CONFORMATIONAL CONVERSION OF ANTITHROMBIN TO A FULLY ACTIVATED SUBSTRATE OF FACTOR XA WITHOUT NEED FOR HEPARIN

Regulation of the inhibitory activity of antithrombin, the principal i nhibitor of the blood-clotting proteinases factor Xa and thrombin, is accomplished by binding to heparin. We report here an antithrombin var iant in which serine at position 380, 14 residues N-terminal from the reactive bond and at a hinge point in the structure, was replaced by c ysteine to test a proposed mechanism of heparin activation of antithro mbin as an inhibitor of factor Xa. By derivatization of this cysteine with a bulky group, fluorescein, the antithrombin became permanently a nd fully activated toward reaction with factor Xa in a manner analogou s to heparin activation, albeit as a substrate. These findings establi sh a structural basis for the mechanism of heparin activation of antit hrombin against factor Xa in agreement with that proposed from an X-ra y structure of antithrombin.