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Glycosylation

There are four main bio molecules involve in various biochemical reactions occurs in living organism. These bio molecules are Carbohydrates, Nucleic acid, Lipids and proteins. They are complex molecules and composed of small molecules like amino acid, mono saccharides, fatty acids, sugar, phosphate group and nitrogen bases. These bio molecules involve in biochemical processes like translation, transcription etc. Remember these molecules cannot complete a biochemical process alone, but with the cooperation and interference of other bio molecules.

Apart from these bio molecules, there are many other bio molecules and derivatives which are essential for various processes and for the production of energy. Like adenosine triphosphate (ATP), adenosine diphosphate (ADP), glycoproteins and other glyconutrients etc.
Glyconutrients provide necessary mono saccharides which are essential for optimizing the immune health at the cellular level. They involve in fundamental structure of the body and are at the forefront of nutritional science. In glyconutrients, the mono saccharide can combine with proteins or fats to form a code which allows each cell to communicate with others.

Deficiencies of glyco nutrients can cause a number of diseases like fibromyalgia, rheumatoid arthritis, AIDS, asthma, chronic fatigue syndrome, inflammation and lupus. The consumption of glyconutrient provides strength to immune system and builds resistance to bacterial and viral infection.

 

Glycosylation Definition

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"The enzymatic process of carbohydrate with another molecule contains a hydroxyl group or other functional group is known as glycosylation."

Here carbohydrate molecule acts as glycosyl donor and other molecule acts as glycosyl acceptor. Another molecule can be protein, lipid or other bio molecules and produces one of the fundamental bio polymers. It is a form of co-translational and post-translational modification and occurs in endoplasmic reticulum (ER) and Golgi body.

Almost all proteins in a cell undergo this modification and covalently bonded with sugar moieties through specific amino acids. In general, soluble and membrane-bonded proteins like secreted proteins, organelle-resident proteins and surface receptors get glycosylated to some extent. The glycosylated proteins are also known as glycoproteins and mainly found in eukaryotes and eubacteria.

Protein Glycosylation

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The process of bonding of carbohydrate moiety to a protein part is known as protein glycosylation results the formation of glycoproteins. This process is generally involved in cell membrane formation. It is a series of enzymatic reactions which involve the formation of glycoproteins at n- and o- sites.

Protein Glycosylation

There are total 16 enzymes who supposed to take part in this reaction and a glycoprotein molecule has at least 41 bonds which involve 8 amino acids and 13 different monosaccharide units and includes the glycophosphatidylinositol (GPI) and phosphoglycosyl linkages. It mainly occurs at ER, cell fluid, Golgi body and nucleus. There are three major types of glycoproteins or glycan which are produced in protein glycosylation.

  • N-linked glycans: These glycans bonded to nitrogen of asparagine or arginine side-chains through N-linked glycosylation. This process requires participation of a special lipid known as dolichol phosphate.
  • O-linked glycans: These are bonded with the the hydroxy oxygen of tyrosine, serine, hydroxylysine, threonine or hydroxyproline side-chains, or to oxygens on lipids like ceramide phospho-glycans linked through the phosphate of a phospho-serine;
  • C-linked glycans: In this glycan, a sugar is added to a carbon on a tryptophan side-chain.

  Glycan

N-Linked Glycosylation

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In n-linked glycosylation, a sugar moiety gets bonded with an asparagine amino acid and composed of glucose, mannose and N-acetylglucosamine molecules.

This n-linked glycan then transferred to the Endoplasmic Reticulum lumen. The process takes place in the presence of oligosaccharyl transferase enzyme and results the formation of a tripeptide sequence shown as Asn-X-Ser or Asn-X-Thr , where X can be any amino acid excluding Proline. After proper folding in protein sequences, it is translocated to the Golgi body where the mannose residue is removed. N-glycans can be three types high-mannose type, complex type and hybrid type. They have common pentasaccharide core and are synthesized from a common precursor oligosaccharide.
N-Linked Glycosylation

This glycosylation is a co-translation process in which the glycan gets bonded with nascent protein and transported to ER. Here N-denotes that the carboxamido nitrogen covalently bonded to asparagine (Asn or N) residues. N-glycosylation process consists of four steps.

  1. Precursor glycan assembly: In this step, oligosaccharides bonded through N-glycosidic linkage. These oligosaccharides are derived from 14-sugar precursor molecule comprised of N-acetylglucosamine (GlcNAc), mannose (Man) and glucose (Glc). These sugar molecules are bonded onto a polyisoprenoid lipid carrier called as dolichol which is embedded in ER membrane.
  2. Glycan attachment: In the presence of protein translocator proteins translated and transported to ER in the presence of oligosaccharide transferase (OSTase) which scan the sequence of polypeptide chain Asn-X-Ser/Thr, where X can be any one amino acid excluding proline.
  3. Glycan trimming: The trimming of oligosaccharide in both ER and golgi takes place in the presence of glycosidases through hydrolysis.
  4. Glycan maturation: After the trimming process in the ER, the glycoprotein is transferred to the Golgi. In Golgi, the mannose sugar part has to remove in the presence of Golgi mannosidase-I and this glycans do not undergo further glycosylation are known as high-mannose oligosaccharides.

O-Linked Glycosylation

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The less common O-glycoproteins play an important role in cell biology and formed through O-glycosylation. The formation of O-glycoproteins is occurring through o-glycosylation process. This process is essential for the biosynthesis of mucins as these glycoproteins are high molecular weight proteins which form mucus secretions. O-glycosylation process is a post-translational process and occurs in the ER, Golgi, cystosol and nucleus.

In this process, monosaccharides bind to the hydroxyl group of serine or threonine in the ER, Golgi, cystosol and nucleus. The process begins with an enzyme mediated addition of N-acetyl-galactosamine followed by other carbohydrates to serine or threonine residues.
O-Linked Glycosylation
This process is not so complicated as N-glycosylation process. Proteins are o-glycosylated by N-acetylgalactosamine (GalNAc) transferases. The O-glycosylation process can also occur in nucleus and cytosol which regulate the gene expression or signal transduction.

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