glycoprotein

Meaning: A glycoprotein is a protein with one or more short, often branched oligosaccharide chains covalently attached. Glycosylation occurs in the rough ER lumen and the Golgi, so glycoproteins are typically secreted, lysosomal, or plasma membrane proteins, with sugars facing the extracellular side. N-linked chains attach to the amide nitrogen of asparagine and begin in the ER. O-linked chains attach to the hydroxyl oxygen of serine or threonine and are built mainly in the Golgi. The sugars aid folding, stability, targeting, and cell-cell recognition. Blood group antigens are an example: their carbohydrate groups sit on glycoproteins and glycolipids.

On the MCAT: Passages ask where glycosylation happens, which residues are modified, or what follows when a receptor or secreted protein lacks its sugars, such as impaired folding, trafficking, or binding. Expect links to the ER → Golgi → vesicle pathway and to lectins, which bind specific carbohydrate groups.

Common confusion: Do not mix up glycoproteins and proteoglycans. A glycoprotein is mostly protein with short, branched sugar chains; a proteoglycan is mostly carbohydrate by mass, with long, unbranched glycosaminoglycan chains on a core protein.