Glycan complexity dictates microbial resource allocation in the large intestine

Abstract

The structure of the human gut microbiota is controlled primarily through the degradation of complex dietary carbohydrates, but the extent to which carbohydrate breakdown products are shared between members of the microbiota is unclear. We show here, using xylan as a model, that sharing the breakdown products of complex carbohydrates by key members of the microbiota, such as Bacteroides ovatus, is dependent on the complexity of the target glycan. Characterization of the extensive xylan degrading apparatus expressed by B. ovatus reveals that the breakdown of the polysaccharide by the human gut microbiota is significantly more complex than previous models suggested, which were based on the deconstruction of xylans containing limited monosaccharide side chains. Our report presents a highly complex and dynamic xylan degrading apparatus that is fine-tuned to recognize the different forms of the polysaccharide presented to the human gut microbiota.

Polysaccharide Utilization Loci: Fueling Microbial Communities

“…2A) (79). Interestingly, this form of syntrophy was observed only during the utilization of simple, linear xylans.…”

“…1 and 2, blue) (78)(79)(80); in the Sus, this role is fulfilled by SusG, a GH13 endo-␣1,4-glucanase (␣-amylase) (65,69). The resulting fragments are actively shuttled into the periplasmic space by the TBDT (e.g., SusC [61,64]), where additional linkage-specific GHs ( Fig.…”

“…A pair of xylan-targeting PULs from B. ovatus, PUL-XylS and PUL-XylL, was found to encode enzymes tailored for individual plant ␤-xylans that varied in their composition and branching (79). Recently, the detailed genetic, biochemical, and enzyme structural characterization of a galactomannanspecific PUL from B. ovatus revealed the interplay of two mannan-specific SGBPs, two GH26 endo-␤-mannanases, and a GH36 exo-␣-galactosidase in the deconstruction of this plant cell wall polysaccharide (110,111).…”

“…A picture that has emerged is that individual Bacteroidetes bacteria in complex environments, such as the human gut, contain partially overlapping sets of PULs, which indicate both the ability to respond dynamically to nutrient availability and niche specialization within a web of species. Further, comparison of syntenic PULs across species highlights the ongoing evolution of PUL specificity through stepwise changes in CAZyme cohorts (78,79,86).…”

GH62 arabinofuranosidases: Structure, function and applications

“…1A). In more complex xylans like glucuronoarabinoxylan from corn, -D-1,4-Xylp backbone in addition to the above mentioned substituents is decorated by -L-galactose and (Rogowski et al, 2015;Scheller and Ulvskov, 2010) (Fig 1B). By contrast, L-arabinan has -L-1,5-Araf backbone that is hydrolyzed by other ABFs than of GH62, but which similarly to the AXs is 5 substituted singly -L-1,3-as well as doubly -L-1,2-Araf -L-1,3-Araf, hydrolyzed off by some GH62 enzymes ( Fig.…”