Glycosylation is the process or result of addition of saccharides to proteins and lipids. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble The process is one of four principal co-translational and post-translational modification steps in the synthesis of membrane and secreted proteins and the majority of proteins synthesized in the rough ER undergo glycosylation. Posttranslational modification (PTM is the chemical modification of a Protein after its translation. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle It is an enzyme-directed site-specific process, as opposed to the non-enzymatic chemical reaction of glycation. Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins Glycation (sometimes called non-enzymatic Glycosylation) is the result of a sugar molecule such as Fructose or Glucose, bonding to a Protein Two types of glycosylation exist: N-linked glycosylation to the amide nitrogen of asparagine side chains and O-linked glycosylation to the hydroxy oxygen of serine and threonine side chains. In Chemistry, an amide is one of three kinds of Compounds (sometimes called acid amide the organic Functional group characterized Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Asparagine (abbreviated as Asn or N; Asx or B represent either asparagine or Aspartic acid) is one of the 20 most common natural A side chain in Organic chemistry and Biochemistry is a part of a Molecule that is attached to a core structure Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Serine (abbreviated as Ser or S) is an Organic compound with the formula H[[oxygen O]]2 CCH NH sub>2CH2OH Threonine (abbreviated as Thr or T) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH(OHCH3

Purpose

The polysaccharide chains attached to the target proteins serve various functions. For instance, some proteins do not fold correctly unless they are glycosylated first. Also, polysaccharides linked at the amide nitrogen of asparagine in the protein confer stability on some secreted glycoproteins. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Asparagine (abbreviated as Asn or N; Asx or B represent either asparagine or Aspartic acid) is one of the 20 most common natural 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. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor

Mechanisms

There are various mechanisms for glycosylation, although all share several common features:

• Glycosylation is an enzymatic process;
• The donor molecule is an activated nucleotide sugar;
• The process is site-specific. Nucleotide sugars are Biochemicals that act as donors of Sugar residues in Nucleotide sugars metabolism.

N-linked glycosylation

Comparative overview of the major types of vertebrate N-glycan subtypes and some representative C. elegans N-glycans. Caenorhabditis elegans (ˌsiːnoʊræbˈdaɪtɪs ˈɛlɪgænz is a free-living Nematode (roundworm about 1  mm in length which

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. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have

For N-linked oligosaccharides, a 14-sugar precursor is first added 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. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Glucose (Glc a Monosaccharide (or simple Sugar) also known as grape sugar, is an important Carbohydrate in Biology. Mannose is a Sugar Monomer of the Hexose series of Carbohydrates Metabolism Mannose enters the carbohydrate Metabolism 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. Dolichol refers to any of a group of long-chain mostly unsaturated Organic compounds which are made up of varying numbers of Isoprene units terminating

There are two major types of N-linked saccharides: high-mannose oligosaccharides, and complex 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 oligos 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. The Golgi apparatus (also called the 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, Asn-X-Thr or Asn-X-Cys, where X could be any amino acid except Pro. An oligosaccharide is a Saccharide polymer containing a small number (typically three to ten of component sugars also known as Simple sugars. Oligosaccharyltransferase or OST ( is a Membrane protein complex that transfers a 14- Sugar Oligosaccharide from Dolichol Asparagine (abbreviated as Asn or N; Asx or B represent either asparagine or Aspartic acid) is one of the 20 most common natural Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino Serine (abbreviated as Ser or S) is an Organic compound with the formula H[[oxygen O]]2 CCH NH sub>2CH2OH Threonine (abbreviated as Thr or T) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH(OHCH3 Not to be confused with Cystine, its oxidized dimer Cysteine (abbreviated as Cys or C) is an α- Amino acid with In Chemistry, an amino acid is a Molecule containing both Amine and Carboxyl Functional groups In Biochemistry, this Proline (abbreviated as Pro or P) is an α- Amino acid, one of the twenty DNA -encoded amino acids This sequence is known as a glycosylation sequon. A sequon is a sequence of three consecutive amino acids in a proteinthat can serve as the attachment site to a polysaccharide (sugar called an N-linked-Glycan. 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. Mannose is a Sugar Monomer of the Hexose series of Carbohydrates Metabolism Mannose enters the carbohydrate Metabolism Galactose (Gal (also called brain sugar) is a type of Sugar which is less sweet than Glucose. N-Acetylglucosamine ( N-Acetyl-D-Glucosamine, or GlcNAc, or NAG) is a Monosaccharide derivative of Glucose. N-Acetylgalactosamine (also called GalNAc, 2-Acetamido-2-deoxy-D-galactopyranose or N-Acetyl-D-galactosamine) is a Monosaccharide derivative Fucose is a Hexose Deoxy sugar with the Chemical formula C6H12O5 Sialic acid is a generic term for the N - or O -substituted derivatives of Neuraminic acid, a Monosaccharide with a nine- Carbon backbone

O-linked glycosylation

O-N-acetylgalactosamine (O-GalNAc)

O-linked glycosylation occurs at a later stage during protein processing, probably in the Golgi apparatus. The Golgi apparatus (also called the 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). Serine (abbreviated as Ser or S) is an Organic compound with the formula H[[oxygen O]]2 CCH NH sub>2CH2OH Threonine (abbreviated as Thr or T) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH(OHCH3 This article is about the Enzyme Commission codes For the European Commission system for coding chemicals see EC-No. Galactose (Gal (also called brain sugar) is a type of Sugar which is less sweet than Glucose. Sialic acid is a generic term for the N - or O -substituted derivatives of Neuraminic acid, a Monosaccharide with a nine- Carbon backbone 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. Proteoglycans represent a special class of Glycoproteins that are heavily glycosylated. " 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. In Biology, the extracellular matrix ( ECM) is the Extracellular part of animal tissue that usually provides structural support to the cells 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. Immunoglobulin A ( IgA) is an Antibody playing a critical role in mucosal immunity Immunoglobulin D ( IgD) is an Antibody isotype that makes up about 1% of Proteins in the Plasma membranes of immature B-lymphocytes C1-inhibitor ( C1-inh, C1 esterase inhibitor) is a Serine protease inhibitor (serpin protein the main function of which is the inhibition of the

O-fucose

O-fucose is added between the second and third conserved cysteines of EGF-like repeats in the Notch protein, and possibly other substrates by GDP-fucose protein O-fucosyltransferase 1, and to Thrombospondin repeats by GDP-fucose protein O-fucosyltransferase 2. GDP-fucose protein O -fucosyltransferase 1 (POFUT1 is an Enzyme responsible for adding Fucose sugars in O linkage to Serine or Thrombospondins (TSP are secreted proteins with Antiangiogenic abilities GDP-fucose protein O -fucosyltransferase 2 (POFUT2 is an Enzyme responsible for adding Fucose sugars in O linkage to Serine or 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. Fucose is a Hexose Deoxy sugar with the Chemical formula C6H12O5 N-Acetylglucosamine ( N-Acetyl-D-Glucosamine, or GlcNAc, or NAG) is a Monosaccharide derivative of Glucose. Galactose (Gal (also called brain sugar) is a type of Sugar which is less sweet than Glucose. Sialic acid is a generic term for the N - or O -substituted derivatives of Neuraminic acid, a Monosaccharide with a nine- Carbon backbone Glucose (Glc a Monosaccharide (or simple Sugar) also known as grape sugar, is an important Carbohydrate in Biology. Both of these fucosyltransferases have been localized to the endoplasmic reticulum, which is unusual for glycosyltransferases, most of which function in the Golgi apparatus. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle

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 an unidentified O-glucosyltransferase.

O-N-acetylglucosamine (O-GlcNAc)

O-GlcNAc is added to serines or threonines by O-GlcNAc transferase. O-GlcNAc appears to occur on serines and threonines that would otherwise be phosphorylated by serine/threonine kinases. Thus, if phosphorylation occurs, O-GlcNAc does not, and vice versa. Phosphorylation is the addition of a Phosphate (PO4 group to a Protein molecule or a small molecule 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 appear to be key regulators of the pathways that are deregulated in diabetes mellitus. The gene encoding the O-GlcNAc removal enzyme has been linked to non-insulin dependent diabetes mellitus. It is the terminal step in a nutrient-sensing hexosamine signaling pathway. Hexosamines are Amino sugars created by adding an Amine group to a Hexose.

GPI anchor

A special form of glycosylation is the GPI anchor. Glycosylphosphatidylinositol ( GPI anchor) is a Glycolipid that can be attached to the C-terminus of a Protein during Posttranslational This form of glycosylation functions to attach a protein to a hydrophobic lipid anchor, via a glycan chain. In Chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) refers to the physical property of (see also prenylation)

C-mannosylation

A mannose sugar is added to tryptophan residues in Thrombospondin repeats. Prenylation or isoprenylation or lipidation is the addition of Hydrophobic molecules to a Protein. Mannose is a Sugar Monomer of the Hexose series of Carbohydrates Metabolism Mannose enters the carbohydrate Metabolism Tryptophan (abbreviated as Trp or W) is one of the 20 standard amino acids, as well as an Essential amino acid in the Human diet Thrombospondins (TSP are secreted proteins with Antiangiogenic abilities This is an unusual modification both because the sugar is linked to a carbon rather than a reactive atom like a nitrogen or oxygen and because the sugar is linked to a tryptophan residue rather than an asparagine or serine/threonine. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Tryptophan (abbreviated as Trp or W) is one of the 20 standard amino acids, as well as an Essential amino acid in the Human diet Asparagine (abbreviated as Asn or N; Asx or B represent either asparagine or Aspartic acid) is one of the 20 most common natural Serine (abbreviated as Ser or S) is an Organic compound with the formula H[[oxygen O]]2 CCH NH sub>2CH2OH Threonine (abbreviated as Thr or T) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH(OHCH3

See also

• Glycation
• Advanced glycation endproduct

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.