Proteoglycans represent a specalist class of glycoprotein that are heavily glycosylated. They consist of a core protein with one or more covalently attached glycosaminoglycan chain(s). These glycosaminoglycan chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions, due to the occurrence of sulphate and uronic acid groups.

Types

Proteoglycans can be categorised depending upon the nature of their glycan chains. These chains may be:

• chondroitin sulfate and dermatan sulfate
• heparin and heparan sulfate
• keratan sulfate

Function

Proteoglycans are a major component of the animal extracellular matrix, the 'filler' subtance exisiting between cells in an organism. Here they form large complexes, both to other proteoglycans and also to fiberous 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.

Synthesis

The protein component of proteoglycans is synthesised 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, with one sugar unit being added with each step. 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 series of genetic disorders, called mucopolysaccharidoses. The inactivity of specific lysozomal enzymes that normally degraded glycosaminoglycans leads to the accumaltion of proteoglycans within cells. This leads to a variety of disease symptoms, depending upon the type of proteoglycan that is not degraded.

Sources

• Functional and Molecular Glycobiology 2002. Brooks SA, Dwek, MV, Schumacher, U. Bios Scientific Publishers.
• Molecular Biology of the Cell (3rd Eddition). Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson JD. Garland Publishing