Glucuronoxylomannogalactans (GXMGals) are characteristic capsular polysaccharides produced by the opportunistic fungus C. neoformans, which are implicated in cryptococcal virulence, via impairment of the host immune response. We determined for the first time the structure of a lipoglucuronomannogalactan (LGMGal), isolated from the surface of a mutant C. neoformans carrying a deletion in the UDP-GlcA decarboxylase gene. Monosaccharide composition and methylation analyses, as well as nuclear magnetic resonance spectroscopy were employed in discerning the structure. Our results show that the polysaccharide structure of the LGMGal differs from GXMGal by the absence of xylose and 2-O-acetylated mannose residues. LGMGal consists of a galactan main chain -[-6-α-Gal-]-, where every second Gal residue is substituted at O-3 with an oligosaccharide α-Man6OAc-3-α-Man-4-(β-GlcA-3)-β-Gal-; components in italic being non-stoichiometric. The substitution rate of β-Galp units by GlcpA is 35%. Additionally, we determined that the glycolipid anchor of the LGMGal is based on an myo-inositol phosphoceramide composed of C18-phytosphingosine and monohydroxylated lignoceric acid (2OHC24:0 fatty acid).

cell wall polysaccharide, Cryptococcus neoformans, cryptococcosis, lipoglucuronomannogalactan

13C NMR data:
Linkage	Residue	C1	C2	C3	C4	C5	C6
6	aDGalp	99.2	69.3	70.5	70.4	69.9	67.5
3,3	%bDGlcpA	105.1	74.3	76.5	72.5	76.2	?
3,4,3,6	%Ac	?	21.5
3,4,3	aDManp	103.4	70.8	71.2	67.6	71.8	64.6
3,4	aDManp	102.7	70.5	79.5	66.7	74.2	61.5
3	bDGalp	105.5	71.8	81.3	77.2	76.3	61.5
	aDGalp	99.0	68.3	81.1	70.2	69.9	67.8


1H NMR data:
Linkage	Residue	H1	H2	H3	H4	H5	H6
6	aDGalp	5.01	3.87	3.93	4.06	4.25	3.72-3.96
3,3	%bDGlcpA	4.77	3.41	3.57	3.59	3.87	-
3,4,3,6	%Ac	-	2.20
3,4,3	aDManp	5.17	4.14	3.95	3.79	4.04	4.37-4.44
3,4	aDManp	4.88	4.22	4.00	3.86	4.14	3.86
3	bDGalp	4.71	3.85	3.91	4.30	3.84	3.77
	aDGalp	5.04	4.07	4.05	4.33	4.25	3.73-3.96


1H/13C HSQC data:
Linkage	Residue	C1/H1	C2/H2	C3/H3	C4/H4	C5/H5	C6/H6
6	aDGalp	99.2/5.01	69.3/3.87	70.5/3.93	70.4/4.06	69.9/4.25	67.5/3.72-3.96
3,3	%bDGlcpA	105.1/4.77	74.3/3.41	76.5/3.57	72.5/3.59	76.2/3.87	
3,4,3,6	%Ac		21.5/2.20
3,4,3	aDManp	103.4/5.17	70.8/4.14	71.2/3.95	67.6/3.79	71.8/4.04	64.6/4.37-4.44
3,4	aDManp	102.7/4.88	70.5/4.22	79.5/4.00	66.7/3.86	74.2/4.14	61.5/3.86
3	bDGalp	105.5/4.71	71.8/3.85	81.3/3.91	77.2/4.30	76.3/3.84	61.5/3.77
	aDGalp	99.0/5.04	68.3/4.07	81.1/4.05	70.2/4.33	69.9/4.25	67.8/3.73-3.96

The spectrum also has 2 signals at unknown positions (not plotted).

There is only one chemically distinct structure:

SVGMWSMILES3D molIonize

 

*OC[C@H]1O[C@H](OC[C@H]2O[C@H](*)[C@H](O)[C@@H](O[C@@H]3O[C@H](CO)[C@H](O[C@H]4O[C@H](CO)[C@@H](O)[C@H](O[C@H]5O[C@H](COC(C)=O)[C@@H](O)[C@H](O)[C@@H]5O)[C@@H]4O)[C@H](O[C@@H]4O[C@H](C(=O)O)[C@@H](O)[C@H](O)[C@H]4O)[C@H]3O)[C@H]2O)[C@H](O)[C@@H](O)[C@H]1O

1028.866 g/mol (C₃₈H₆₀R₂O₃₂, R = next and previous repeats, not counted in mol weight)