Complex N-glycan breakdown by gut Bacteroides involves an extensive enzymatic apparatus encoded by multiple co-regulated genetic loci

Abstract

Glycans are the major carbon sources available to the human colonic microbiota. Numerous N-glycosylated proteins are found in the human gut, from both dietary and host sources, including immunoglobulins such as IgA which are secreted into the intestine at high levels. Here we show that many mutualistic gut Bacteroides spp. have the capacity to utilise complex N-glycans (CNGs) as nutrients, including those from immunoglobulins. Detailed mechanistic studies using transcriptomic, biochemical, structural and genetic techniques reveal the pathway employed by B. thetaiotaomicron (Bt) for CNG degradation. The breakdown process involves an extensive enzymatic apparatus encoded by multiple non-adjacent loci and comprises 19 different carbohydrate-active enzymes (CAZymes) from different families, including a CNG specific endo-glycosidase activity. Furthermore, CNG degradation involves the activity of CAZymes that have previously been implicated in the degradation of other 2 classes of glycan. This complex and diverse apparatus provides Bt with the capacity to access the myriad different structural variants of CNGs likely to be found in the intestinal niche.

High levels of fucosylation and sialylation of milk N‐glycans from mothers with gestational diabetes mellitus alter the offspring gut microbiome and immune balance in mice

“…Although direct mother‐to‐child bacterial transmission significantly contributes to the establishment of the neonatal microbiome, feeding pattern also has a major influence . Because gut microorganisms have differing abilities to utilize different types of glycans as carbon sources for growth and metabolism, differences in milk glycans may also affect the composition of the gut microbiome in the offspring. It has been shown that the milk N‐glycans of secretory immunoglobulin A (sIgA) and lactoferrin are affected by GDM, but not the HMOs .…”

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

“…See Supplementary Figs. 9, 10 and 19 and Briliūtė et al 2019 for activities of exo-acting enzymes against defined oligosaccharides.…”

“…Inclusion of further exo-glycosidases with the BF4058 GH16 , sialidase and fucosidase digests reveal further insight into the oligosaccharide structures released by the GH16. The addition of a β1,4-galactosidase (BT0461 GH2 ) 25 removes one of the glycan 5 peaks, indicating this saccharide is capped with a β1,4-galactose, while both glycan 5 peaks disappear with the addition of a β1,3/4-galactosidase (BF4061 GH35 ; see Supplementary discussion and supplementary Fig. 10), indicating the other glycan 5 peak is capped with a β1,3-galactose.…”

“…10), indicating the other glycan 5 peak is capped with a β1,3-galactose. Interestingly, the β-GlcNAc’ase (BT0459 GH20 ) 25 could degrade the sulfated GlcNAc-Gal disaccharide, although the position of the sulfation in not known so it is unclear at which position the enzyme can accommodate this modification (Fig. 2b, glycan 3 ).…”

Growing, evolving and sticking in a flowing environment: understanding IgA interactions with bacteria in the gut

“…First, bacteria can interact with sIgA in a non‐cognate way by binding to the extensive O‐glycans at the hinge regions and N‐glycans attached to the secretory component . Indeed many Bacteroides strains carry the complete machinery required to degrade mammalian N‐glycans and may reasonably use these as a carbon source in the gut lumen, similar to the accepted role of mucins as a bacterial carbon source. Non‐cognate binding of sIgA to B. thetaiotaomicron mediated an effect on the expression of polysaccharide utilization loci and facilitated symbiosis with other phyla such Firmicutes .…”