A substantial and increasing number of human diseases are associated with changes in the gut microbiota, and discovering the molecules and mechanisms underlying these associations represents a major research goal. Multiple studies associate Ruminococcus gnavus, a prevalent gut microbe, with Crohn's disease, a major type of inflammatory bowel disease. We have found that R. gnavus synthesizes and secretes a complex glucorhamnan polysaccharide with a rhamnose backbone and glucose sidechains. Chemical and spectroscopic studies indicated that the glucorhamnan was largely a repeating unit of five sugars with a linear backbone formed from three rhamnose units and a short sidechain composed of two glucose units. The rhamnose backbone is made from 1,2- and 1,3-linked rhamnose units, and the sidechain has a terminal glucose linked to a 1,6-glucose. This glucorhamnan potently induces inflammatory cytokine (TNFα) secretion by dendritic cells, and TNFα secretion is dependent on toll-like receptor 4 (TLR4). We also identify a putative biosynthetic gene cluster for this molecule, which has the four biosynthetic genes needed to convert glucose to rhamnose and the five glycosyl transferases needed to build the repeating pentasaccharide unit of the inflammatory glucorhamnan.

inflammatory bowel disease; microbiome; polysaccharide

13C NMR data:
Linkage	Residue	C1	C2	C3	C4	C5	C6
3,3	aLRhap	101.02	78.55	?	?	?	?
3,2,6	aDGlcp	98.64	73.60	73.69	70.22	72.74	61.17
3,2	aDGlcp	99.18	72.12	70.67	?	?	65.63
3	aLRhap	100.36	77.84	75.94	?	?	?
	aLRhap	102.64	70.54	78.92	72.12	70.09	17.41


1H NMR data:
Linkage	Residue	H1	H2	H3	H4	H5	H6
3,3	aLRhap	5.32	4.07	?	?	?	?
3,2,6	aDGlcp	4.98	3.55	3.74	3.43	3.70	3.77-3.84
3,2	aDGlcp	5.05	3.55	3.81	?	?	3.71-4.13
3	aLRhap	5.20	4.14	4.04	?	?	?
	aLRhap	4.95	4.20	3.88	3.56	3.75	1.27


1H/13C HSQC data:
Linkage	Residue	C1/H1	C2/H2	C3/H3	C4/H4	C5/H5	C6/H6
3,3	aLRhap	101.02/5.32	78.55/4.07	?/?	?/?	?/?	?/?
3,2,6	aDGlcp	98.64/4.98	73.60/3.55	73.69/3.74	70.22/3.43	72.74/3.70	61.17/3.77-3.84
3,2	aDGlcp	99.18/5.05	72.12/3.55	70.67/3.81	?/?	?/?	65.63/3.71-4.13
3	aLRhap	100.36/5.20	77.84/4.14	75.94/4.04	?/?	?/?	?/?
	aLRhap	102.64/4.95	70.54/4.20	78.92/3.88	72.12/3.56	70.09/3.75	17.41/1.27

The spectrum also has 9 signals at unknown positions (not plotted).

There is only one chemically distinct structure:

SVGMWSMILES3D molIonize

 

*O[C@H]1[C@H](O[C@@H]2[C@@H](O)[C@H](C)O[C@@H](O[C@H]3[C@@H](O)[C@H](*)O[C@@H](C)[C@@H]3O)[C@@H]2O[C@H]2O[C@H](CO[C@H]3O[C@H](CO)[C@@H](O)[C@H](O)[C@H]3O)[C@@H](O)[C@H](O)[C@H]2O)O[C@@H](C)[C@H](O)[C@H]1O

762.708 g/mol (C₃₀H₅₀R₂O₂₂, R = next and previous repeats, not counted in mol weight)