Glycoconjugates: Proteoglycans, Glycoproteins, and Glycosphingolipids:
Polysaccharides and oligosaccharides are information carriers. Some provide
communication between cells and their extracellular surroundings; others label
proteins for transport to and localization in specific organelles, or for destruction
when the protein is malformed or superfluous; and others serve as recognition
sites for extracellular signal molecules or extracellular parasites.
On almost every eukaryotic cell, specific oligosaccharide chains attached to
components of the plasma membrane form a carbohydrate layer (the glycocalyx),
that serves as an information-rich surface that is cell shows to its surroundings.
These oligosaccharides are central players in cell-cell recognition and adhesion,
cell migration during development. Blood clotting, the immune response, wound
healing, and other cellular processes. In most of these cases, the informational
carbohydrate is covalently joined to a protein or a lipid to form a glycoconjugate,
which is the biologically active molecule.
They are macromolecules of the cell surface or ECM in which one or more
sulfated glycosaminoglycan chains are joined covalently to a membrane protein
or a secreted protein. The glycosaminoglycan chain can bind to extracellular
proteins through electrostatic interactions between the protein and negatively
charged sugar moieties on the proteoglycan. Proteoglycans are major
components of all extracellular matrices.
Some proteoglycans can form proteoglycan aggregates, enormous
supramolecular assemblies of many core proteins all bound to a single molecule
of hyaluroan. Aggregan core protein has multiple chains of chondroitin sulfate
and keratan sulfate, joined to Ser residues in the core protein through
trisaccharide linkers. Aggregan interacts strongly with collagen in the ECM of
cartilage, contributing to the development, tensile strength, and resilience of this
Interwoven with these enormous extracellular proteoglycans are fibrous matrix
proteins such as collagen, elastin, and fibronectin, forming a cross-linked
meshwork that gives the whole extracellular matrix strength and resilience.
Proteoglycans Are Glycoaminoglycan-Containing Macromolecules of the
Cell Surface and ECM:
The basic proteoglycan unit consists of a ‘core protein’ with covalently attached
glycosaminoglycan(s). The point of attachment is a Ser residue, to which the
glycosaminoglycan is joined through a tetrasaccharide bridge. Many
proteoglycans are secreted into the ECM, but some