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Proteoglycans represent a special class of glycoproteins that are heavily glycosylated. They consist of a core protein with one or more covalently attached glycosaminoglycan chain(s). These glycosaminoglycan (GAG) chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions, due to the occurrence of sulphate and uronic acid groups.
Proteoglycans can be categorised depending upon the nature of their glycosaminoglycan chains. These chains may be:
Proteoglycans can also be categorised by size. Examples of large proteoglycans are aggrecan, the major proteoglycan in cartilage, and versican, present in many adult tissues including blood vessels and skin. The small leucine rich repeat proteoglycans (SLRPs) include decorin, biglycan, fibromodulin and lumican.
Proteoglycans are a major component of the animal extracellular matrix, the "filler" substance existing between cells in an organism. Here they form large complexes, both to other proteoglycans, to hyaluronan and to fibrous matrix proteins (such as collagen). They are also involved in binding cations (such as sodium, potassium and calcium) and water, and also regulating the movement of molecules through the matrix. Evidence also shows they can affect the activity and stability of proteins and signalling molecules within the matrix. Individual functions of proteoglycans can be attributed to either the protein core or the attached GAG chain.
The protein component of proteoglycans is synthesized by ribosomes and translocated into the lumen of the rough endoplasmic reticulum. Glycosylation of the proteoglycan occurs in the Golgi apparatus in multiple enzymatic steps. First a special link tetrasaccharide is attached to a serine side chain on the core protein to serve as a primer for polysaccharide growth. Then sugars are added one at the time by glycosyl transferase. The completed proteoglycan is then exported in secretory vesicles to the extracellular matrix of the cell.
Proteoglycans and disease
An inability to break down proteoglycans is characteristic of a group of genetic disorders, called mucopolysaccharidoses. The inactivity of specific lysozomal enzymes that normally degrade glycosaminoglycans leads to the accumulation of proteoglycans within cells. This leads to a variety of disease symptoms, depending upon the type of proteoglycan that is not degraded.
- Functional and Molecular Glycobiology 2002. Brooks SA, Dwek, MV, Schumacher, U. Bios Scientific Publishers.
- Molecular Biology of the Cell (3rd Edition). Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson JD. Garland Publishing