Everything about Proteoglycan totally explained
Proteoglycans represent a special class of
glycoproteins that are heavily
glycosylated. They consist of a core
protein with one or more
covalently attached
glycosaminoglycan (GAG) chain(s). These glycosaminoglycan chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions, due to the occurrence of
sulfate and
uronic acid groups.
Types
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.
Function
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.
Synthesis
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 isn't degraded.
Sources
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 PublishingFurther Information
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