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تسجيل

Polysaccharides:- Glycolipids and Lipopolysaccharides Are Membrane Components

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  260

2026-04-30

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Polysaccharides:- Glycolipids and Lipopolysaccharides Are Membrane Components

Glycoproteins are not the only cellular components that bear complex oligosaccharide chains; some lipids, too, have covalently bound oligosaccharides. Gangliosides are membrane lipids of eukaryotic cells in which the po lar head group, the part of the lipid that forms the outer surface of the membrane, is a complex oligosaccharide containing sialic acid (Fig. 7–9) and other monosaccha ride residues. Some of the oligosaccharide moieties of gangliosides, such as those that determine human blood groups , are identical with those found in certain glycoproteins, which therefore also contribute to blood group type determination. Like the oligosaccharide moieties of glycoproteins, those of membrane lipids are generally, perhaps always, found on the outer face of the plasma membrane. Lipopolysaccharides are the dominant surface feature of the outer membrane of gram-negative bacteria such as Escherichia coli and Salmonella typhimurium. These molecules are prime targets of the antibodies produced by the vertebrate immune system in response to bacterial infection and are therefore important determinants of the serotype of bacterial strains (serotypes are strains that are distinguished on the ba sis of antigenic properties). The lipopolysaccharides of S. typhimurium contain six fatty acids bound to two glucosamine residues, one of which is the point of attachment for a complex oligosaccharide (Fig. 7–32). E. coli has similar but unique lipopolysaccharides. The lipopolysaccharides of some bacteria are toxic to hu mans and other animals; for example, they are responsible for the dangerously lowered blood pressure that occurs in toxic shock syndrome resulting from gram negative bacterial infections.