Proteoglycans areproteins[1] that are heavilyglycosylated. The basic proteoglycan unit consists of a "coreprotein" with one or morecovalently attachedglycosaminoglycan (GAG) chain(s).[2] The point of attachment is aserine (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 butproline), although not every protein with this sequence has an attached glycosaminoglycan. The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions due to the occurrence ofsulfate anduronic acid groups. Proteoglycans occur inconnective tissue.
Proteoglycans are categorized by their relative size (large and small) and the nature of their glycosaminoglycan chains.[3] Types include:
| Type | Glycosaminoglycans (GAGs) | Small proteoglycans | Large proteoglycans |
|---|---|---|---|
| chondroitin sulfate/dermatan sulfate | decorin, 36 kDa biglycan, 38 kDa | aggrecan, 220 kDa, the major proteoglycan incartilage | |
| 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 |
| Keratan sulfate proteoglycan | keratan sulfate | fibromodulin, 42 kDa lumican, 38 kDa |
Certain members are considered members of the "small leucine-rich proteoglycan family" (SLRP).[4] These includedecorin,biglycan,fibromodulin andlumican.
Proteoglycans are a major component of the animalextracellular matrix, the "filler" substance existing betweencells in an organism. Here they form large complexes, both to other proteoglycans, tohyaluronan, and to fibrous matrix proteins, such ascollagen. The combination of proteoglycans and collagen formcartilage, a sturdy tissue that is usually heavily hydrated (mostly due to the negatively charged sulfates in the glycosaminoglycan chains of the proteoglycans).[5] They are also involved in bindingcations (such assodium,potassium andcalcium) andwater, 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.[6][7] 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.
The protein component of proteoglycans is synthesized byribosomes andtranslocated into the lumen of therough endoplasmic reticulum. Glycosylation of the proteoglycan occurs in theGolgi apparatus in multipleenzymatic steps. First, a special link tetrasaccharide is attached to aserine 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 secretoryvesicles to the extracellular matrix of the tissue.
An inability to break down the proteoglycans is characteristic of a group ofgenetic disorders, calledmucopolysaccharidoses. The inactivity of specificlysosomal 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 galactosyltransferaseB4GALT7 result in a reduced substitution of theproteoglycansdecorin andbiglycan withglycosaminoglycan chains, and cause a spondylodysplastic form ofEhlers–Danlos syndrome.[8]
Quoting from recommendations forIUPAC:[9]
Aglycoprotein is a compound containing carbohydrate (or glycan) covalently linked to protein. The carbohydrate may be in the form of a monosaccharide, disaccharide(s), oligosaccharide(s), polysaccharide(s), or their derivatives (e.g. sulfo- or phospho-substituted). One, a few, or many carbohydrate units may be present.Proteoglycans are a subclass of glycoproteins in which the carbohydrate units are polysaccharides that contain amino sugars. Such polysaccharides are also known as glycosaminoglycans.
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