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Glycosylation (see also biopolymers found in cells (along with DNA, RNA, and proteins). Glycosylation is a form of co-translational and post-translational modification. Glycans serve a variety of structural and functional roles in membrane and secreted proteins.[1] The majority of proteins synthesized in the rough ER undergo glycosylation. It is an enzyme-directed site-specific process, as opposed to the non-enzymatic chemical reaction of glycation. Glycosylation is also present in the cytoplasm and nucleus as the O-GlcNAc modification. Aglycosylation is a feature of engineered antibodies to bypass glycosylation.[2][3] Five classes of glycans are produced:
Glycosylation is the process by which a sugar is covalently attached to a target protein. This modification serves various functions.[4] For instance, some proteins do not fold correctly unless they are glycosylated.[1] In other cases, proteins are not stable unless they contain oligosaccharides linked at the amide nitrogen of certain asparagines. Experiments have shown that glycosylation in this case is not a strict requirement for proper folding, but the unglycosylated protein degrades quickly. Glycosylation also plays a role in cell-cell adhesion (a mechanism employed by cells of the immune system) via sugar-binding proteins called lectins, which recognize specific carbohydrate moieties.[1] Glycosylation is an important parameter in the optimization of many glycoprotein-based drugs such as monoclonal antibodies.[5]
Glycosylation increases diversity in the proteome, because almost every aspect of glycosylation can be modified, including:
There are various mechanisms for glycosylation, although most share several common features:[1]
N-linked glycosylation is the most common type of glycosidic bond and is important for the folding of some eukaryotic proteins and for cell-cell and cell-extracellular matrix attachment. The N-linked glycosylation process occurs in eukaryotes in the lumen of the endoplasmic reticulum and widely in archaea, but very rarely in bacteria. In addition to their function in protein folding and cellular attachment, the N-linked glycans of a protein can modulate a protein's function, in some cases acting as an on-off switch.[7]
O-linked glycosylation is a form of glycosylation that occurs in eukaryotes in the Golgi apparatus,[8] but also occurs in archaea and bacteria.
Xylose, fucose, mannose, and GlcNAc phosphoserine glycans have been reported in the literature. Fucose and GlcNAc have been found only in Dictyostelium discoideum, mannose in Leishmania mexicana, and xylose in Trypanosoma cruzi. Mannose has recently been reported in a vertebrate, the mouse, Mus musculus, on the cell-surface laminin receptor alpha dystroglycan4. It has been suggested this rare finding may be linked to the fact that alpha dystroglycan is highly conserved from lower vertebrates to mammals.[9]
A mannose sugar is added to the first tryptophan residue in the sequence W-X-X-W (W indicates tryptophan; X is any amino acid). Thrombospondins are one of the most commonly C-modified proteins, although this form of glycosylation appears elsewhere as well. C-mannosylation is unusual because the sugar is linked to a carbon rather than a reactive atom such as nitrogen or oxygen. Recently, the first crystal structure of a protein containing this type of glycosylation has been determined - that of human complement component 8, PDB ID 3OJY.
A special form of glycosylation is the formation of a GPI anchor. In this kind of glycosylation a protein is attached to a lipid anchor, via a glycan chain. (See also prenylation.)
Over 40 disorders of glycosylation have been reported in humans.[10] These can be divided into four groups: disorders of protein N-glycosylation, disorders of protein O-glycosylation, disorders of lipid glycosylation and disorders of other glycosylation pathways and of multiple glycosylation pathways. No effective treatment is known for any of these disorders. 80% of these affect the nervous system.
Oxygen, Argon, Hydrogen, Helium, Gold
Citric acid cycle, Metabolism, Glucose, Hexokinase, Glyceraldehyde 3-phosphate
Nitrogen, Hydrogen, Helium, Sulfur, Fluorine
Archaea, Anthrax, Cheese, Cyanobacteria, Cholera
Bacteria, Crenarchaeota, Euryarchaeota, Sulfur, Acid
Methylation, Lysine, Protein, Cysteine, Tyrosine
Methylation, Amino acid, Biomolecular structure, Protein, Cysteine
Glycolysis, Metabolism, Methylation, Amino acid, Posttranslational modification
Connective tissue, Gelatin, FACIT collagen, Protein, Horse
Metabolism, Biochemistry, Glycolysis, Cholesterol, Phospholipid