| Sphingolipids containing covalently bound sugars are known as glycosphingolipids, or glycolipids. The cell's complement of carbohydrate structures on glycolipids (and glycoproteins) does not depend on a template mechanism for synthesis, as do nucleic acids and proteins. Instead, it is determined by the enzymatic makeup of the cell (that is, its content of glycosyltransferases), and on how these various enzymes are expressed with respect to each other. The glycosyltransferase distribution and glycosphingolipid content of cells varies during development and in response to regulatory processes.
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| Glycolipids can be classified into four different groups: cerebrosides, sulfatides, globosides, and gangliosides. In all of these compounds, the polar head-group - comprising the sugars - is attached to ceramide by a glycosidic bond at the terminal hydroxyl group of sphingosine; Figure 26.11 illustrates this, together with the pathways involved in the biosynthesis of some of the more complex members of the group. The simplest of the glycosphingolipids are the gluco- and galacto-cerebrosides (glucosyl and galactosyl ceramides), which have a single sugar attached to sphingosine. Sulfatides are formed by addition of sulfate from the sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate (PAPS) (see Fig. 26.6), yielding, for example, galactocerebroside 3-sulfate. When the carbohydrate portion of the cerebroside contains two or more neutral sugars plus an N-acetyl-galactosamine (GalNAc), it is referred to as a globoside. Finally, glycolipids containing sialic acids (N-acetylneuraminic acid, NANA) are termed gangliosides.
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