<p>Proteoglycans are macromolecules consisting of a core protein to which one or more glycosaminoglycan (GAG) chains are covalently attached. The core protein can be either a glycerolipid or a sphingolipid. The GAG chains are long, linear carbohydrate polymers with negative charges due to sulfate and uronic acid groups. The point of attachment is typically a serine (Ser) residue, forming a tetrasaccharide bridge. These are found in our bodies, particularly in connective tissues, bone, cartilage, and cell surfaces.</p>