Glycosylation

Glycosylation is the reaction in which a carbohydrate (or 'glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not always in chemistry), glycosylation usually refers to an enzyme-catalysed reaction, whereas glycation (also 'non-enzymatic glycation' and 'non-enzymatic glycosylation') may refer to a non-enzymatic reaction.^[1]

Glycosylation is a form of co-translational and

• N-linked glycans attached to a nitrogen of asparagine or arginine side-chains. N-linked glycosylation requires participation of a special lipid called dolichol phosphate.
• O-linked glycans attached to the
  hydroxyl oxygen of serine, threonine, tyrosine, hydroxylysine, or hydroxyproline side-chains, or to oxygens on lipids such as ceramide
  .
• Phosphoglycans linked through the phosphate of a phosphoserine.
• C-linked glycans, a rare form of glycosylation where a sugar is added to a carbon on a tryptophan side-chain. Aloin is one of the few naturally occurring substances.
• Glypiation, which is the addition of a GPI anchor that links proteins to lipids through glycan linkages.

Purpose

Glycosylation is the process by which a

Overall, glycosylation needs to be understood by the likely evolutionary selection pressures that have shaped it. In one model, diversification can be considered purely as a result of endogenous functionality (such as cell trafficking). However, it is more likely that diversification is driven by evasion of pathogen infection mechanism (e.g. Helicobacter attachment to terminal saccharide residues) and that diversity within the multicellular organism is then exploited endogenously.

Glycosylation can also module the thermodynamic and kinetic stability of the proteins.^[9]

Glycoprotein diversity

Glycosylation increases diversity in the proteome, because almost every aspect of glycosylation can be modified, including:

• Glycosidic bond—the site of glycan linkage
• Glycan composition—the types of sugars that are linked to a given protein
• Glycan structure—can be unbranched or branched chains of sugars
• Glycan length—can be short- or long-chain oligosaccharides

Mechanisms

There are various mechanisms for glycosylation, although most share several common features:^[2]

• Glycosylation, unlike
  post-translational modification, because of the large number of enzymatic steps involved.^[10]
• The donor molecule is often an activated nucleotide sugar.
• The process is non-templated (unlike DNA
  transcription or protein translation); instead, the cell relies on segregating enzymes into different cellular compartments (e.g., endoplasmic reticulum, cisternae in Golgi apparatus
  ). Therefore, glycosylation is a site-specific modification.

N-linked glycosylation is a very prevalent form of glycosylation and is important for the folding of many eukaryotic glycoproteins and for cell–cell and cell–

O-linked glycosylation is a form of glycosylation that occurs in

A

The non-enzymatic glycosylation is also known as glycation or non-enzymatic glycation. It is a spontaneous reaction and a type of post-translational modification of proteins meaning it alters their structure and biological activity. It is the covalent attachment between the carbonil group of a reducing sugar (mainly glucose and fructose) and the amino acid side chain of the protein. In this process the intervention of an enzyme is not needed. It takes place across and close to the water channels and the protruding tubules.^[21]

At first, the reaction forms temporary molecules which later undergo different reactions (Amadori rearrangements, Schiff base reactions, Maillard reactions, crosslinkings...) and form permanent residues known as Advanced Glycation end-products (AGEs).

AGEs accumulate in long-lived extracellular proteins such as collagen^[22] which is the most glycated and structurally abundant protein, especially in humans. Also, some studies have shown lysine may trigger spontaneous non-enzymatic glycosylation.^[23]

Role of AGEs

AGEs are responsible for many things. These molecules play an important role especially in nutrition, they are responsible for the brownish color and the aromas and flavors of some foods. It is demonstrated that cooking at high temperature results in various food products having high levels of AGEs.^[24]

Having elevated levels of AGEs in the body has a direct impact on the development of many diseases. It has a direct implication in diabetes mellitus type 2 that can lead to many complications such as: cataracts, renal failure, heart damage...^[25] And, if they are present at a decreased level, skin elasticity is reduced which is an important symptom of aging.^[22]

They are also the precursors of many hormones and regulate and modify their receptor mechanisms at the DNA level.^[22]

Deglycosylation

There are different

• sialic acids
  .
• glycoproteins
  .
• β-N-Acetylglucosaminidase (from Streptococcus pneumoniae): cleaves all non-reducing terminal β-linked N-acetylglucosamine residues from complex carbohydrates and glycoproteins.
• endo-α-N-Acetylgalactosaminidase (O-glycosidase from Streptococcus pneumoniae): removes O-glycosylation. This enzyme cleaves serine- or threonine-linked unsubstituted Galβ1,3GalNAc
• PNGase F: cleaves asparagine-linked oligosaccharides unless α1,3-core fucosylated.

Regulation of Notch signalling

Some of the specific modulators that control this process are glycosyltransferases located in the endoplasmic reticulum and the Golgi apparatus.^[28] The Notch proteins go through these organelles in their maturation process and can be subject to different types of glycosylation: N-linked glycosylation and O-linked glycosylation (more specifically: O-linked glucose and O-linked fucose).^[26]

All of the Notch proteins are modified by an O-fucose, because they share a common trait: O-fucosylation consensus sequences.^[26] One of the modulators that intervene in this process is the Fringe, a glycosyltransferase that modifies the O-fucose to activate or deactivate parts of the signalling, acting as a positive or negative regulator, respectively.^[28]

Clinical

There are three types of glycosylation disorders sorted by the type of alterations that are made to the glycosylation process: congenital alterations, acquired alterations and non-enzymatic acquired alterations.

• Congenital alterations: Over 40 congenital disorders of glycosylation (CGDs) have been reported in humans.^[29] These can be divided into four groups: disorders of protein N-glycosylation, disorders of protein O-glycosylation, disorders of lipid glycosylation and disorders of other glycosylation pathways and of multiple glycosylation pathways. No effective treatment is known for any of these disorders. 80% of these affect the nervous system.^{[citation needed]}
• Acquired alterations: In this second group the main disorders are infectious diseases,
  mutations on the enzymes that control the glycosylation of Notch proteins, such as Alagille syndrome.^[28]
• Non-enzymatic acquired alterations: Non-enzymatic disorders, are also acquired, but they are due to the lack of enzymes that attach oligosaccharides to the protein. In this group the illnesses that stand out are Alzheimer's disease and diabetes.^[30]

All these diseases are difficult to diagnose because they do not only affect one organ, they affect many of them and in different ways. As a consequence, they are also hard to treat. However, thanks to the many advances that have been made in next-generation sequencing, scientists can now understand better these disorders and have discovered new CDGs. ^[31]

Effects on therapeutic efficacy

It has been reported that mammalian glycosylation can improve the therapeutic efficacy of

• Advanced glycation endproduct
   – Proteins or lipids that become glycated as a result of exposure to sugarsPages displaying short descriptions of redirect targets
• Chemical glycosylation – Reaction of a glycosyl donor and acceptor
• Fucosylation – The covalent attachment of a fucosyl group to an acceptor molecule.
• Glycation – Attachment of a sugar to a protein or lipid
• Glycorandomization – Technology enabling rapid molecule diversification

References