Glycoprotein formation

Figure 25.7 Glycoprotein formation occurs by initial phosphorylation of the starting carbohydrate to a glycosyl phosphate, followed by reaction with UTP to form a glycosyl uridine 5 -diphosphate. Nucleophilic substitution by an -OH (or -NH2) group on a protein then gives the glycoprotein.

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

Glycoprotein formation is a complex process that involves multiple enzymatic reactions occurring in the endoplasmic reticulum and the Golgi apparatus. 

Polysaccharide formation is carried out by sequential addition of glycosidic residues (one or more monosaccharide molecules) at a time to different acceptors. Glycoprotein formation is also carried out similarly, but how is the first sugar resioue introduced It is thought that a spatial and temporal union between protein and glycosylation mechanisms most be established. 

The mechanism of inhibition has not been characterized, but it is probably related to the ionophoretic properties of these antibiotics. Monensin has been shown to inhibit the intracellular transport of viral membrane proteins of cells infected with Semliki Forest vims (169). The formation of syncytia, normally observed when T-lymphoblastoid cell line (CEM) cells are cocultivated with human immunodeficiency vims (HlV-l)-infected T-ceU leukemia cell line (MOLT-3) cells, was significantly inhibited in the presence of monensin (170). This observation suggests that the viral glycoproteins in the treated cells were not transported to the cell surface from the Golgi membrane. 

Coagulation Factors II, III, VII, IX, X, XI, and Xlla fragments, thrombin, and plasmin are classified as serine proteases because each possesses a serine residue with neighboring histidine and asparagine residues at its enzymatically active site (Table 3). Factors II, VII, IX, and X, Protein C, Protein S, and Protein Z are dependent on the presence of vitamin K [84-80-0] for their formation as biologically functionally active procoagulant glycoproteins. 

Tissue-type plasminogen activator (tPA) is a glycoprotein (68 kDa), synthesized by endothelial and tumor-cells. As a serine protease, tPA hydrolyses Arg561-Val562 peptide bond in plasminogen, resulting in plasmin formation. It needs cofactors for efficient plasminogen activation. 

The same ceUs that secrete collagen also secrete fi-bronectin, a large glycoprotein present on cell surfaces, in the extracellular matrix, and in blood (see below). Fi-bronectin binds to aggregating precollagen fibers and alters the kinetics of fiber formation in the pericellular matrix. Associated with fibronectin and procollagen in... 

At cellular level each stage of atheroma development is accompanied by the expression of specific glycoproteins by endothelial cells which mediate the adhesion of monocytes and T-lymphocytes. Their recruitment and migration is triggered by various cytokines released by leukocytes and possibly by smooth muscle cells. Atheroma development continues with the activation of macrophages, which accumulate lipids and become, together with lymphocytes, so-called fatty streaks. The continuous influx, differentiation and proliferation finally leads to more advanced lesion and to the formation of the fibrous plaque. ... 

Yoshida H, Koga Y, Moroi Y, Kimura G, Nomoto K. The effect of p561ck, a lymphocyte specific protein tyrosine kinase, on the syncytium formation induced by human immunodeficiency vims envelope glycoprotein. Int Immunol 1992 4(2) 233-242. 

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