CALCIUM ENHANCES HEPARIN CATALYSIS OF THE ANTITHROMBIN FACTOR XA REACTION BY A TEMPLATE MECHANISM - EVIDENCE THAT CALCIUM ALLEVIATES GLA DOMAIN ANTAGONISM OF HEPARIN-BINDING TO FACTOR XA

It is believed that heparin accelerates factor Xa (FXa) inactivation b y antithrombin (AT) by conformationally activating the inhibitor rathe r than by bridging AT and FXa in a ternary complex (template effect). This is derived from kinetic studies done in the absence of Ca2+ or in the presence of EDTA. To test the possibility that the anionic Gla do main of FXa, when not neutralized by Ca2+ ions, prevents heparin bindi ng to FXa, the heparin and pentasaccharide dependence of FXa inactivat ion by AT in both the absence (100 mu M EDTA) and presence of Ca2+ (2. 5 mM) was studied using wild-type FXa and a FXa derivative that lacks the Gla domain (GDFXa). AT inactivated both FXa derivatives similarly in both the absence and presence of Ca2+ (k(2) = 1.7-2.5 x 10(3) m(-1) s(-1)). The active AT-binding pentasaccharide also accelerated the in activation rates of both derivatives similarly in both the absence and presence of Ca2+ (K-2 = 5.7- 8.0 x 10(5) M-1 s(-1)). However, in the presence of an optimum concentration of heparin (similar to 50 nM) the inactivation rate constant of FXa in the presence of Ca2+ (k(2) = 4.4 x 10(7) M-1 s(-1)) was 13-fold higher than the rate constant in the a bsence of Ca2+ (k(2) = 3.5 x 10(6) M-1 s(-1)). Heparin acceleration of GDFXa inactivation by AT was rapid and insensitive to the presence or absence of Ca2+ (k(2) = 5.1-5.9 x 10(7) M-1 s(-1)). The additional co factor effect of heparin with all FXa derivatives was a bell-shaped cu rve, which disappeared if the ionic strength of the reaction was incre ased to similar to 0.4. These results suggest that although the major effect of heparin in acceleration of FXa inactivation is through a hep arin-induced conformational change in the reactive site loop of AT, th e template effect of heparin, nevertheless, contributes significantly to rapid FXa inactivation at physiological Ca2+.