Glycosylation

Similarities and Differences in the Glycosylation Mechanisms in Prokaryotes and Eukaryotes

Glycosylation of proteins occurs in both prokaryotic and eukaryotic cells. More than 70% of eukaryotic proteome is said to be glycosylated. While the percent of proteins that are glycosylated in prokaryotes is unknown, it is a common occurrence. The article presents four major glycosylation pathways that are present in archaea, eukarya, and bacteria, which vary in their abundance depending on what kinds of organism is being presented. For example, N-glycosylation, mediated by oligosaccharyltransferase, is fairly abundant in archae and eukaryea, but not in bacteria.


N-glycosylation starts with the formation of sugars in the cytoplasm, which assembles into a oligosaccharide precursor that is attached through pyrophosphate to a lipid carrier. Once the oligosaccharide is assembled, the lipid-linked oligosaccharide (LLO) is flipped, and faces the ER's lumen. The oligosaccharide is then moved from to the acceptor protein from the lipid carrier. This step, in which the LLO is attached to an asparagine amino acid is catalysed by an enzyme.


The other major type of glycosylation is called O-glycosylation, which also occurs in a systematic fashion. It begins when the linking monosaccharide is attached to serine or threonine, which are acceptors. Sugars are subsequently added one at a time to form a mature glycan. Most O-glycosylation events occur in the Golgi apparatus, although there are some that occur in the endoplasmic reticulum as well. There is a tremendous variety of sequences that can be attached, causing there to be much diversity in O-glycosylation.


All glycoproteins in eukaryotes go through substantial remodeling in the Golgi apparatus upon exiting the endoplasmic reticulum. There is no equivalent to the Golgi apparatus in prokaryotes, so glycoproteins there do not undergo remodeling. The article goes on to describe how the different types of glycosylation differ in the different domains. For a more in-depth explanation, feel free to check out the link for my article that I posted above!