Figure 13–31 Oligosaccharide processing in the ER and the Golgi apparatus. The processing pathway is highly ordered, so that each step shown depends on the previous one. step 1: processing begins in the er with the removal of the glucoses from the oligosaccharide initially transferred to the protein. Then a mannosidase in the er membrane removes a specific mannose. The remaining steps occur in the golgi stack. step 2: golgi mannosidase I removes three more mannoses. step 3: n-acetylglucosamine transferase I then adds an n-acetylglucosamine. step 4: Mannosidase II then removes two additional mannoses. This yields the final core of three mannoses that is present in a complex oligosaccharide. at this stage, the bond between the two n-acetylglucosamines in the core becomes resistant to attack by a highly specific endoglycosidase (Endo H). since all later structures in the pathway are also endo h-resistant, treatment with this enzyme is widely used to distinguish complex from high-mannose oligosaccharides. step 5: finally, as shown in figure 13–30, additional n-acetylglucosamines, galactoses, and sialic acids are added. These final steps in the synthesis of a complex oligosaccharide occur in the cisternal compartments of the golgi apparatus: three types of glycosyl transferase enzymes act sequentially, using sugar substrates that have been activated by linkage to the indicated nucleotide; the membranes of the golgi cisternae contain specific carrier proteins that allow each sugar nucleotide to enter in exchange for the nucleoside phosphates that are released after the sugar is attached to the protein on the lumenal face.
note that, as a biosynthetic organelle, the golgi apparatus differs from the er: all sugars in the golgi are assembled inside the lumen from sugar nucleotide, whereas in the er, the n-linked precursor oligosaccharide is assembled partly in the cytosol and partly in the lumen, and most lumenal reactions use dolichol-linked sugars as their substrates (see figure 12–51).
complex oligosaccharides are of special importance because they bear a negative charge. Whether a given oligosaccharide remains high-mannose or is processed depends largely on its position in the protein. If the oligosaccharide is accessible to the processing enzymes in the Golgi apparatus, it is likely to be converted to a complex form; if it is inaccessible because its sugars are tightly held to the protein’s surface, it is likely to remain in a high-mannose form. The processing that generates complex oligosaccharide chains follows the highly ordered pathway shown in Figure 13–31.
Beyond these commonalities in oligosaccharide processing that are shared among most cells, the products of the carbohydrate modifications carried out in the Golgi apparatus are highly complex and have given rise to a new field of study called glycobiology. The human genome, for example, encodes hundreds of different Golgi glycosyl transferases and many glycosidases, which are expressed differently from one cell type to another, resulting in a variety of glycosylated forms of a given protein or lipid in different cell types and at varying stages of differentiation, depending on the spectrum of enzymes expressed by the cell. The complexity of modifications is not limited to N-linked oligosaccharides but also occurs on O-linked sugars, as we discuss next.
proteoglycans are assembled in the golgi apparatus
In addition to the N-linked oligosaccharide alterations made to proteins as they pass through the Golgi cisternae en route from the ER to their final destinations, many proteins are also modified in the Golgi apparatus in other ways. Some proteins have sugars added to the hydroxyl groups of selected serines or threonines,