Antithrombin (AT) inhibits most of the serine proteases generated in the blood coagulation cascade, but its principal targets are factors IXa, Xa, and thrombin. Heparin binding to AT, via a specific pentasaccharide sequence, alters the conformation of AT in a way that promotes efficient inhibition of factors IXa and Xa, but not of thrombin. The conformational change most likely to be relevant to protease recognition is the expulsion of the N-terminal portion of the reactive center loop (hinge region) from the main beta-sheet A. Here we investigate the hypothesis that the exosites on the surface of AT are accessible for interaction with a protease only when the hinge region is fully extended, as seen in the related Michaelis complex between heparin cofactor II and thrombin. We engineered a disulfide bond between residues 222 on strand 3A and 381 in the reactive center loop to prevent the extension of the hinge region upon pentasaccharide binding. The disulfide bond did not significantly alter the ability of the variant to bind to heparin or to inhibit thrombin. Although the basal rate of factor Xa inhibition was not affected, that of factor IXa inhibition was reduced to the limit of detection. In addition, the disulfide bond completely abrogated the pentasaccharide accelerated inhibition of factors Xa and IXa. We conclude that AT hinge region extension is the activating conformational change for inhibition of factors IXa and Xa, and propose models for the progressive and activated AT Michaelis complexes with thrombin, factor Xa, and factor IXa.