Xylose is the first saccharide added to the serine or threonine in the proteoglycan type O-glycosylation, and, so, it is the first saccharide in biosynthetic pathways of most anionic polysaccharides such as heparan sulfate and chondroitin sulfate.

• Buskas, Therese; Ingale, Sampat; Boons, Geert-Jan (2006), “Glycopeptides as versatile tool for glycobiology”, Glycobiology, 16 (8): 113R–36R, doi:10.1093/glycob/cwj125, PMID 16675547

Definitions

Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a “core protein” with one or more covalently attached glycosaminoglycan (GAG) chain(s). 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). 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. The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions due to the occurrence of sulfate and uronic acid groups. Proteoglycans occur in connective tissue.

• Proteoglycans at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
• Gerhard Meisenberg; William H. Simmons (2006). Principles of medical biochemistry. Elsevier Health Sciences. pp. 243–. ISBN 978-0-323-02942-1. Retrieved 6 February 2011.

Distinction between proteoglycans and glycoproteins

Quoting from recommendations for IUPAC:

A glycoprotein 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.

“Nomenclature of glycoproteins, glycopeptides and peptidoglycans, Recommendations 1985”. www.qmul.ac.uk.

O-linked glycosylation is the attachment of a sugar molecule to the oxygen atom of serine (Ser) or threonine (Thr) residues in a protein. O-glycosylation is a post-translational modification that occurs after the protein has been synthesised. In eukaryotes, it occurs in the endoplasmic reticulum, Golgi apparatus and occasionally in the cytoplasm; in prokaryotes, it occurs in the cytoplasm.

• Van den Steen P, Rudd PM, Dwek RA, Opdenakker G (1998). “Concepts and principles of O-linked glycosylation”. Critical Reviews in Biochemistry and Molecular Biology. 33 (3): 151–208. doi:10.1080/10409239891204198. PMID 9673446.

Several different sugars can be added to the serine or threonine, and they affect the protein in different ways by changing protein stability and regulating protein activity. O-glycans, which are the sugars added to the serine or threonine, have numerous functions throughout the body, including trafficking of cells in the immune system, allowing recognition of foreign material, controlling cell metabolism and providing cartilage and tendon flexibility.

• Hounsell EF, Davies MJ, Renouf DV (February 1996). “O-linked protein glycosylation structure and function”. Glycoconjugate Journal. 13 (1): 19–26. doi:10.1007/bf01049675. PMID 8785483. S2CID 31369853

Because of the many functions they have, changes in O-glycosylation are important in many diseases including cancer, diabetes and Alzheimer’s. O-glycosylation occurs in all domains of life, including eukaryotes, archaea and a number of pathogenic bacteria including Burkholderia cenocepacia, Neisseria gonorrhoeae and Acinetobacter baumannii.

• Lithgow KV, Scott NE, Iwashkiw JA, Thomson EL, Foster LJ, Feldman MF, Dennis JJ (April 2014). “A general protein O-glycosylation system within the Burkholderia cepacia complex is involved in motility and virulence”. Molecular Microbiology. 92 (1): 116–37. doi:10.1111/mmi.12540. PMID 24673753. S2CID 25666819.
• Vik A, Aas FE, Anonsen JH, Bilsborough S, Schneider A, Egge-Jacobsen W, Koomey M (March 2009). “Broad spectrum O-linked protein glycosylation in the human pathogen Neisseria gonorrhoeae”. Proceedings of the National Academy of Sciences of the United States of America. 106 (11): 4447–52. Bibcode:2009PNAS..106.4447V. doi:10.1073/pnas.0809504106. PMC 2648892. PMID 19251655.
• Iwashkiw JA, Seper A, Weber BS, Scott NE, Vinogradov E, Stratilo C, et al. (2012). “Identification of a general O-linked protein glycosylation system in Acinetobacter baumannii and its role in virulence and biofilm formation”. PLOS Pathogens. 8 (6): e1002758. doi:10.1371/journal.ppat.1002758. PMC 3369928. PMID 22685409.