Role of Glycosylation in Organisms

Protein glycosylation influences the function of protein (Cassey, 1995) and is of great importance in many cellular processes such as immunoprotection, virus replication, cell growth, intracellular adhesion, and inflammation. Known O-glycosylated proteins (Comer and Hart, 2000) include transcription factors, nuclear-pore and heat-shock proteins, RNA polymerase II, and the transcription product of carcinogen and enzymes (Fig. 13.4). Aberrant glycosylation always results in the occurrence of diseases.

Higher glycosylation on transferrin has been demonstrated in patients with Alzheimer's disease, rheumatoid arthritis, and other diseases associated with a free radical etiology. Transferrin is a glycosylated metal-carrying serum protein. One of the biological functions of glycosylation is to stabilize the transferrin and indirectly influence iron homeostasis (van Rensburg et al., 2004). Also, there are many studies concerning the relationship between glycosylation and rapid-developed muscular dystrophy. Muntoni et al. (2004) demonstrated a novel mechanism responsible for muscular dystrophy. Aberrant glycosylation of a-dystroglycan appears to be a common finding in all these cases.

Non-Caps id Cytoskeletal Viral Proteins Proteins

Cytoplasmic Tails of Membrane Proteins

Parasite Proteins

Cytosolic Enzymes

C y topi a si Vesicle

Non-Caps id Cytoskeletal Viral Proteins Proteins

Cytoplasmic Tails of Membrane Proteins

Parasite Proteins

Cytosolic Enzymes

C y topi a si Vesicle

Nuclear Pore Proteins

Protein Translation Regulatory Factors

Proteins

Tumor Suppressors & Oncogenes

RNA Polymerase II & Transcription Factors

Figure 13.4 Glycoproteins in cell.

Proteins

Tumor Suppressors & Oncogenes

RNA Processing Proteins

Nuclear Pore Proteins

Protein Translation Regulatory Factors

RNA Polymerase II & Transcription Factors

Figure 13.4 Glycoproteins in cell.

Chen et al. (2004) synthesized the O-GlcNAcylated polypeptides and studied the effects of O-GlcNAcylation on the configuration and biological function of the polypeptides. They also studied the different regulatory mechanisms between O-GlcNAcylation and O-phosphorylation.

Since protein glycosylation is vital to fundamental cellular processes, targeting glycosylation for drug design is likely to be a necessity. Therapeutic strategies are to regulate relative enzymes. Imino sugars are monosaccharide mimics that have a nitrogen atom in place of the ring oxygen. Members of the imino-sugar family can inhibit several glycosylation enzymes, including ER a-glucosidases I and II and the ceramide-specific glucosyltransferase. Targeting ER a-glucosidases at a low level could be a potential strategy for treating virus infections without compromising the host cell. Imino sugar N-nonyl-DNI has antivirus activity to an animal model of hepatitis B virus, a model of neonatal calf diarrhea coronavirus, and an in vitro model of HCV (Dwek et al., 2002).

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