Great Googly Mogly, It's Glycosylation!

This article explains the process of glycosylation. Glycosylation is the process by carbohydrates are attached to protein molecules. It is a common post translational modification for protein molecules that are a part of the plasma membrane. Glycosylation involves the linking of monosaccharide units to amino acid chains. This sets up the stage for a series of enzymatic reactions which results in the formation of glycoproteins. There are 16 known enzymes that facilitate these enzymatic reactions. A typical glycoprotein has at least 41 bonds involving 8 amino acids and 13 different monosaccharide units. The major sites of protein glycosylation are the ER, Golgi , nucleus and cytoplasm.

There are 2 main types of gylcosylation.

 N-Linked Glycosylation- N-Linked Glycosylation begins with the addition of a 14-sugar precursor to an asparagine amino acid. It contains glucose, mannose and n-acetylglucosamine molecules. The whole entity is then transferred to the ER lumen. Oligosaccharyl transferase enzyme attaches the oligosaccharide chain to asparagine in the tripeptide sequence, Asn-X-Ser or Asn-X-Thr where X can be any amino acid other than Proline. The oligosaccharide attached protein sequence will now fold correctly and is translocated to the Golgi. Mannose residue is removed in the golgi apparatus.

 O-Linked Glycosylation- O-linked glycosylation begins with the addition of N-acetyl-galactosamine to the molecule by an enzyme and other carbohydrates to serine or threonine residues. O-linked glycosylation occurs at a later stage in protein processing, probably in the Golgi apparatus.