requires the minimal heparin pentasaccharide sequence. The conformational changes that occur within antithrombin in response to pentasaccharide binding are well documented.

In the absence of heparin, amino acids P14 and P15 (see Figure 3) from the reactive site loop are embedded within the main body of the protein (specifically the top of beta sheet A). This feature is in common with other serpins such as heparin cofactor II,