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Old 09-05-2014
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Default O linked & N linked glycosylation

I didn't understand those what are they? what their function?
read everywhere but still
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Old 09-08-2014
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Default Explained

Hi, let me try to explain...

After protein has been synthesized in the cytoplasm of cells (translated from mRNA; involving the sequential addition of amino acids with the creation of peptide bonds between them and a growing polypeptide chain), depending on its function, it may undergo modifications to create the final active protein that's ready to perform. One of such modifications is glycosylation and as the name implies, involves the addition of sugar (oligosaccharide group) to the polypeptide chain (protein). This creates a glyco-protein.

In the case of O-glycosylation, the sugar group is added to the -OH group of an amino acid on the peptide chain and this is usually Serine or Threonine. This takes place in the Golgi and the group added is that of a hexosamine, N-acetylgalactosamine. Glycosyltransferases are involved in this process.

As for N-glycosylation, the sugar group is added to the amide, NH+ group of a specific Asparagine amino acid on the protein. This one takes place in the RER and is a bit more complex than that of O-linked in that here, a lipid in the RER membrane called dolichol (very long long chain lipid, abt 80-100C long) is first phosphorylated to dolichol pyrophosphate and linked with an oligosaccharide group containing N-acetylglucosamine (a hexosamine) + mannose + glucose. These groups are later transferred from the dolichol onto the asparagine.
That's it

The glycosylated polypeptide chains may undergo further modifications in RER and/or Golgi to produce a complex glycoprotein which can then be released via secretory vesicles extracellularly or to cell membrane or to lysosomes (it is important to note that it's just N-linked ones that can be redirected to the lysosomes after one of their mannose groups earlier added is first phosphorylated by a phosphotransferase.)
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