Cerebrosides

 Cerebrosides

Cerebroside is the collective name for group of glycosphingolipids called monoglycosylceramides which are very important components in animal muscle and nerve cell membranes.

These consist of a ceramide with a single sugar residue at the 1-hydroxyl moiety. The sugar residue can be either glucose or galactose; the two major types are therefore called glucocerebrosides (glucosylceramides) and galactocerebrosides (galactosylceramides). Galactocerebrosides are typically found in neural tissue, while glucocerebrosides are found in other tissues.

Structure

The fundamental structure of a cerebroside is ceramide. Monoglycosyl and oligoglycosylceramides having a mono or polysaccharide bonded glycosidically to the terminal OH group of ceramide are defined as cerebrosides. Sphingosine is the main long-chain base present in ceramide.

Galactosylceramide is the principal glycosphingolipid in brain tissue. Galactosylceramides are present in all nervous tissues, and can compose up to 2% dry weight of grey matter and 12% of white matter. They are major constituents of oligodendrocytes. Glucosylceramide is found at low levels in animal cells such as the spleen, erythrocytes, and nervous tissues, especially neurons.

Glucosylceramide is a major constituent of skin lipids, where it is essential for lamellar body formation in the stratum corneum and to maintain the water permeability barrier of the skin. Glucosylceramide is the only glycosphingolipid common to plants, fungi and animals. It is usually considered to be the principal glycosphingolipid in plants. It is a major component of the outer layer of the plasma membrane. Galactosyl ceramides have not been found in plants.

Monogalactosylceramide is the largest single component of the myelin sheath of nerves. Cerebroside synthesis can therefore give a measurement of myelin formation or remyelination. The sugar moiety is linked glycosidically to the C-1 hydroxyl group of ceramide, such as in lactosylceramide.

Cerebrosides containing a sulfuric ester (sulfate) group, known as sulfatides, also occur in the myelin sheath of nerves. These compounds are preferably named as sulfates of the parent glycosphingolipid.

Synthesis

The biosynthesis of monoglycosylceramides requires a direct transfer of the carbohydrate moiety from a sugar-nucleotide, such as uridine 5-diphosphate (UDP)-galactose, or UDP-glucose to the ceramide unit. The glycosyl-transferase catalyzed reaction results in an inversion of the glycosidic bond stereochemistry, changing from α →β. Synthesis of galactosylceramide, and glucosylceramide occurs on the lumenal surface of the endoplasmic reticulum, and on the cytosolic side of the early Golgi membranes respectively.

Fate

Degradation of glycosphingolipids occurs in the lysosome, which contains digestive enzymes in animal cells. The lysosome breaks down the glycosphingolipid to its primary components, fatty acids, sphingosine and saccharide -