Glycoproteins are proteins with oligosaccharide chains (glycans) covalently bound to polypeptide side-chains. The carbohydrate is attached by a process called glycosylation. Proteins that have extending segments in an extracellular manner are often glycosylated. Glycosylation occurs in one of two modes:
Note that these glycosylation reactions of a protein will only occur when the protein has the following order: Asn-X-Ser or Asn-X-Thr. X in the sequence can be any protein except proline. Glycoproteins are often important integral membrane proteins, such that they allow cell-cell interactions. Glycoproteins appear in the cytosol as well, but there is a lack of understanding in this subject.