N-linked glycosylation
Encyclopedia
N-linked glycosylation is important for the folding of some eukaryotic proteins. The N-linked glycosylation process occurs in eukaryotes and widely in archaea
, but very rarely in bacteria
.
In Eukaryotes, most N-linked oligosaccharides begin with addition of a 14-sugar precursor to an asparagine in the polypeptide chain of the target protein. The structure of this precursor is common to most eukaryote
s, and contains 3 glucose
, 9 mannose
, and 2 N-acetylglucosamine molecules. A complex set of reactions attaches this branched chain to a carrier molecule called dolichol
, and then it is transferred to the appropriate point on the polypeptide chain as it is translocated into the ER lumen.
There are three major classes of N-linked saccharides resulting from this core: high-mannose oligosaccharides, complex oligosaccharides and hybrid oligosaccharides.
Proteins can be glycosylated by both types of oligosaccharides on different portions of the protein. Whether an oligosaccharide is high-mannose or complex is thought to depend on its accessibility to saccharide-modifying proteins in the Golgi
. If the saccharide is relatively inaccessible, it will most likely stay in its original high-mannose form. If it is accessible, then it is likely that many of the mannose residues will be cleaved off and the saccharide will be further modified by the addition of other types of group as discussed above.
The oligosaccharide
chain is attached by oligosaccharyltransferase
to asparagine
occurring in the tripeptide
sequence Asn
-X-Ser
or Asn
-X-Thr
where X could be any amino acid
except Pro
. This sequence is known as a glycosylation sequon
. After attachment, once the protein is correctly folded, the three glucose residues are removed from the chain and the protein is available for export from the ER. The glycoprotein thus formed is then transported to the Golgi where removal of further mannose residues may take place. However, glycosylation itself does not seem to be as necessary for correct transport targeting of the protein, as one might think. Studies involving drugs that block certain steps in glycosylation, or mutant cells deficient in a glycosylation enzyme, still produce otherwise-structurally-normal proteins that are correctly targeted, and this interference does not seem to interfere severely with the viability of the cells. Mature glycoproteins may contain a variety of oligomannose N-linked oligosaccharides containing between 5 and 9 mannose residues. Further removal of mannose residues leads to a 'core' structure containing 3 mannose
, and 2 N-acetylglucosamine residues, which may then be elongated with a variety of different monosaccharides including galactose
, N-acetylglucosamine
, N-acetylgalactosamine
, fucose
and sialic acid
.
GalNAc, glucose, and rhamnose linked to asparagines have been observed as well, although mostly in less complex organisms or bacteria. Glucose linked to the guanidinium group of arginine in sweet corn amyelogenin is the only reported example of N-linked glycosylation on an amino acid other than asparagine.
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, but very rarely in bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
.
In Eukaryotes, most N-linked oligosaccharides begin with addition of a 14-sugar precursor to an asparagine in the polypeptide chain of the target protein. The structure of this precursor is common to most eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s, and contains 3 glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, 9 mannose
Mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....
, and 2 N-acetylglucosamine molecules. A complex set of reactions attaches this branched chain to a carrier molecule called dolichol
Dolichol
Dolichol refers to any of a group of long-chain mostly unsaturated organic compounds that are made up of varying numbers of isoprene units terminating in an α-saturated isoprenoid group, containing an alcohol functional group.-Functions:...
, and then it is transferred to the appropriate point on the polypeptide chain as it is translocated into the ER lumen.
There are three major classes of N-linked saccharides resulting from this core: high-mannose oligosaccharides, complex oligosaccharides and hybrid oligosaccharides.
- High-mannose is, in essence, just two N-acetylglucosamines with many mannose residues, often almost as many as are seen in the precursor oligosaccharides before it is attached to the protein.
- Complex oligosaccharides are so named because they can contain almost any number of the other types of saccharides, including more than the original two N-acetylglucosamines.
Proteins can be glycosylated by both types of oligosaccharides on different portions of the protein. Whether an oligosaccharide is high-mannose or complex is thought to depend on its accessibility to saccharide-modifying proteins in the Golgi
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
. If the saccharide is relatively inaccessible, it will most likely stay in its original high-mannose form. If it is accessible, then it is likely that many of the mannose residues will be cleaved off and the saccharide will be further modified by the addition of other types of group as discussed above.
The oligosaccharide
Oligosaccharide
An oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...
chain is attached by oligosaccharyltransferase
Oligosaccharyltransferase
Oligosaccharyltransferase or OST is a membrane protein complex that transfers a 14-sugar oligosaccharide from dolichol to nascent protein. It is a type of glycosyltransferase...
to asparagine
Asparagine
Asparagine is one of the 20 most common natural amino acids on Earth. It has carboxamide as the side-chain's functional group. It is not an essential amino acid...
occurring in the tripeptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
sequence Asn
Asparagine
Asparagine is one of the 20 most common natural amino acids on Earth. It has carboxamide as the side-chain's functional group. It is not an essential amino acid...
-X-Ser
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
or Asn
Asparagine
Asparagine is one of the 20 most common natural amino acids on Earth. It has carboxamide as the side-chain's functional group. It is not an essential amino acid...
-X-Thr
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
where X could be any amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
except Pro
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
. This sequence is known as a glycosylation sequon
Sequon
A sequon is a sequence of three consecutive amino acids in a proteinthat can serve as the attachment site to a polysaccharide called anN-linked-Glycan. This is a polysaccharidelinked to the protein via the nitrogen atom in the side chain of...
. After attachment, once the protein is correctly folded, the three glucose residues are removed from the chain and the protein is available for export from the ER. The glycoprotein thus formed is then transported to the Golgi where removal of further mannose residues may take place. However, glycosylation itself does not seem to be as necessary for correct transport targeting of the protein, as one might think. Studies involving drugs that block certain steps in glycosylation, or mutant cells deficient in a glycosylation enzyme, still produce otherwise-structurally-normal proteins that are correctly targeted, and this interference does not seem to interfere severely with the viability of the cells. Mature glycoproteins may contain a variety of oligomannose N-linked oligosaccharides containing between 5 and 9 mannose residues. Further removal of mannose residues leads to a 'core' structure containing 3 mannose
Mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....
, and 2 N-acetylglucosamine residues, which may then be elongated with a variety of different monosaccharides including galactose
Galactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....
, N-acetylglucosamine
N-Acetylglucosamine
N-Acetylglucosamine is a monosaccharide derivative of glucose. It is an amide between glucosamine and acetic acid...
, N-acetylgalactosamine
N-Acetylgalactosamine
N-Acetylgalactosamine , is an amino sugar derivative of galactose.-Function:In humans it is the terminal carbohydrate forming the antigen of blood group A....
, fucose
Fucose
Fucose is a hexose deoxy sugar with the chemical formula C6H12O5. It is found on N-linked glycans on the mammalian, insect and plant cell surface, and is the fundamental sub-unit of the fucoidan polysaccharide...
and sialic acid
Sialic acid
Sialic acid is a generic term for the N- or O-substituted derivatives of neuraminic acid, a monosaccharide with a nine-carbon backbone. It is also the name for the most common member of this group, N-acetylneuraminic acid...
.
GalNAc, glucose, and rhamnose linked to asparagines have been observed as well, although mostly in less complex organisms or bacteria. Glucose linked to the guanidinium group of arginine in sweet corn amyelogenin is the only reported example of N-linked glycosylation on an amino acid other than asparagine.