Oligosaccharides

An oligosaccharide (from the Greek olígos, "a few", and σάκχαρ sácchar, "sugar") is a saccharide polymer containing a small number (typically two to ten) of simple sugars (monosaccharides). Oligosaccharides can have many functions including cell recognition and cell binding. For example, glycolipids have an important role in the immune response.

In general, they are found either N- or O-linked to compatible amino acid side-chains in proteins or to lipid moieties (see glycans). N-linked oligosaccharides are found attached to asparagine via a beta linkage to the amine nitrogen of the side chain. Alternately, O-linked oligosaccharides are generally attached to threonine or serine on the alcohol group of the side chain.

In biology, glycosylation is the co-translational process by which a carbohydrate is covalently attached to an organic molecule – creating structures such as glycoproteins and glycolipids.

N-linked glycosylation involves oligosaccharide attachment to asparagine via a beta linkage to the amine nitrogen of the side chain. The process of N-linked glycosylation occurs cotranslationally, or concurrently while the proteins is being translated. Since it is added cotranslationally, it is believed that N-linked glycosylation helps determine the folding of polypeptides due to the hydrophilic nature of sugars. All N-linked Oligosaccharides are composed of a core of five sugars - known as a pentasaccharide.

In N-glycosylation for eukaryotes, the oligosaccharide substrate is assembled right at the membrane of the ER. For prokaryotes, this process occurs at the plasma membrane. In both cases, the acceptor substrate is an asparagine residue.They are small carbohydrates which are formed by condensation of 2-9 monosacchrides. This asparagine has increased nucleophilicity in the amide group. The unique arrangement of N-linked oligosaccharides usually has the oligosaccharide linked to the amide nitrogen of the Asn residue, in the sequence Asn-X-Ser/Thr. X can be any amino acid except for proline (though it is rare to see Asp, Glu, Leu, or Trp).

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