The structural differences between the repeating units of heparin (a) and heparan sulfate (b) allow them to be involved in different physiological processes.  Key differences include:

- a sulfo group in heparin and an acetyl group in heparan sulfate at the N-position of the glucosamine
- sulfation at different positions
- epimerization (i.e. spatial rearrangement to chiral analog) of the C-5 uronic acid

Michigan State University

Heparin and heparan sulfate are structurally similar glycosaminoglycans that are physiologically relevant and have been used as pharmacological targets, especially in regards to cancer.  Heparin interacts with the protein antithrombin III (ATIII) and causes a conformation change that increases its ability to inhibit coagulation factors.  Consequently, heparin is used as an anticoagulant.  Heparan sulfate is involved in many processes.  It is notably involved in cell-to-cell extracellular membrane organization - the process in which cells adhere together - and cell proliferation via growth factor signaling.  These roles make it and its related enzymes key targets for drugs meant to prevent cancer metastasis.  

Heparin and Heparan Sulfate

Shriver, Z., Capila, I., Venkataraman, G., & Sasisekharan, R. (2012). Heparin and heparan sulfate: analyzing structure and microheterogeneity. In Heparin-A Century of Progress (pp. 159-176). Springer, Berlin, Heidelberg.
https://link.springer.com/chapter/10.1007/978-3-642-23056-1_8
doi: 10.1007/978-3-642-23056-1_8

Heparin and Heparan Sulfate: Analyzing Structure and Microheterogeneity

Anticoagulants like heparin may improve outcomes in COVID-19 patients. Many patients exhibit unusual clotting patterns that may be alleviated by use of anticoagulants.  Heparin has been shown to directly bind to the S1 subunit of SARS-CoV-2 S protein which may interfere with its ability to infect cells.

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