Proteoglycan

Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to which the glycosaminoglycan is joined through a tetrasaccharide bridge (e.g. chondroitin sulfate-GlcA-Gal-Gal-Xyl-PROTEIN). The Ser residue is generally in the sequence -Ser-Gly-X-Gly- (where X can be any amino acid residue, but proline is most common). The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions, due to the occurrence of sulfate and uronic acid groups. Proteoglycans occur in the connective tissue.

Structure

Proteoglycans are categorized by their relative size (large and small) and the nature of their glycosaminoglycan chains. Types include:

• Chondroitin sulfate proteoglycans (e.g. aggrecan, versican, neurocan, phosphacan)
• Dermatan sulfate proteoglycans (e.g. decorin, biglycan)
• Heparan sulfate proteoglycans (e.g. perlecan, syndecans, glypicans)
• Keratan sulfate proteoglycans (e.g. lumican, keratocan, mimecan, PRELP, osteoadherin)

Function

Proteoglycans play a key role in organizing the extracellular matrix and they also have important roles 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 and serve as lubricants.

Clinical significance

Defects in proteoglycan structure or metabolism can lead to a wide variety of disorders, including multiple types of cancer, fibrosis, osteoarthritis, and Atherosclerosis.

See also

• Glycosaminoglycan
• Extracellular matrix
• Hyaluronan
• Mucopolysaccharidosis

References

Proteoglycan Resources
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