CoachSugars are attached covalently to proteins and lipids to form glycoconjugates. A glycoprotein carries short branched chains (a dozen sugars) on some of its asparagine, serine or threonine residues, added in the ER and Golgi (Chapter 6): nearly every secreted and membrane protein is one. A proteoglycan is a protein core bearing long unbranched chains of repeating acidic disaccharides, the glycosaminoglycans (hyaluronan, chondroitin sulfate, heparin), which bind enormous amounts of water and give cartilage, the vitreous body and the extracellular matrix their resilience. Glycolipids carry sugars on a lipid tail in the outer leaflet of the plasma membrane. Together with the sugars of glycoproteins they form the glycocalyx, the sugar coat by which cells are recognised.
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Example 10.12 (Blood groups)
The ABO blood groups are three versions of one sugar chain on the red cell’s surface glycolipids and glycoproteins: the O chain ends in fucose; A adds an N-acetylgalactosamine to it, B adds a galactose, by two versions of one enzyme; AB cells carry both. A single sugar residue is enough for the immune system to distinguish self from foreign, and for a transfusion to succeed or kill.