Carbohydrates Occur In Cell Membranes In Lipoproteins

In addition to the lipid of cell membranes (see Chapters 14 and 41), approximately 5% is carbohydrate in glyco-proteins and glycolipids. Carbohydrates are also present in apo B of lipoproteins. Their presence on the outer surface of the plasma membrane (the glycocalyx) has been shown with the use of plant lectins, protein agglutinins that bind with specific glycosyl residues. For example, concanavalin A binds a-glucosyl and a-man-nosyl residues. Glycophorin is a major integral membrane glycoprotein of human erythrocytes and spans the lipid membrane, having free polypeptide portions

Figure 13-16. Structure of W-acetylneuraminic acid, a sialic acid (Ac = CH3— CO —).

outside both the external and internal (cytoplasmic)

surfaces. Carbohydrate chains are only attached to the amino terminal portion outside the external surface (Chapter 41).

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