protein glycosylation
/ gly-co-suh-LAY-shun /
Many of the proteins on the outside of your cells and floating in your blood are not bare — they are decorated with branching chains of sugar molecules, like a plain coat covered in intricate beadwork. Glycosylation is the cell's process of attaching these sugar chains to a protein. The result, a glycoprotein, can look and behave very differently from the bare chain it started as.
Unlike making the protein itself, glycosylation is not coded letter-by-letter in a gene; instead a relay of enzymes builds and trims the sugar trees as the protein travels through the endoplasmic reticulum and the Golgi apparatus. There are two main flavors: in N-linked glycosylation a pre-assembled sugar block is attached to the nitrogen of an asparagine side chain, then sculpted; in O-linked glycosylation sugars are added one at a time to the oxygen of a serine or threonine. Because so many enzymes can act in so many combinations, the same protein can carry a whole population of different sugar patterns — a diversity far beyond what the underlying gene specifies. These sugars help the protein fold correctly, protect it from being chewed up, hold water around it, and serve as recognition labels that other cells and molecules read.
Glycosylation matters everywhere proteins meet the outside world: your blood type is determined by which sugars decorate red-blood-cell proteins, many hormones and antibodies must be glycosylated to work, and viruses like HIV and the coronavirus hide behind dense sugar coats that shield them from the immune system. It is also a practical headache and opportunity in medicine — therapeutic proteins must be glycosylated correctly to be safe and effective. The caveat to keep straight: glycosylation is added after translation by enzymes in a pathway, so it varies with cell type and conditions and is not simply readable from the DNA sequence.
The ABO blood groups come down to glycosylation: people with type A make an enzyme that adds one particular sugar to a protein on red blood cells, type B add a different sugar, type AB add both, and type O add neither. A mismatched transfusion triggers an immune attack on those foreign sugars.
Your blood type is a glycosylation pattern.
Glycosylation is not directly encoded in the gene. The same protein from different cells or conditions can carry different sugar patterns, so its glycosylation cannot be simply read off the DNA sequence.