a series of electron micrographs shows the golgi apparatus (a) unstained, (b) stained with osmium, which preferentially labels the cisternae of the cis compartment, and (c and d) stained to reveal the location of specific enzymes. nucleoside diphosphatase is found in the trans golgi cisternae (c), while acid phosphatase is found in the trans golgi network (d). note that usually more than one cisterna is stained. The enzymes are therefore thought to be highly enriched rather than precisely localized to a specific cisterna. (courtesy of daniel s. friend.) 1 µm
Two broad classes of N-linked oligosaccharides, the complex oligosaccharides and the high-mannose oligosaccharides, are attached to mammalian glycoproteins. Sometimes, both types are attached (in different places) to the same polypeptide chain. Complex oligosaccharides are generated when the original N-linked oligosaccharide added in the ER is trimmed and further sugars are added; by contrast, high-mannose oligosaccharides are trimmed but have no new sugars added to them in the Golgi apparatus (Figure 13–30). The sialic acids in the = N-acetylneuraminic acid (sialic acid, or NANA) (C) HIGH-MANNOSE OLIGOSACCHARIDE
Figure 13–30 The two main classes of asparagine-linked (N-linked) oligosaccharides found in mature mammalian glycoproteins. (a) both complex oligosaccharides and high-mannose oligosaccharides share a common core region derived from the original n-linked oligosaccharide added in the er (see figure 12–50) and typically containing two n-acetylglucosamines (glcnac) and three mannoses (Man). (b) each complex oligosaccharide consists of a core region, together with a terminal region that contains a variable number of copies of a special trisaccharide unit (n-acetylglucosamine–galactose–sialic acid) linked to the core mannoses. frequently, the terminal region is truncated and contains only glcnac and galactose (gal) or just glcnac. In addition, a fucose may be added, usually to the core glcnac attached to the asparagine (asn). Thus, although the steps of processing and subsequent sugar addition are rigidly ordered, complex oligosaccharides can be heterogeneous. Moreover, although the complex oligosaccharide shown has three terminal branches, two and four branches are also common, depending on the glycoprotein and the cell in which it is made. (c) high-mannose oligosaccharides are not trimmed back all the way to the core region and contain additional mannoses. hybrid oligosaccharides with one Man branch and one glcnac and gal branch are also found (not shown).
The three amino acids indicated in (a) constitute the sequence recognized by the oligosaccharyl transferase enzyme that adds the initial oligosaccharide to the protein. ser, serine; Thr, threonine;  x, any amino acid, except proline.
Figure 13–29 Oligosaccharide processing in Golgi compartments.
The localization of each processing step shown was determined by a combination of techniques, including biochemical subfractionation of the golgi apparatus membranes and electron microscopy after staining with antibodies specific for some of the processing enzymes. processing enzymes are not restricted to a particular cisterna; instead, their distribution is graded across the stack, such that early-acting enzymes are present mostly in the cis golgi cisternae and later-acting enzymes are mostly in the trans golgi cisternae. Man, mannose; glcnac, n-acetylglucosamine; gal, galactose; nana, n-acetylneuraminic acid (sialic acid).
KEY: = N-acetylneuraminic acid (sialic acid, or NANA)