text: N-glycan processing is carried out in endoplasmic reticulum and the Golgi body. Initial trimming of the precursor molecule occurs in the ER and the subsequent processing occurs in the Golgi.
  Upon transferring the completed glycan onto the nascent polypeptide, two glucose residues are removed from the structure. Enzymes known as glycosidases remove some sugar residues. These enzymes can break glycosidic linkages by using a water molecule. These enzymes are exoglycosidases as they only work on monosaccharide residues located at the non-reducing end of the glycan. This initial trimming step is thought to act as a quality control step in the ER to monitor protein folding.
  Once the protein is folded correctly, two glucose residues are removed by glucosidase I and II. The removal of the final third glucose residue signals that the glycoprotein is ready for transit from the ER to the cis-Golgi. ER mannosidase catalyses the removal of this final glucose. However, if the protein is not folded properly, the glucose residues are not removed and thus the glycoprotein can't leave the endoplasmic reticulum. A chaperone protein (calnexin/calreticulin) binds to the unfolded or partially folded protein to assist protein folding.
  The next step involves further addition and removal of sugar residues in the cis-Golgi. These modifications are catalyzed by glycosyltransferases and glycosidases respectively. In the cis-Golgi, a series of mannosidases remove some or all of the four mannose residues in α-1,2 linkages. Whereas in the medial portion of the Golgi, glycosyltransferases add sugar residues to the core glycan structure, giving rise to the three main types of glycans: high mannose, hybrid and complex glycans.
  *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.
  *Hybrid oligosaccharides contain a mannose residues on one side of the branch, while on the other side a N-acetylglucosamine initiates a complex branch.
  The order of addition of sugars to the growing glycan chains is determined by the substrate specificities of the enzymes and their access to the substrate as they move through secretory pathway. Thus, the organization of this machinery within a cell plays an important role in determining which glycans are made.