Chemical structure of heparin.

Heparin/Heparan Sulfate

Brief summary

Heparan sulfate (HS) is a component of the extracellular matrix^1,2. It is a glycosaminoglycan that is covalently attached to core proteins to form proteoglycans. Its structure is amazingly diverse since during synthesis it undergoes extensive sulfation and epimerization¹. Heparin is distinct from HS in that it is produced primarily by mast cells, whereas, HS is produced by all cell types¹. Note that there is not a single HS or heparin structure. Instead, the basic polysaccharide components that make up these molecules vary depending upon the tissue and the developmental stage.

Nomenclature

Heparin ≠ Heparan sulfate
Heparan sulfate = HS
Heparan sulfate proteoglycan = HSPG

Heparin vs. heparan sulfate

Heparin is only produced by mast cells. It functions as an anticoagulant. HS is made by almost all cell types and also has anticoagulant activity, but it is much lower than that of heparin. HS further varies from heparin in the degree of modification of the sugar residues¹. Table 1 summarizes the differences between the two molecules.

Table 1. Differences between heparan sulfate and heparin¹

Chemical structure

Heparin and HS are glycosaminoglycans (GAGs), which are linear polysaccharides composed of two basic saccharides: an amino sugar and an uronic acid^2,3,4. The amino sugar is typically either N-acetyl-D-glucosamine (D-GlcNAc) or N-acetyl-D-galactosamine (D-GalNAc). The uronic acid is either D-glucuronic acid (D-GlcA) or L-iduronic acid (L-IdoA)³. These basic components are further varied by epimerization, sulfation, and deacetylation. Elongation and modification of the polysaccharides are thought to occur simultaneously during biosynthesis¹. Heparin and HS are based on a polysaccharide unit of¹:

1      GlcNAc (α1-4)
2      GlcA (β1-4)
3      GlcNAc (α1-4)
4      GlcA (β1-4)
5      GlcNAc-N-sulfate-6-sulfate α1-4
6      GlcA (β1-4)
7      GlcNAc-N-sulfate-6-sulfate α1-4
8      IdoA-2-sulfate (α1-4)
9      GlcNAc-N-sulfate (α1-4)
10    GlcA (β1-4)

Where:

GlcNAc = N-acetylglucosamine
lcA = Glucuronic acid
IdoA = Iduronic acid

They are typically covalently attached to proteins to form proteoglycans^2,4.

Synthesis and chemical diversity

The heparin disaccharide is synthesized by α-GlcNAc Transferase II and β-GlcA Transferase II. The sugar moities are then modified by at least four sulfotransferases (N-Sulfotransferase, 2-O-Sulfotransferase, 3-O-Sulfotransferase and 6-O-Sulfotransferase) and one epimerase, GlcA-C5-Epimerase. 3’-phosphoadenyl-5’-phosphosulfate (PAPS) donates the high energy sulfate groups that are covalently attached to the HS. These enzymes do not consistently modify the HS chain. Instead they act through regions so that the resulting HS polymers have areas with high degrees of sulfation and other areas with none. This results in HS chains with a staggering degree of heterogeneity. Furthermore, the patterns of epimerization and sulfation set the ligand binding sites thus changing the functionality of the chains¹.

Heparan sulfate proteoglycans

A proteoglycan (PG) is composed of a core protein with one or more covalently attached GAGs. PGs are stored in secretory granules, inserted into the plasma membrane or secreted into the ECM¹. Table 1 lists several well characterized HS proteoglycans (HSPG) along with their molecular weight and tissue localization.

Table 1.  Heparan sulfate proteoglycans present in the ECM¹

Binding partners

Heparin/HS have several well characterized binding proteins (see Table 3). Of these, one of the most extensively studied is the interaction between heparin and FGF2 (bFGF). A crystal structure of this interaction has been solved¹. The heparan sulfate proteoglycan (HSPG) perlecan sequesters and regulates the release of FGF, VEGF and other cytokines involved in neovascularization⁴.

Table 3. Heparin and heparan sulfate binding proteins¹

Heparin in Extracel-HP/HPG

The heparin used in Extracel-HP and Extracel-HPG is derived from porcine intestinal mucosa. Its structure is the disaccharide units consisting of 1,4-α L-iduronic acid and D-glucosamine. The iduronic acid residues are O-sulfated at position 2, and the glucosamine residues are N-sulfated and O-sulfated at position 6. The repeating block can be interrupted or extended by residues of α-D-glucuronic acid and 6-O-sulfated N-acetyl-a-D-glucosamine. Heparin is a mixture of polyanion chains in a relatively wide range of molecular weights, although most chains fall in the range of 17,000-19,000 Da (Sigma Aldrich). The heparin is thiol-modified so that it chemically crosslinks with the Extralink and is thus covalently bound within the hydrogel.

References

1. Essentials of Glycobiology. A. Varti, R. cummings, J. Esko, H. Freeze, G. Hart, J. Marth (editors) Cold Spring Harbor Laboratory Press (1999)
2. Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. B. Alberts, D. Bray, A. Johnson, J. Lewis, M. Raff, K. Roberts, P. Walter (editor) Garland Publishing, Inc.
3. K.R. Taylor, R.L. Gallo, “Glycosaminoglycans and their proteoglycans: host-associated molecular patterns for initiation and modulation of inflammantion,” FASEB Journal 20:9-22 (2006).
4. J.M. Rhodes, M. Simons, “The extracellular matrix and blood vessel formation: not just a scaffold,” J. Cell. Mol. Med. 11(2): 176-205 (2007)