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How much do you know about Ganglioside GM1 (a Dominant Molecular Determinants in the Brain)? The glycome of each animal tissue is comprised of glycoproteins, glycolipids, and proteoglycans in varying proportions. The brain is unusual in that its glycome is dominated by glycolipids. These include galactosylceramide and its 3-O-sulfated form sulfatide, which are major myelin lipids, and gangliosides, which are enriched on nerve cells. Together, glycosphingolipids represent ∼80% of the total glycan mass in the brain, and gangliosides carry ∼75% of the brain's sialic acid. In addition to expressing a much higher proportion of its sialome as gangliosides, the adult brain expresses gangliosides of higher complexity than those typical in other tissues . This section describes the structures, biosynthesis, and expression of brain ganglioside GM1; their functions are discussed later in this review.
Glycans in the adult rat brain. The mass of constituent monosaccharides (μmol/g fresh brain wt) for each glycan class and subclass were calculated from published data as follows. 1) Galactosylceramide and sulfatide together represent 19.4 mg/g brain fresh wt, with 76% (by weight) being galactosylceramide . This represents ∼20.3 μmol galactosylceramide and ∼5.8 μmol sulfatide/g fresh wt. 2) Ganglioside sialic acid is expressed at 1.15 mg (3.9 μmol)/g fresh wt. Since there is an average of 2.9 ganglioside monosaccharides per ganglioside sialic acid, gangliosides represent 11.2 μmol monosaccharide/g fresh wt . 3) Glycoproteins represent 71 μmol monosaccharide/g lipid-free dry wt. Since lipid-free dry weight is ∼11% of brain fresh weight, glycoproteins represent ∼7.8 μmol monosaccharide/g fresh wt . Of these, O-linked GalNAc is expressed at 0.28 μmol/g fresh wt. Since there is an average of 3.2 O-linked monosaccharides per O-linked GalNAc, O-linked glycoproteins represent 0.88 μmol/g fresh wt. Proteoglycan 1
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hexosamine is expressed at 4.1 μmol/g lipid-free dry wt in the adult rat brain (∼8.1 μmol total monosaccharide/g dry wt). Based on lipid-free dry weight being 11% of fresh weight, proteoglycans represent 0.89 μmol/g fresh wt .
Sialoglycoproteins and gangliosides in liver and brain. A: protein-bound sialic acid is equivalent in the two tissues, whereas expression of lipid-bound sialic acids (gangliosides) is >10-fold higher in the brain. B: ganglioside patterns (mol%) in human brain and liver.
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