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Not to be confused with peptidoglycan, glycoprotein, or glycopeptide. Proteoglycan_sentence_0

Proteoglycans are proteins that are heavily glycosylated. Proteoglycan_sentence_1

The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). Proteoglycan_sentence_2

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). Proteoglycan_sentence_3

The Ser residue is generally in the sequence -Ser-Gly-X-Gly- (where X can be any amino acid residue but proline), although not every protein with this sequence has an attached glycosaminoglycan. Proteoglycan_sentence_4

The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions due to the occurrence of sulfate and uronic acid groups. Proteoglycan_sentence_5

Proteoglycans occur in connective tissue. Proteoglycan_sentence_6

Types Proteoglycan_section_0

Proteoglycans are categorized by their relative size (large and small) and the nature of their glycosaminoglycan chains. Proteoglycan_sentence_7

Types include: Proteoglycan_sentence_8


TypeProteoglycan_header_cell_0_0_0 Glycosaminoglycans (GAGs)Proteoglycan_header_cell_0_0_1 Small proteoglycansProteoglycan_header_cell_0_0_2 Large proteoglycansProteoglycan_header_cell_0_0_3
Proteoglycan_cell_0_1_0 chondroitin sulfate/dermatan sulfateProteoglycan_cell_0_1_1 decorin, 36 kDa

biglycan, 38 kDaProteoglycan_cell_0_1_2

aggrecan, 220 kDa, the major proteoglycan in cartilageProteoglycan_cell_0_1_3
Heparan sulfate proteoglycan


heparan sulfate/chondroitin sulfateProteoglycan_cell_0_2_1 testican, 44 kDaProteoglycan_cell_0_2_2 perlecan, 400–470 kDa

betaglycan, >300 kDaProteoglycan_cell_0_2_3

Chondroitin sulfate proteoglycan


chondroitin sulfateProteoglycan_cell_0_3_1 bikunin, 25 kDaProteoglycan_cell_0_3_2 neurocan, 136 kDa

versican, 260–370 kDa, present in many adult tissues including blood vessels and skin

brevican, 145kDaProteoglycan_cell_0_3_3

Keratan sulfate proteoglycanProteoglycan_cell_0_4_0 keratan sulfateProteoglycan_cell_0_4_1 fibromodulin, 42 kDa

lumican, 38 kDaProteoglycan_cell_0_4_2


Certain members are considered members of the "small leucine-rich proteoglycan family" (SLRP). Proteoglycan_sentence_9

These include decorin, biglycan, fibromodulin and lumican. Proteoglycan_sentence_10

Function Proteoglycan_section_1

Proteoglycans are a major component of the animal extracellular matrix, the "filler" substance existing between cells in an organism. Proteoglycan_sentence_11

Here they form large complexes, both to other proteoglycans, to hyaluronan, and to fibrous matrix proteins, such as collagen. Proteoglycan_sentence_12

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). Proteoglycan_sentence_13

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. Proteoglycan_sentence_14

Evidence also shows they can affect the activity and stability of proteins and signalling molecules within the matrix. Proteoglycan_sentence_15

Individual functions of proteoglycans can be attributed to either the protein core or the attached GAG chain. Proteoglycan_sentence_16

They can also serve as lubricants, by creating a hydrating gel that helps withstand high pressure. Proteoglycan_sentence_17

Synthesis Proteoglycan_section_2

The protein component of proteoglycans is synthesized by ribosomes and translocated into the lumen of the rough endoplasmic reticulum. Proteoglycan_sentence_18

Glycosylation of the proteoglycan occurs in the Golgi apparatus in multiple enzymatic steps. Proteoglycan_sentence_19

First, a special link tetrasaccharide is attached to a serine side chain on the core protein to serve as a primer for polysaccharide growth. Proteoglycan_sentence_20

Then sugars are added one at a time by glycosyl transferase. Proteoglycan_sentence_21

The completed proteoglycan is then exported in secretory vesicles to the extracellular matrix of the tissue. Proteoglycan_sentence_22

Clinical significance Proteoglycan_section_3

An inability to break down the proteoglycans is characteristic of a group of genetic disorders, called mucopolysaccharidoses. Proteoglycan_sentence_23

The inactivity of specific lysosomal enzymes that normally degrade glycosaminoglycans leads to the accumulation of proteoglycans within cells. Proteoglycan_sentence_24

This leads to a variety of disease symptoms, depending upon the type of proteoglycan that is not degraded. Proteoglycan_sentence_25

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. Proteoglycan_sentence_26

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