Question

Describe When and How Oligosaccharides are added to proteins in the ER.

Answer 1

Protein glycosylation is a chemical reaction directed by enzymes that takes place in the ER(Endoplasmic Reticulum) and in the Golgi Apparatus body of the cell. General glycosylation within the ER helps with folding. The ribosomes on the cytoplasmic surface of the ER membrane synthesize the protein and peptid chain is sent into the lumen of the ER. There are N-linked and O-linked glycosylation processes which determined by whether the sugars in glycoproteins are attached to the amide nitrogen on the amino acid asparagine, or to the oxygen on the side chains of either serine or threonine. The N-linkage glycosylations happens in both ER and in the Golgi complex while the O-linked glycosylation only occurs in the Golgi complex. Proteins derived from the glycoproteins diffuse into vesicles and are transported into different places according to the signals instructed by the amino acid sequence and the three-dimensional structures.

N-linked glycosylation mostly takes place in eukaryotes and archaea, but rarely in bacteria. When a 14-sugar chain, including 2 N-acetylglucosamine molecules, 3 glucose, and 9 mannose, is attached to the asparagine amino acid in the target protein, dolichol molecle is carried by reaction and sent into the ER lumen. There are two kinds of N-linked oligosaccarides: High-mannose oligosaccharides, and complex oligosaccharides. High-mannose oligosaccharides is a combination of the 2 N-acetylglucosamine molecules and many numbers of mannose residues attached. This is the most common chain. The complex oligosaccharides is the combination of any number and of any kinds of saccharides attaching together. The modification of both two types depends on the accessibility of the modified proteins in the Golgi complex. If the oligosaccharides are not accessible, then the high-mannose will not be cleaved for further modification.

The cytoplasm is not a place for protein glycosylation, because sugars and complex enzymes are stored in the lumenal side of the ER, so the proteins are not glycosylated as they are above.

Significance of Protein Glycosylation

Glycosylation can avoid the incorrect folding of the original proteins. Many proteins do not fold correctly unless they undergo glycosylation. It also increases the stability of the protein structures in blood so that they will not degrade as quickly as those unglycosylated proteins. For example, glycoproteins linked at the amide nitrogen in asparagine in the protein have increased stability. N-linked glycosylation of this sort occurs when the protein sequence Asn-X-Thr or Asn-X-Ser is reached. X, in this case can be any amino acid except for proline. Glycosylation helps to adhere between cells. This mechanism of cell to cell adhesion is especially vital in cells of the immune system. [