Heparin, a highly negatively charged glycosaminoglycan, is used clinically as an anticoagulant. It acts by binding several plasma proteins, including antithrombin III, an inhibitor of blood clotting. The 1:1 binding of heparin to antithrombin III seems to cause a conformational change in the protein that greatly increases its ability to inhibit clotting. What amino acid residues of antithrombin III are likely to interact with heparin

Positively charged amino acids

Explanation:

Heparin is an acid polysaccharide that has a strong negative charge due to the sulfate groups on its glucosamine residues. This negative charge confers to heparin anticoagulant activity. The negatively charged groups in the heparin molecule can interact with positively charged (basic) residues, such as arginine (Arg) and lysine (Lys) of plasma proteins. For example, it has been shown that lysine amino acid residues of antithrombin III may be involved in the binding with heparin.