely associated with health, development and 34 disease. The thick intestinal mucus layer, especially in the colon, is the key barrier between 35 the contents of the lumen and the epithelial cells, providing protection against infiltration by 36 the microbiota as well potential pathogens. The upper layer of the colonic mucus is a niche for 37 a subset of the microbiota which utilise the mucin glycoproteins as a nutrient source and mucin 38 grazing by the microbiota appears to play a key role in maintaining barrier function as well as 39 community stability. Despite the importance of mucin breakdown for gut health, the 40 mechanisms by which gut bacteria access this complex glycoprotein are not well understood. 41The current model for mucin degradation involves exclusively exo-acting glycosidases that 42 sequentially trim monosaccharides from the termini of the glycan chains to eventually allow 43 access to the mucin peptide backbone by proteases. However, this model is in direct contrast 44 to the Sus paradigm of glycan breakdown used by the Bacteroidetes which involves 45 extracellular cleavage of glycans by surface located endo-acting enzymes prior to import of 46 the oligosaccharide products. Here we describe the discovery and characterisation of endo-47 acting family 16 glycoside hydrolases (GH16s) from prominent mucin degrading gut bacteria 48 that specifically target the oligosaccharide side chains of intestinal mucins from both animals 49 and humans. These endo-acting O-glycanases display β1,4-glactosidase activity and in 50 several cases are surface located indicating they are involved in the initial step in mucin 51 breakdown. The data suggest a new paradigm for mucin breakdown by the microbiota and 52 the endo-mucinases provide a potential tool to explore changes that occur in mucin structure 53 in intestinal disorders such as inflammatory bowel disease and colon cancer. 54 suggesting endo-like cleavage of the O-glycan chains. To investigate the identity of these 126 products in more detail, the products were labelled with the fluorophore procainamide and 127 analysed by liquid chromatography-fluorescence-detection-electrospray-mass spectrometry 128 (LC-FLD-ESI-MS) and the glycan structures determined by MS/MS (Fig. 2a). The data show 129 that oligosaccharides are produced by the GH16 enzymes with between 2 and 6 alternating 130 hexose and HexNAc sugars that are likely to be sections of the polyLacNAc repeats that 131 form the repeating unit of O-glycan chains (Fig. 1b). The reducing ends were all hexoses, 132 11 Desai, M. S. et al. A Dietary Fiber-Deprived Gut Microbiota Degrades the Colonic Mucus 595 Barrier and Enhances Pathogen Susceptibility. Egan, M. et al. Cross-feeding by Bifidobacterium breve UCC2003 during co-cultivation with 598Bifidobacterium bifidum PRL2010 in a mucin-based medium.

Structural basis of mammalian mucin processing by the human gut O-glycopeptidase OgpA from Akkermansia muciniphila

“…This indicates that O-glycopeptidases can work together with other enzymes (e.g., glycoside hydrolases) to process mucins. Very recently, crystal structures of endo-acting GH16 O-glycanases from B. caccae and A. muciniphila were reported 85 . The authors propose that these periplasmic O-glycanases initiate the degradation of O-glycans in mucins, followed by the action of exo-acting glycoside hydrolases before peptidases and O-glycopeptidases trim the remaining mucin backbone.…”

Combined whole cell wall analysis and streamlined in silico carbohydrate-active enzyme discovery to improve biocatalytic conversion of agricultural crop residues

“…Additionally, the combination of glycomics and multiomics provides direct and indirect insights into plant cell wall structure and saccharification of recalcitrant biomass. The use of glycomics in conjugation with -omics has been used to determine the activity and saccharification products of CAZymes in a variety of fields (e.g., human health [219], soil health/carbon sinks [220], novel enzyme discovery [221], and recalcitrant biomass saccharification [222]). However, this strategy is challenged by the complexityof host dynamic microbial ecosystems, CAZymes, and complex carbohydrate structures.…”

Communal living: glycan utilization by the human gut microbiota

“…Bacteroides species (Bacteroidetes phylum) can encode 100 to over 300 CAZymes (El Kaoutari et al ., 2013) and are therefore considered glycan utilization ‘generalists’, in that their deep arsenal of CAZymes allows them to access a wide range of structurally diverse glycans ( Table 1) through rapid transcriptional switching in response to substrate availability (Martens et al ., 2011; Rogers et al ., 2013; Patnode et al ., 2019). Notably, the vast majority of novel CAZyme families, activities, and tertiary structures disclosed in recent years have come from functional analyses of the HGM (Cartmell et al ., 2011; 2018; Larsbrink et al ., 2014; Cuskin et al ., 2015; Rogowski et al ., 2015; Hemsworth et al ., 2016; Bagenholm et al ., 2017; Munoz‐Munoz et al ., 2017; Ndeh et al ., 2017; 2020; Luis et al ., 2018; Briliute et al ., 2019; Helbert et al ., 2019; Crouch et al ., 2020; Dejean et al ., 2020). Key recent advances in the structural biology of the HGM have been reviewed (Tamura and Brumer, 2021).…”

“…Both species possess PULs comprising GH and PL families that enable pectin deconstruction (Despres et al ., 2016; Ndeh et al ., 2017; Luis et al ., 2018). In contrast, exclusive PUL complements (Martens et al ., 2011) allow B. thetaiotaomicron to access host‐derived O ‐glycans (Crouch et al ., 2020) and α‐mannan (Cuskin et al ., 2015), whereas B. ovatus is distinctly equipped to access plant‐derived hemicelluloses, including xyloglucan, xylan, mixed‐linkage β‐glucan, and galactomannan (Valentine and Salyers, 1992; Larsbrink et al ., 2014; Rogowski et al ., 2015; Reddy et al ., 2016; Bagenholm et al ., 2017; Tamura et al ., 2017).…”