Enzyme action in glycoprotein synthesis

Abstract

Just a few decades ago, the saccharides bound to glycoproteins were considered little more than an irritation. They increased the difficulty of purifying and characterizing proteins, making proteins run as several bands on gels and smearing them on columns. They were considered a nuisance and were typically cleaved away to reveal the 'important part', the protein moiety, for structural (e.g. via X-ray crystallography or nuclear magnetic resonance) and functional studies. We now realize that that the saccharide is often as important as the protein itself, and that glycosylation can have many effects on the function, structure, physical properties and targeting of a protein. There are a myriad of reviews and books on this subject, reflecting the nearly overwhelming number of articles in print discussing saccharide structures, glycoprotein processing enzymes and the biological implication of glycosylation. This review discusses, in turn, the extent and biological relevance of glycosylation; the structures observed; how glycosylated proteins are formed in vivo; the clinical relevance of glycosylation, in terms of the correlations between disease states and unusual glycosylation patterns; and, finally, the molecules, both natural and synthetic, that can be used to study the role of carbohydrates in glycoprotein structure and function or to disrupt various carbohydrate recognition processes and enzymatic reactions in the glycoprotein synthetic pathway.

Effect of Extension of the Cytoplasmic Domain of Human Immunodeficiency Type 1 Virus Transmembrane Protein gp41 on Virus Replication

“…HeLa cells transfected with the wild-type and 6His proviruses were grown in the presence or absence of 1 mM DMM, which inhibits mannosidase I in the Golgi apparatus, generating proteins containing only highmannose N-glycans lacking sialic acid (43). As previously noted (18), the N-glycans of the wild-type gp120 were only partially converted to the complex type, and DMM blocked this process, resulting in higher electrophoretic mobility, as marked by band a in Fig.…”

Golgi GDP-fucose Transporter-deficient Mice Mimic Congenital Disorder of Glycosylation IIc/Leukocyte Adhesion Deficiency II

“…Preparation of [2][3] H]Mannose-radiolabeled Glycopeptides-MEF, hepatocytes, and mesangial cells in 6-cm dishes were labeled with 125 Ci of [2-3 H]mannose for 20 h in the presence or absence of fucose. Glycoproteins were extracted sequentially as described (41).…”

“…Glycan structures affect the physicochemical properties and the function of proteins in a variety of biological processes, including folding, solubility, sorting, proteolytic stability, and receptor-ligand interactions. In mammalian organisms, the biosynthesis of the oligosaccharide chains requires a broad spectrum of glycosyltransferases, glycosidases, transport proteins, and 13 different monosaccharides (2)(3)(4).…”

Impaired Lysosomal Trimming of N-Linked Oligosaccharides Leads to Hyperglycosylation of Native Lysosomal Proteins in Mice with α-Mannosidosis

“…Hyperglycosylation of N-linked oligosaccharides due to an impaired trimming had been described for Golgi ␣-mannosidase II deficiency, also called HEMPAS (hereditary erythroblastic multinuclearity with a positive acidified serum lysis test), leading to a reduced ability to form complex-type oligosaccharides (11). The unpredictability of the glycosylation impact on the function of glycoproteins has to be ascribed to the different tasks and functions in terms of its influence on the conformational properties and solubility of glycoproteins and its function in control of half-life by conferring resistance to denaturation and proteolysis as well as its effect on enzymatic activities and proteinsubstrate interactions (22,42,54).…”