Proteins that are intended to function within the cytoplasm are synthesized on ribosomes present in the cytosol. However, proteins, including glycoproteins, that have a purpose in cellular membranes, lysosomes, or require export from the cell, are produced on ribosomes that are attached to the endoplasmic reticulum (ER). These proteins have specific signal sequences that serve as molecular addresses, guiding them to their appropriate destinations. Initially, an N-terminal hydrophobic sequence directs these proteins to the ER, allowing the growing polypeptide to be released into the lumen. Subsequently, the proteins are transported to the Golgi via secretory vesicles, which acts as a sorting center. Within the Golgi, the glycoproteins intended for secretion or lysosomes are enclosed in vesicles that fuse with the respective cell or lysosomal membrane to release their contents. On the other hand, those meant to become components of the cell membrane become integrated into the Golgi membrane. Vesicles then bud off from the Golgi, adding their membrane-bound glycoproteins to the cell membrane. It is important to note that the membrane glycoproteins are oriented with the carbohydrate portion on the outside of the cell.

A.
The nucleotide sugars UDP-glucose, UDP-galactose, UDP-GlcNAc, and UDP-GalNAc serve as the precursors for the carbohydrate components of glycoproteins. Additionally, guanosine diphosphate (GDP)-mannose, GDP-L-fucose (derived from GDP-mannose), and CMP-NANA can also contribute sugars to the growing chain. The attachment of the oligosaccharides to specific amino acid side chains within the protein is dependent on the three-dimensional structure of the protein itself.

B. O-Linked glycoprotein synthesis
The synthesis of O-linked glycoproteins follows a similar process to that of glycosaminoglycans (GAGs). Initially, the protein to which sugars will be attached is synthesized on the endoplasmic reticulum (ER) and then released into the ER lumen. The glycosylation process begins with the transfer of GalNAc from UDP-GalNAc to specific serine or threonine residues. The Golgi membranes contain glycosyltransferases that are responsible for stepwise synthesis of the oligosaccharides using individual sugars. Unlike DNA, RNA, and protein synthesis, the glycosyltransferases do not require a template, but instead recognize the growing oligosaccharide structure as the appropriate substrate.

C. N-Linked glycoprotein synthesis
The synthesis of N-linked glycoproteins occurs within the lumen of the endoplasmic reticulum (ER) and involves the phosphorylated form of dolichol, a lipid found in the ER membrane. The initial product undergoes processing in both the ER and Golgi.
Proteins destined to function in the cytoplasm are synthesized on free cytosolic ribosomes. However, proteins, including glycoproteins, that are destined for cellular membranes, lysosomes, or to be exported from the cell, are synthesized on ribosomes attached to the RER. These proteins contain specific signal sequences that act as molecular addresses, targeting the proteins to their proper destinations. An N-terminal hydrophobic sequence initially directs these proteins to the RER, allowing the growing polypeptide to be extruded into the lumen (see p.
459). The proteins are then transported via secretory vesicles to the Golgi, which acts as a sorting center (Fig. 14.15). In the Golgi, those glycoproteins that are to be secreted from the cell (or are targeted for lysosomes) are packaged into vesicles that fuse with the cell (or lysosomal) membrane and release their contents. Those that are destined to become components of the cell membrane are integrated into the Golgi membrane, which buds off, forming vesicles that add their membrane-bound glycoproteins to the cell membrane. [Note: Therefore, the membrane glycoproteins are oriented with the carbohydrate portion on the outside of the cell (see Fig. 14.15).]
A.
The precursors of the carbohydrate components of glycoproteins are nucleotide sugars, which include UDP-glucose, UDP-galactose, UDP-GlcNAc, and UDP-GalNAc. In addition, guanosine diphosphate (GDP)mannose, GDP-L-fucose (which is synthesized from GDP-mannose), and CMP-NANA may donate sugars to the growing chain. [Note: When the acidic NANA is present, the oligosaccharide has a negative charge at physiologic pH.] The oligosaccharides are covalently attached to the side chains of specific amino acids in the protein, where the three-dimensional structure of the protein determines whether or not a specific amino acid is glycosylated.
B. O-Linked glycoprotein synthesis
Synthesis of the O-linked glycoproteins is very similar to that of the GAG (see p. 158). First, the protein to which sugars are to be attached is synthesized on the RER and extruded into its lumen. Glycosylation begins with the transfer of GalNAc (from UDP-GalNAc) to the hydroxyl group of a specific serine or threonine residues. The glycosyltransferases responsible for the stepwise synthesis (from individual sugars) of the oligosaccharides are bound to the membranes of the Golgi. They act in a specific order, without using a template as is required for DNA, ribonucleic acid (RNA), and protein synthesis (see Unit VII), but instead by recognizing the actual structure of the growing oligosaccharide as the appropriate substrate.
C. N-Linked glycoprotein synthesis
Synthesis of N-linked glycoproteins occurs in the lumen of the RER and requires the participation of the phosphorylated form of dolichol (dolichol pyrophosphate), a lipid of the RER membrane (Fig. 14.16). The initial product is processed in the RER and Golgi.
= terminal group (fucose or Nacetylneuraminic acid); mRNA = messenger RNA; Asn = asparagine.