Glycoproteins are proteins to which oligosaccharides are covalently attached to their polypeptide chain. Glycoproteins contain a much shorter carbohydrate chain than proteoglycans. The difference between glycoproteins and proteoglycans might be based on the quantity of carbohydrates. Glycoproteins contain less than 4 percent carbohydrate in the molecule. Proteoglycans consist of more than 4 percent carbohydrate.
Because of the -OH groups of sugars, glycoproteins are more hydrophilic than simple proteins. This means glycoproteins are more attractive to water than regular proteins. The hydrophilic nature of the molecule also results in the characteristic folding of the protein’s tertiary structure.
The carbohydrate is a short particle, often branched, and might consist of:
- Simple sugars (e.g., glucose, galactose, mannose, xylose).
- Amino sugars (sugars that have an amino group, such as N-acetylglucosamine or N-acetyl galactosamine).
- Acidic sugars (sugars that have a carboxyl group, such as sialic acid or N-acetylneuraminic acid).
Glycoproteins are classified according to the attachment site of the carbohydrate to an amino acid in the protein.
O-linked glycoproteins are ones in which the carbohydrate bonds to the oxygen atom (O) of the hydroxyl group (- OH) of the R group of either the amino acid threonine or serine. O-linked carbohydrates might also bond to hydroxylysine or hydroxyproline. The procedure is called O-glycosylation. O-linked glycoproteins are bound to sugar within the Golgi complex.
N-linked glycoproteins have actually a carbohydrate bonded to the nitrogen (N) of the amino group (- NH2) of the R group of the amino acid asparagine. The R group is generally the amide side chain of asparagine. The bonding procedure is called N-glycosylation. N-linked glycoproteins acquire their sugar from the endoplasmic reticulum membrane and after that are transferred to the Golgi complex for modifications.
While O-linked and N-linked glycoproteins are the most common types, other connections are also possible:
- P-glycosylation occurs when the sugar connects to the phosphorus of phosphoserine.
- C-glycosylation is when the sugar attaches to the carbon atom of amino acid. An example is when the sugar mannose bonds to the carbon in tryptophan.
- Glypiation is when a glycophosphatidylinositol (GPI) glycolipid attaches to the carbon terminus of a polypeptide.
Lysosomal metabolism of glycoproteins
The lysosomal catabolism of glycoproteins belongs to the normal turnover of cellular constituents and the cellular homeostasis of glycosylation. Glycoproteins are delivered to lysosomes for catabolism either by endocytosis from outside the cell or by autophagy within the cell. Once inside the lysosome, glycoproteins are broken down by a combination of proteases and glycosidases, with the characteristic properties of soluble lysosomal hydrolases.
The proteases consist of a mixture of endopeptidases and exopeptidases, which act in concert to produce a mixture of amino acids and dipeptides, which are transferred across the lysosomal membrane into the cytosol by a combination of diffusion and carrier-mediated transportation. Although the glycans of all fully grown glycoproteins are most likely broken down in lysosomes, the breakdown of N-linked glycans has actually been studied most intensively.
The catabolic pathways for high-mannose, hybrid, and intricate glycans have actually been developed. They are bidirectional with concurrent sequential elimination of monosaccharides from the nonreducing end by exoglycosidases and proteolysis and digestion of the carb– polypeptide linkage at the reducing end. The process is started by the elimination of any core and peripheral fucose, which is a requirement for the action of the peptide N-glycanase aspartylglucosaminidase, which hydrolyzes the glycan– peptide bond.
This enzyme likewise requires totally free alpha carboxyl and amino groups on the asparagine residue, suggesting comprehensive prior proteolysis. The structures of a few of the oligosaccharides that accumulate in these diseases are not digestion intermediates in the lysosomal catabolic pathways but represent intermediates in the biosynthetic pathway for N-linked glycans, recommending another path of delivery of glycans to the lysosome.
Improperly folded or glycosylated proteins that are declined by the quality control system are broken down in the ER and cytoplasm and the complete product of the cytosolic destruction of N-glycans is provided to the lysosomes. This path is boosted in cells actively secreting glycoproteins or producing increased quantities of aberrant glycoproteins. Hence interaction between the lysosome and proteasome is essential for the regulation of the biosynthesis and circulation of N-linked glycoproteins.
Functions of Glycoproteins
Glycoproteins work in the structure, reproduction, immune system, hormones, and defense of cells and organisms.
- Glycoproteins are found on the surface of the lipid bilayer of cell membranes. Their hydrophilic nature permits them to work in the liquid environment, where they act in cell-cell recognition and binding of other particles. Cell surface glycoproteins are likewise essential for cross-linking cells and proteins (e.g., collagen) to include strength and stability to a tissue. Glycoproteins in plant cells are what permits plants to stand upright against the force of gravity.
- Hormones might be glycoproteins. Examples include human chorionic gonadotropin (HCG) and erythropoietin (EPO).
- Glycophorin A is likewise crucial because it’s the attachment site for Plasmodium falciparum, a human blood parasite.
- Glycoproteins are important for reproduction due to the fact that they enable the binding of the sperm cell to the surface area of the egg.
- Mucins are glycoproteins found in mucous. The molecules safeguard sensitive epithelial surface areas, consisting of the breathing, urinary, digestion, and reproductive systems.
- The immune action relies on glycoproteins. The carb of antibodies (which are glycoproteins) determines the particular antigen it can bind. B cells and T cells have surface area glycoproteins which bind antigens, as well.