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The different types of glycans in different organisms

In the field of biochemistry, N-linked glycosylation is the attachment of a sugar molecule (known as glycan) to an amide nitrogen of asparagine (Asn) residue of a protein, giving rise to a glycoprotein. This type of linkage is important for both the structure and function of some eukaryotic proteins. The N-linked glycosylation process occurs in eukaryotes and widely in archaea, but very rarely in prokaryotes. The nature of N-linked glycans attached to a glycoprotein is determined by the protein and the cell in which it is expressed. It also varies across species. Different species synthesises different types of N-linked glycoproteins.

Bond formation and Energetics

There are two types of bonds involved in a glycoprotein: bonds between the saccharides residues in the glycan and the linkage between the glycan chain and the protein molecule.

The sugar moieties are linked to one another in the glycan chain via glycosidic bonds. These bonds are formed between carbon 1 and 4 of the sugar molecules. The formation of glycosidic bond is energetically unfavourable, therefore the reaction is coupled to the hydrolysis of two ATP molecules.

On the other hand, the attachment of a glycan residue to a protein requires the recognition of a consensus sequence. N-linked glycans are almost always attached to the nitrogen atom of an Asn side chain that is present as a part of Asn-X-Ser/Thr consensus sequence, where X is any amino acid except proline.

In animal cells, the glycan attached to the Asn is almost inevitably, N-Acetylglucosamine (GlcNAc), in the β-configuration. This β-linkage is similar to glycosidic bond between the sugar moieties in the glycan structure as described above. The anomeric carbon atom instead of being attached to a sugar hydroxyl group is attached to amide nitrogen. The energy required for this linkage comes from the hydrolysis of a pyrophosphate molecule.

Pathway of N-linked glycan biosynthesis

Biosynthesis pathway of N-linked glycoproteins

The biosynthesis of N-linked glycans occurs via 3 major steps

1. Synthesis of dolichol-linked precursor oligosaccharide
2. En block transfer of precursor oligosaccharide to protein
3. Processing of the oligosaccharide

Synthesis, en bloc transfer and initial trimming of precursor oligosaccharide occurs in the Endoplasmic Reticulum (ER). Whereas, subsequent processing and modification of the oligosaccharide chain is carried out in the Golgi apparatus.

The synthesis of glycoproteins is thus, spatially separated in different cellular compartments. Therefore, the type of N-glycan synthesised depends on the accessibility of the glycan to the different enzymes present within these cellular compartments.

However, in spite of the diversity, all N-glycans are synthesised through a common pathway with a common core glycan structure.

The core glycan structure consist of two N-Acetyl Glucosamine and three mannose residues. These core glycan is then elaborated and modified further, resulting in a diverse range of N-glycan structures.

Synthesis of precursor oligosaccharide

The process of N-linked glycosylation starts with the formation of dolichol-linked sugar (GlcNAc). Dolichol is a lipid molecule composed of repeating isoprene units. This molecule is found attached to the membrane of the ER. Sugar molecules are attached to the dolichol through a pyrophosphate linkage (One phosphate was originally linked to dolichol, and the second phosphate came from the nucleotide sugar). The oligosaccharide chain is then extended through the addition of various sugar molecules in a step-wise manner to form a precursor oligosaccharide.

The assembly of this precursor oligosaccharide essentially occurs in two phases: Phase I and II. Phase I takes place on the cytoplasmic side of the ER and Phase II takes place on the luminal side of the ER.

The precursor molecule, ready to be transferred to a protein, consist of 2 GlcNAc, 9 mannose and 3 glucose moieties.

Diagram shows the precursor oligosaccharide synthesis in the ER lumen during N-linked glycosylation

Phase I
Steps	Location
Two UDP-GlcNAc residues are attached to the dolichol molecule embedded in the ER membrane. The sugar and dolichol form a pyrophosphate linkage. Five GDP-Man residues are attached to the GlcNAc disaccharide .These steps are performed by glycosyltransferases. Product: Dolichol - GlcNAc2 - Man5	Cytoplasmic side of ER
At this point, the lipid-linked glycan is translocated across the membrane making it accessible to enzymes in the ER lumen. This translocation process is still poorly understood, but it is suggested to be performed by an enzyme known as flipase.
Phase II
The growing glycan is exposed on the luminal side of the ER membrane and subsequent sugars (4 mannose and 3 glucose) are added. These additional sugars are transported into the lumen from the cytoplasm of the ER via attachment to the dolichol molecule and subsequent translocation into the lumen with the help of flippase enzyme. (Various dolichols in the membrane are used to translocate multiple sugars at once). Product: Dolichol - GlcNAc2 – Man9-Glc3	Luminal side of ER

En bloc transfer of glycan to protein

Once the precursor oligosaccharide is formed, the completed glycan is then transferred to the nascent polypeptide in the lumen of the ER membrane. This reaction is driven by the energy released from the cleavage of the pyrophosphate bond between the dolichol-glycan molecule. There are three conditions to fulfill before a glycan is transferred to a nascent polypeptide:

• Asn must be located in a specific consensus sequence in the primary structure (Asn-X-Ser or Asn-X-Thr or in rare instances Asn-X-Cys).
• Asn must be located appropriately in the three dimensional structure of the protein (Sugars are polar molecules and thus need to be attached to Asn located on surface of the protein and not buried within the protein)
• Asn must be found in the luminal side of the ER for N-linked glycosylation to be initiated. (Target Asn residues are either found in secretory proteins or in the regions of transmembrane protein that faces the lumen.)

Oligosaccharyltransferase is the enzyme responsible for the recognition of the consensus sequence and the transfer of the precursor glycan to a polypeptide acceptor which is being translated in the ER lumen. N-linked glycosylation is therefore, is a co-translational event

Processing of glycan

Glycan processing in the ER and Golgi

N-glycan processing is carried out in ER and the Golgi. Initial trimming of the precursor molecule occurs in the ER and the subsequent processing occurs in the Golgi.

Upon transferring the completed glycan onto the nascent polypeptide, three glucose residues are removed from the structure. Enzymes known as glycosidases remove some sugar residues. These enzymes can break glycosidic linkages by using a water molecule. These enzymes are exoglycosidases as they only work on monosaccharide residues located at the non-reducing end of the glycan. This initial trimming step is thought to act as a quality control step in the ER to monitor protein folding.

Once protein is folded correctly, the three glucose residues are removed by Glucosidase I and II. The removal of the final glucose residue signals that the glycoprotein is ready for transit from the ER to the cis-Golgi. However, if the protein is not folded properly, the glucose residues are not removed and thus the glycoprotein can’t leave the ER. A chaperone protein (Calnexin/Calreticulin) binds to the unfolded or partially folded protein to assist protein folding.

The next step involves further addition and removal of sugar residues in the golgi. These modifications are catalyzed by glycosyltransferases and glycosidases respectively. In the cis-golgi, a series of mannosidases remove some or all of the four mannose residues in alpha 1,2 linkages. Whereas, in the medial portion of golgi, glycosyltransferases add sugar residues, to the core glycan structure giving rise to the three main types of glycans: high mannose, hybid and complex glycans.

The three different types of glycans

• 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.
• Hybrid oligosaccharides contain a mannose residues on one side of the branch, while on the other side a N-Acetylglucosamine initiates a complex branch.

The order of addition of sugars to the growing glycan chains is determined by the substrate specificities of the enzymes and their access to the substrate as they move through secretory pathway. Thus, the organization of this machinery within a cell plays an important role in determining which glycans are made.

Organization of enzymes in the Golgi

Golgi enzymes plays a key role in determining the synthesis of the various types of glycan. The order of action of the enzymes is reflected in their position in the Golgi stack:

Enzymes	Location within Golgi
Mannosidase I	cis-Golgi
GlcNAc transferases	medial Golgi
Galactosyltransferase and Sialyltransferase	trans-Golgi

Biosynthesis pathway comparison to archaea and prokaryotes

Similar N-glycan biosynthesis pathway have been found in prokaryotes and Archaea. However, compared to eukaryotes, the final glycan structure in prokaryotes and Achaea does not seem to differ much from the initial precursor made in the ER. In eukaryotes, the original precursor oligosaccharide is extensively modified en route to the cell surface.

Functions of N-glycans

N-linked glycans have intrinsic and extrinsic functions.

Functions of N-linked Glycans
Intrinsic	Provides structural components to the cell wall and extracellular matrix. Modify protein properties such as stability and solubility. (More stable to high temperature, pH and etc.)
Extrinsic	Directs trafficking of glycoproteins. Mediates cell signalling. (Cell-Cell and Cell-Matrix interactions)

Clinical significance

Mutations in eighteen genes involved in N-linked glycosylation result in a variety of diseases, most of which involve the nervous system.

Importance in therapeutic proteins

Many “blockbuster” therapeutic proteins in the market now are N-linked glycoproteins. For examples Etanercept, Infliximab and Rituximab are N-glycosylated therapeutic proteins.

Difference between the glycan produced by humans and animal cells

The importance of N-linked glycosylation is becoming increasingly evident in the field of pharmaceuticals. Proteins are usually produced using bacterial or yeast protein expression systems due to high yield and low cost, however, problems arise when the desired protein is glycosylated in its native state. Most prokaryotic expression systems such as E.coli cannot carry out post-translational modifications. Eukaryotic expression hosts such as yeast and animal cells on the other hand, have different glycosylation patterns. The proteins produced in these expression hosts are often not identical to human protein and thus, causes immunogenic reactions in patients. For example S.cerevisiae (yeast) often produce high-mannose glycans which are immunogenic.

Animal expression systems on the other hand, have the machinery required to carry out complex glycosylation process but their glycans differ from human glycans in their terminal sugar molecule. The glycans in animal cells are cappped with N-Glycolylneuraminic acid (Neu5Gc) instead of N-Acetylneuraminic acid (Neu5Ac) (humans).

Studies are being conducted to tackle this problem in protein expression. Among the efforts being taken to produce therapeutic glycoproteins with human-like N-linked glycoproteins includes using CHO cell line and glycoengineered Pichia pastoris( a type of yeast) as expression hosts.

See also

• Glycosylation
• O-linked glycosylation
• Protein expression