The major nutrients available to human colonic
Bacteroides species are glycans exemplified by pectins, a
network of covalently linked plant cell wall polysaccharides containing
galacturonic acid (GalA). Metabolism of complex carbohydrates by the
Bacteroides genus is orchestrated by polysaccharide
utilisation loci or PULs. In Bacteroides thetaiotaomicron, a
human colonic bacterium, the PULs activated by the different pectin domains have
been identified, however, the mechanism by which these loci contribute to the
degradation of these GalA-containing polysaccharides is poorly understood. Here
we show that each PUL orchestrates the metabolism of specific pectin molecules,
recruiting enzymes from two previously unknown glycoside hydrolase (GH)
families. The apparatus that depolymerizes the backbone of rhamnogalacturonan-I
(RGI) is particularly complex. This system contains several GHs that trim the
remnants of other pectin domains attached to RGI, while nine enzymes contribute
to the degradation of the backbone comprising a rhamnose-GalA repeating unit.
The catalytic properties of the pectin degrading enzymes are optimized to
protect the glycan cues that activate the specific PULs ensuring a continuous
supply of inducing molecules throughout growth. The contribution of
Bacteroides spp. to the metabolism of the pectic network is
illustrated by cross-feeding between organisms.
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