Thermodynamic linkage between the S1 site, the Na+ site, and the Ca2+ sitein the protease domain of human coagulation factor Xa - Studies on catalytic efficiency and inhibitor binding

The serine protease domain of factor Xa (FXa) contains a sodium as well as a calcium-binding site. Here, we investigated the functional significance o f these two cation-binding sites and their thermodynamic links to the S1 si te. Kinetic data reveal that Na+ binds to the substrate bound FXa with K-d similar to 39 mM in the absence and similar to9.5 mM in the presence of Ca2 +. Sodium-bound FXa (sodium-Xa) has similar to 18-fold increased catalytic efficiency (similar to4.5-fold decrease in K-m and similar to4-fold increas e in k(cat)) in hydrolyzing S-2222 (benzoyl-Ile-Glu-Gly-Arg-p-nitroanilide) , and Ca2+ further increases this k(cat) similar to1.4-fold. Ca2+ binds to the protease domain of substrate bound FXa with K-d similar to 705 par in t he absence and similar to 175 muM in the presence of Na+. Ca2+ binding to t he protease domain of FXa (Xa-calcium) has no effect on the K-m but increas es the k(cat) similar to4-fold in hydrolyzing S-2222, and Na+ further incre ases this k(cat) similar to1.4-fold. In agreement with the K-m data, sodium -Xa has similar to5-fold increased affinity in its interaction with p-amino benzamidine (S1 site probe) and similar to4-fold increased rate in binding to the two-domain tissue factor pathway inhibitor; Ca2+ (+/-Na+) has no eff ect on these interactions. Antithrombin binds to Xa-calcium with a similar to4-fold faster rate, to sodium-Xa with a similar to 24-fold faster rate an d to sodium-Xa-calcium with a similar to 28-fold faster rate. Thus, Ca2+ an d Na+ together increase the catalytic efficiency of FXa similar to 28-fold. Na+ enhances Ca2+ binding, and Ca2+ enhances Na+ binding. Further, Na+ enh ances S1 site occupancy, and S1 site occupancy enhances Na+ binding. Theref ore, Na+ site is thermodynamically linked to the S1 site as well as to the protease domain Ca2+ site, whereas Ca2+ site is only linked to the Na+ site . The significance of these findings is that during physiologic coagulation , most of the FXa formed will exist as sodium-Xa-calcium, which has maximum biologic activity.