ABSTRACT
Glycoproteins are classified according to the type of oligosaccharide chains they carry, and also their site of attachment to the protein molecule. N-Linked glycosylation of proteins is a co-translational event. The presence of a consensus amino acid sequence in the nascent polypeptide chain is a prerequisite for N-linked oligosaccharide synthesis. The glycosyltransferases build the oligosaccharide structure, whereas the glycosidases trim it. There are many classical chaperone molecules involved in protein folding; an example is protein disulfide isomerase, the molecule responsible for the production of disulfide linkages. Misfolded/mutant proteins are transported from the ER into proteasomes in the cytosol for degradation. There are three main classes of N-linked oligosaccharides which can be built on the trimannosyl core: high mannose type, complex type; and hybrid type. The functions of N-linked oligosaccharides on serum proteins include correct folding and conformation of the protein, maintaining protein stability and metabolic turnover.
