Glycosylation

Glycosylation is the enzymatic process that links saccharides to produce glycans, attached to proteins, lipids, or other organic molecules. This enzymatic process produces one of the fundamental 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. Five classes of glycans are produced:

• N-linked glycans attached to a nitrogen of asparagine or arginine side chains;
• O-linked glycans attached to the hydroxy oxygen of serine, threonine, tyrosine, hydroxylysine, or hydroxyproline side chains, or to oxygens on lipids such as ceramide;
• phospho-glycans linked through the phosphate of a phospho-serine;
• C-linked glycans, a rare form of glycosylation where a sugar is added to a carbon on a tryptophan side chain;
• glypiation, which is the addition of a GPI anchor that links proteins to lipids through glycan linkages.

Purpose

The carbohydrate chains attached to the target proteins serve various functions.^[2] For instance, some proteins do not fold correctly unless they are glycosylated first.^[1] Also, polysaccharides linked at the amide nitrogen of asparagine in the protein confer stability on some secreted glycoproteins. Experiments have shown that glycosylation in this case is not a strict requirement for proper folding, but the unglycosylated protein degrades quickly. Glycosylation may play a role in cell-cell adhesion (a mechanism employed by cells of the immune system), as well.

Mechanisms

There are various mechanisms for glycosylation, although most share several common features:^[1]

• Glycosylation, unlike glycation, is an enzymatic process;
• The donor molecule is often an activated nucleotide sugar;
• The process is site-specific.

Types of Glycosylation

N-linked glycosylation

Comparative overview of the major types of vertebrate N-glycan subtypes and some representative C. elegans N-glycans.

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 the asparagine in the polypeptide chain of the target protein. The structure of this precursor is common to most eukaryotes, 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.^[2]

• 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. 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.

O-linked glycosylation

O-N-acetylgalactosamine (O-GalNAc)

O-linked glycosylation occurs at a later stage during protein processing, probably in the Golgi apparatus. This is the addition of N-acetyl-galactosamine to serine or threonine residues by the enzyme UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase (EC 2.4.1.41), followed by other carbohydrates (such as galactose and sialic acid). This process is important for certain types of proteins such as proteoglycans, which involves the addition of glycosaminoglycan chains to an initially unglycosylated "proteoglycan core protein." These additions are usually serine O-linked glycoproteins, which seem to have one of two main functions. One function involves secretion to form components of the extracellular matrix, adhering one cell to another by interactions between the large sugar complexes of proteoglycans. The other main function is to act as a component of mucosal secretions, and it is the high concentration of carbohydrates that tends to give mucus its "slimy" feel. Proteins that circulate in the blood are not normally O-glycosylated, with the exception of IgA1 and IgD (two types of antibody) and C1-inhibitor.

O-fucose

O-fucose is added between the second and third conserved cysteines of EGF-like repeats in the Notch protein, and other substrates by GDP-fucose protein O-fucosyltransferase 1, and to Thrombospondin repeats by GDP-fucose protein O-fucosyltransferase 2. In the case of EGF-like repeats, the O-fucose may be further elongated to a tetrasaccharide by sequential addition of N-acetylglucosamine (GlcNAc), galactose, and sialic acid, and for Thrombospondin repeats, may be elongated to a disaccharide by the addition of glucose. Both of these fucosyltransferases have been localized to the endoplasmic reticulum, which is unusual for glycosyltransferases, most of which function in the Golgi apparatus.

O-glucose

O-glucose is added between the first and second conserved cysteines of EGF-like repeats in the Notch protein, and possibly other substrates by protein:O-glucosyltransferase (Poglut). This enzyme is known as Rumi in Drosophila, and is also localized to the ER like the O-fucosyltransferases. The O-glucose modification appears to be necessary for proper folding of the EGF-like repeats of the Notch protein, and increases secretion of this receptor.

O-N-acetylglucosamine (O-GlcNAc)

O-GlcNAc is added to serines or threonines by O-GlcNAc transferase. O-GlcNAc appears to occur on most serines and threonines that would otherwise be phosphorylated by serine/threonine kinases. Thus, if phosphorylation occurs, O-GlcNAc does not, and vice versa. This is an incredibly important finding because phosphorylation/dephosphorylation has become a scientific paradigm for the regulation of signaling within cells. A massive amount of cancer research is focused on phosphorylation. Ignoring the involvement of this form of glycosylation, which clearly appears to act in concert with phosphorylation, means that a lot of current research is missing at least half of the picture. O-GlcNAc addition and removal also appears to be a key regulator of the pathways that are disrupted in diabetes mellitus. The gene encoding the O-GlcNAcase enzyme has been linked to non-insulin dependent diabetes mellitus. It is the terminal step in a nutrient-sensing hexosamine signaling pathway.

O-N-acetylglucosamine In Other Contexts

Recently, O-GlcNAc was reported to occur between the fifth and sixth conserved cysteines in some EGF-like repeats from the Notch protein. It would seem unlikely that this modification would be due to the same enzyme involved with addition of O-GlcNAc to cytoplasmic and nuclear localized proteins. Considering that O-fucose and O-glucose addition to EGF-like repeats is due to ER localized enzymes, presumably an ER localized protein O-GlcNAc transferase exists.

O-mannose

During O-mannosylation, a mannose residue is transferred from mannose-p-dolichol to a serine/threonine residue in secretory pathway proteins^[3]. O-mannosylation is common to both prokaryotes and eukaryotes.

Collagen Glycosylation

Many lysines in collagen are hydroxylated to form hydroxylysine, and many of these hydroxylysines are then glycosylated by the addition of galactose. This galactose monosaccharide can then be further elongated by the addition of a glucose. This glycosylation is required for the proper functioning of collagen. Glycosylation of hydroxlysine occurs in the ER.

Hydroxyproline Glycosylation

Proline is also hydroxylated in collagen, however, no glycosylation occurs here as the hydroxyprolines are necessary for hydrogen bonding in the collagen triple helix. There is one protein named Skp1 in Dictyostelium that carries a GlcNAc on hydroxyproline, but this would appear to be an extremely rare form of glycosylation. Otherwise, only plants appear to carry glycans on hydroxyproline, with both galactose and arabinose glycans being reported in the literature.

Glycosylation of Glycogenin

Liver and muscle glycogenin carries a glucose on a tyrosine side chain. This is the only known example of glycosylated tyrosine in nature.

Glycosylation of Ceramide

Either a galactose or a glucose can be added to a hydroxyl on the lipid ceramide. The glucose can be further elongated to a disaccharide by the addition of a galactose.

Proteoglycans

The large and complex glycans that modify proteoglycans are initiated by addition of xylose to serine. This is the only form of glycan so far reported to begin with xylose addition directly to protein apart from the xylose seen on phospho-serine in Dictyostelium described below.

Phospho-Serine Glycosylation

Xylose, fucose, mannose, and GlcNAc phospho-serine glycans have been reported in the literature. Fucose and GlcNAc have been found only in Dictyostelium, mannose in Leishmania mexicana, and xylose in Trypanosoma cruzi.

C-mannosylation

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 modified proteins, however this form of glycosylation appears elsewhere as well. This is an unusual modification because the sugar is linked to a carbon rather than a reactive atom like a nitrogen or oxygen.

GPI Anchors (Glypiation)

A special form of glycosylation is the GPI anchor. This form of glycosylation functions to attach a protein to a hydrophobic lipid anchor, via a glycan chain. (see also prenylation)

See also

• Glycorandomization
• Glycation
• Advanced glycation endproduct
• Chemical glycosylation
• Fucosylation

References

External links

• Online textbook of glycobiology with chapters about glycosylation
• GlyProt: In-silico N-glycosylation of proteins on the web
• NetNGlyc: The NetNglyc server predicts N-Glycosylation sites in human proteins using artificial neural networks that examine the sequence context of Asn-Xaa-Ser/Thr sequons.
• Supplementary Material of the Book "The Sugar Code"