Proteoglycans are
Types
Proteoglycans are categorized by their relative size (large and small) and the nature of their glycosaminoglycan chains.
Types include:
|
|
| | chondroitin sulfate/dermatan sulfate | decorin, 36 kDa
biglycan, 38 kDa | aggrecan, 220 kDa, the major proteoglycan in cartilage |
Heparan sulfate proteoglycan
(HSPGs) | heparan sulfate/chondroitin sulfate | testican, 44 kDa | perlecan, 400–470 kDa
betaglycan, >300 kDa
agrin, >500 kDa |
Chondroitin sulfate proteoglycan
(CSPGs) | chondroitin sulfate | bikunin, 25 kDa | neurocan, 136 kDa
versican, 260–370 kDa, present in many adult tissues including blood vessels and skin
brevican, 145kDa |
| Keratan sulfate proteoglycan | keratan sulfate | fibromodulin, 42 kDa
lumican, 38 kDa | |
Certain members are considered members of the "small leucine-rich proteoglycan family" (SLRP).
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. The combination of proteoglycans and collagen form
cartilage, a sturdy tissue that is usually heavily hydrated (mostly due to the negatively charged sulfates in the glycosaminoglycan chains of the proteoglycans).
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. They can also serve as lubricants, by creating a hydrating gel that helps withstand high pressure.
Synthesis
The protein component of proteoglycans is synthesized by
and
translocon into the lumen of the rough endoplasmic reticulum. Glycosylation of the proteoglycan occurs in the
Golgi apparatus in multiple
enzyme 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 a time by glycosyl transferase. The completed proteoglycan is then exported in secretory vesicles to the extracellular matrix of the tissue.
Clinical significance
An inability to break down the proteoglycans is characteristic of a group of
, called mucopolysaccharidoses. The inactivity of specific
lysosome 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. Mutations in the gene encoding the galactosyltransferase B4GALT7 result in a reduced substitution of the
proteoglycans decorin and
biglycan with glycosaminoglycan chains, and cause a spondylodysplastic form of Ehlers–Danlos syndrome.
Distinction between proteoglycans and glycoproteins
Quoting from recommendations for
IUPAC:
External links
