H H NHCOCH3 remainder of H NHCOCH3 oligosaccharide side chain remainder of oligosaccharide side chain or, in some cases—such as collagens—to hydroxylated proline and lysine side chains. This O-linked glycosylation (Figure 13–32), like the extension of N-linked oligosaccharide chains, is catalyzed by a series of glycosyl transferase enzymes that use the sugar nucleotides in the lumen of the Golgi apparatus to add sugars to a protein one at a time. Usually, N-acetylgalactosamine is added first, followed by a variable number of additional sugars, ranging from just a few to 10 or more.
The Golgi apparatus confers the heaviest O-linked glycosylation of all on mucins, the glycoproteins in mucus secretions, and on proteoglycan core proteins, which it modifies to produce proteoglycans. As discussed in Chapter 19, this process involves the polymerization of one or more glycosaminoglycan chains (long, unbranched polymers composed of repeating disaccharide units; see Figure 19–35) onto serines on a core protein. Many proteoglycans are secreted and become components of the extracellular matrix, while others remain anchored to the extracellular face of the plasma membrane. Still others form a major component of slimy materials, such as the mucus that is secreted to form a protective coating on the surface of many epithelia.
The sugars incorporated into glycosaminoglycans are heavily sulfated in the Golgi apparatus immediately after these polymers are made, thus adding a significant portion of their characteristically large negative charge. Some tyrosines in proteins also become sulfated shortly before they exit from the Golgi apparatus. In both cases, the sulfation depends on the sulfate donor 3ʹ-phosphoadenosine-5ʹ-phosphosulfate (PAPS) (Figure 13–33), which is transported from the cytosol into the lumen of the trans Golgi network.
what Is the purpose of glycosylation?
There is an important difference between the construction of an oligosaccharide and the synthesis of other macromolecules such as DNA, RNA, and protein. Whereas nucleic acids and proteins are copied from a template in a repeated series of identical steps using the same enzyme or set of enzymes, complex carbohydrates require a different enzyme at each step, each product being recognized as the exclusive substrate for the next enzyme in the series. The vast abundance of glycoproteins and the complicated pathways that have evolved to synthesize them emphasize that the oligosaccharides on glycoproteins and glycosphingolipids have very important functions.
N-linked glycosylation, for example, is prevalent in all eukaryotes, including yeasts. N-linked oligosaccharides also occur in a very similar form in archaeal cell wall proteins, suggesting that the whole machinery required for their synthesis is evolutionarily ancient. N-linked glycosylation promotes protein folding in two ways. First, it has a direct role in making folding intermediates more soluble, thereby preventing their aggregation. Second, the sequential modifications of the N-linked oligosaccharide establish a “glyco-code” that marks the progression of
Figure 13–32 Nand O-linked glycosylation. In each case, only the single sugar group that is directly attached to the protein chain is shown.
Figure 13–33 The structure of PAPS.
protein folding and mediates the binding of the protein to chaperones (discussed in Chapter 12) and lectins—for example, in guiding ER-to-Golgi transport. As we discuss later, lectins also participate in protein sorting in the trans Golgi network.