16.5: Glycoproteins and Glycolipids
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Membrane proteins are often covalently linked to oligosaccharides, which are branched glycoside-linked sugars (averaging around fifteen sugar residues). As glycans, they are the sugars linked to glycoproteins and glycolipids.
Glycoproteins are rare in the cytosol but common on secreted and membrane proteins. Oligosaccharides are typically linked to proteins via the hydroxyl group on serine or threonine amino acids (O-glycosylation) and are occasionally to modified amino acids (e.g., hydroxylysine, hydroxyproline) or to the amide nitrogen on asparagine (N-glycosylation). As a major feature of the glycocalyx, oligosaccharide domains of glycoproteins and glycolipids are the “face” of a plasma membrane that communicates with the extracellular world. Figure 16.18 illustrates the glycocalyx.
Oligosaccharides begin their synthesis in the rough endoplasmic reticulum (RER), with the creation of a core glycoside. These partial glycans are enzymatically linked to compatible amino acids of a membrane protein. As these proteins travel through the Golgi vesicles of the endomembrane system, terminal glycosylation attaches more sugars to the core glycoside to complete glycoprotein synthesis. When vesicles budding from the trans-Golgi vesicles fuse with the plasma membrane, the sugars on the glycoproteins end up on the exterior cell surface. This is illustrated in the following link.
291 The Path to Sugar-Coated Cells
Like glycoproteins, glycolipids are only found on the extracellular surface. Glycolipids are synthesized in much the same way as glycoproteins. Specific enzymes catalyze initial glycosylation of phospholipids and polypeptides, followed by terminal glycosylation reactions. Glycoproteins, glycolipids, and proteoglycans on cell surfaces play critical roles in cell-cell recognition and the formation of tissues. They recognize and bind to carbohydrate receptors (lectins) on adjacent cells, leading to cell-cell attachment as well as intracellular responses in the interacting cells.
Glycoproteins and glycolipids also mediate the interaction of cells with extracellular molecular signals and with chemicals of the ECM, or extracellular matrix (Figure 16.19). The ECM includes components of connective tissue, basement membranes, and, in fact, any surface to which cells attach.
