Biosynthesis of glycoproteins

Glycoproteins are synthesized by passing through two organelles, the ER and the Golgi apparatus [4], The biosynthesis of N-linked oligosaccharides initiates with the cytosolic transfer of GlcNAc from its uridine diphosphate (UDP) form onto dolichol 

Interaction with chaperone (CNX/CRT) T and folding sensor enzyme (UGGT) I 

Two pools of L-fucosylated glycoproteins appear to be present in HeLa cells. From one of them is derived plasma-membrane L-fucose, which eventually enters the growth medium (mostly as free L-fucose), whereas macromolecular L-fucose by-passes the cell surface to enter the growth medium directly from the other. The L-fucosyl- and o-galactosyl-transferases in scrapings from murine gastric epithelia have been characterized.  

Kedinger, K. Haflen, A. Freiburghaus, J. F. Grenier, and B. Hadorn, Biochim. Biophys. Acta, 1977,467, 327. 

At least two forms of sialyltransferase are present in human liver—one of them is thermolabile and associated with the membrane, whereas the other is thermostable and located elsewhere. Whereas sialyltransferases in the sera of humans 

Buscher, J. Casals-Stenzel, R. Schauer, and P. Mestres-Ventura, European J. Biochem., 1977, 77, 297. 

Amphomycin specifically inhibits the formation of dolichyl D-mannosyl phosphate in pig aorta microsomal preparations, but does not inhibit the further transfer of b-mannose from dolichyl D-mannosyl phosphate or from lipid-linked oligosaccharides. However, some of the D-mannosyl residues in the lipid-linked oligosaccharides come directly from GDP-D-mannose without the involvement of dolichyl D-mannosyl phosphate. 

The enzyme responsible for the transfer of oligosaccharide from lipid-linked intermediates to endogenous proteins of rat liver is a structural component of 

Cancer-associated D-galactosyltransferase (GT-II) and the normal isoenzyme (GT-I) have been purified from pooled effusions from patients with various cancers.The purified cancer-associated enzyme appears to be structurally and kinetically distinct from the normal isoenzyme. 

Optimal conditions for the determination of UDP-D-galactosyltransferase in the Golgi system of rat liver have been reported.In contrast to other reports the enzyme is not stimulated by non-ionic detergents. 

Idoyaga-Vargas, M. Perelmuter, O, Burrone, and H. Carminatti, Mol Cell Biochem., 1979,26,123. 

Dolichol phosphate (Dol-P) is the carrier lipid involved in the assembly of oligosaccharides for transfer to the amide nitrogen of L-asparagine of proteins. The precursor oligosaccharide pyrophosphoryl dolichol is biosynthesized in the endoplasmic reticulum or in the Golgi bodies of eukaryotes from dolichol phosphate and nucleotide diphospho sugars, UDP-GlcNAc, GDP-Man, and UDP-Glc. 

Golgi carry out the modifications. The 0-linked glycosides are formed directly by the action of specific enzymes with nucleotide diphospho sugars [117]. 

The biosynthesis and functions of the membrane glycoproteins of eukaryotic cells have been reviewed. The general features of the biosynthesis of these glycoproteins closely resemble those involved in the biosynthesis of complex bacterial glycans, especially the O-antigen of lipopolysaccharides. Syntheses of polyisoprenyl jS-o-mannopyranosyl phosphate and P -di-A-acetylchitobiosyl P -dolichyl pyrophosphate have been reported. A preparation from human-lymphocyte membranes was able to transfer D-mannose from GPD-o-mannose to the chitobiosyl derivative, whereas dolichyl D-mannosyl phosphate did not act as a D-mannosyl donor. It appears that D-mannosyl residues at various positions of the glycoprotein have different anomeric configurations, so that different D-mannosyl donors are required in their synthesis. 

A particulate fraction from calf thyroid catalysed the transfer of D-mannose from GDP-D-mannose to exogenous glycopeptides and to methyl or aryl glycosides to form sequences of a-D-mannopyranosyl-D-mannose. Transfer to the simple glycosides required a single, non-reducing D-mannosyl residue 

Inhibition by 2-deoxy-D-arai mn-hexose of the biosynthesis of the cell-wall mannan in Saccharomyces cerevisiae appears to involve a complex mechanism that disrupts the synthesis of protein rather than glycosylation. Studies using a particulate enzyme from Mycobacterium smegmatis have demonstrated that polyisoprenyl D-mannosyl phosphates are obligatory donors, while preparations obtained from Hansenula holstii have shown that a D-mannosyl-lipid is involved in the synthesis of mannosylated serine (or threonine) residues.  

A preparation from pig-liver microsomes catalysed the formation of P -dolichyl P -2-acetamido-2-deoxy-D-gIucosyl and P -2-acetamido-2-deoxy-D-mannosyl 

Morelis, P. Broquet, and P. Louisot, Biochim. Biophys. Acta, 1974, 373, 10. 

Glycosylation and oligosaccharide processing play an indispensable role in the sorting/ targeting/transport of proteins to their proper cellular locations and functions. Cell surface 

1 N-linked glycoproteins. The core saccharide is synthesized as a precursor and transferred en bloc to proteins in the ER. The subsequent processing and terminal elaborations take place in the Golgi apparatus. The synthesis occurs in four steps (Komfeld and Komfeld, 1985). 

Synthesis of a lipid-linked core precursor. N-Unked oligosaccharides are initially synthesized as Upid-linked precursors. The lipid component is dolichol (Dol), a long chain polyisoprenol of n = 14-24 isoprene units, which is linked to the oligosaccharide precursor via pyrophosphate bridge. 

3 Glycosylphosphatidylinositol membrane anchor. Glycosylphospho-tidylinositol (GPI) groups function to anchor a wide variety of proteins to exterior surface of the eukaryotic plasma membrane as a transmembrane polypeptide domain. The core GPI is synthesized on the lumenal side of ER (Tartakoff and Singh, 1992), as shown in 

Sialyl- and L-fucosyl-transferases have been shown to transfer their respective sugars to the terminal /S-D-Gal/ -(l 4)-D-Glc/7iVAc sequence of L-asparagine-linked oligosaccharides forming a-Neu/7VAc-(2 6)-j8-D-Gal/ -(l 4)-d- 

Granier, J. van Rietschoten, and S. Bouchilloux, Biochem. Biophys. Res. 

---

Long-chain polyisoprenoid. molecules with a terminal alcohol moiety are called, polyprenols. The dolichols, one class of polyprenols (Figure 8.18), consist of 16 to 22 isoprene units and, in the form of dolichyl phosphates, function to carry carbohydrate units in the biosynthesis of glycoproteins in animals. Polyprenyl groups serve to anchor certain proteins to biological membranes (discussed in Chapter 9). 

Table 47-15. Some diseases due to or involving abnormalities in the biosynthesis of glycoproteins.

Mucins are a class of O-linked glycoproteins that are distributed on the surfaces of epithelial cells of the respiratory, gastrointestinal, and reproductive tracts. The Golgi apparatus plays a major role in glycosyla-tion reactions involved in the biosynthesis of glycoproteins. 

Besides cell-wall polysaccharides, plants contain glycoproteins, as do animal cells, and it is in the biosynthesis of glycoprotein that evidence for involvement of lipid-linked sugars has accumulated. Most of the data have been obtained for animal cells, and the subject is well covered in several reviews.1113178 Since 1964, when glycoproteins were first detected in plants,179 their wide distribution has become evident, and reviews are available.15,16,180 We shall describe aspects of this subject not as yet covered. 

Nojirimycin (5-amino-5-deoxy-D-glucose) is an antibiotic used in studies of the biosynthesis of glycoproteins. Write its open-chain and pyranose ring structures. 

Fig. 8.—Pathway of Biosynthesis of Some Sugar Nucleotides That Function as Gly-cosyl Donors for Biosynthesis of Glycoproteins and Glycoenzymes. (The heavy arrows indicate the sites of feedback control of the reactions.135)...

Fig. 16. Schematic presentation of the present hypotheses on the biosynthesis of glycoprotein of halobacteria (for details see the text). From ref. [117].

Sadler, JE, Biosynthesis of glycoproteins formation of O-linked oligosaccharides, Biol. Carbohydr., 2, 199-288, 1984. 

---