Biological Chemistry - Berezov, T. T., & Korovkin, B. F. 1998

Chemistry of Lipids
Glycolipids (Glycosphingolipids)

Glycolipids are widely distributed in various tissues, especially in Nervous Tissue, notably the Brain. The primary form of glycolipids in animal tissues is glycosphingolipids. The latter consist of a ceramide—composed of the amino alcohol sphingosine and a fatty acid residue—and one or more sugar residues.

The simplest glycosphingolipids are galactosylceramides and glucosylceramides.

Galactosylceramides* are the major Sphingolipids of the brain and other neural tissues, though they also occur in small amounts in many other tissues. They contain a hexose (typically D-galactose) linked via an ether bond to the hydroxyl group of the amino alcohol sphingosine. In addition, galactosylceramides contain a fatty acid, most commonly lignoceric, nervonic, or cerebronic acid—i.e., Fatty acids with 24 carbon atoms.

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Sulfogalactosylceramides differ from galactosylceramides by the presence of a sulfuric acid residue attached to the third carbon atom of the hexose. In the mammalian brain, sulfogalactosylceramides are localized mainly in the White matter, although their concentration is much lower than that of galactosylceramides.

Glucosylceramides are simple glycosphingolipids found predominantly in non-neural tissues. They are also present in trace amounts in brain tissue. Unlike galactosylceramides, they contain a glucose residue instead of galactose.

More complex glycosphingolipids are gangliosides, which are derived from glucosylceramides. Gangliosides additionally contain one or more molecules of sialic acid. In human tissues, the predominant sialic acid is neuraminic acid. Furthermore, instead of a single glucose residue, they typically contain a complex oligosaccharide chain. Gangliosides are abundant in nervous tissue, where they appear to function as receptors and perform other roles. One of the simplest gangliosides is hematoside, originally isolated from erythrocyte stroma; it contains a ceramide (acylsphingosine), one molecule of glucose, and one molecule of N-acetylneuraminic acid.

* Derivatives of galactosylceramides are frequently referred to as cerebrosides.