Heparin (see here) is a heterogeneous polysaccharide polymer of D-iduronate-2-sulfate linked 1->4 to N-sulfo-D-glucosamine-6-sulfate. This repeating unit of the polymer is shown at the right.

The sulfates in the molecule make it very negatively charged.

Heparin is found in mast cells lining the walls of arteries. Heparin blocks blood clotting and is used clinically for that purpose. In the body, release of heparin plays a role in preventing a clot from becoming dangerously large.