Proteoglycan

Types

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

Certain members are considered members of the "small leucine-rich proteoglycan family" (SLRP). These 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. 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 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 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 genetic disorders, called mucopolysaccharidoses. The inactivity of specific lysosomal 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:

References

References

1. {{MeshName. Proteoglycans
2. (2006). "Principles of medical biochemistry". Elsevier Health Sciences.
3. (March 2015). "Proteoglycan form and function: A comprehensive nomenclature of proteoglycans.". Matrix Biology.
4. (February 2002). "Glycosciences: Status and Perspectives". John Wiley and Sons.
5. (2016). "Fundamentals of Biochemistry: Life at the Molecular Level". John Wiley & Sons.
6. (2017). "Syndecan-1 is a novel molecular marker for triple negative inflammatory breast cancer and modulates the cancer stem cell phenotype via the IL-6/STAT3, Notch and EGFR signaling pathways.". Molecular Cancer.
7. (2013). "Syndecan-1 (CD138) modulates triple-negative breast cancer stem cell properties via regulation of LRP-6 and IL-6-mediated STAT3 signaling.". PLOS ONE.
8. (2006). "Defective glycosylation of decorin and biglycan, altered collagen structure, and abnormal phenotype of the skin fibroblasts of an Ehlers-Danlos syndrome patient carrying the novel Arg270Cys substitution in galactosyltransferase I (beta4GalT-7).". Journal of Molecular Medicine.
9. "Nomenclature of glycoproteins, glycopeptides and peptidoglycans, Recommendations 1985".