Predominant contribution of surface approximation to the mechanism of heparin acceleration of the antithrombin-thrombin reaction. Elucidation from salt concentration effects.

Heparin has been shown to accelerate the inactivation of alpha-thrombin by antithrombin III (AT) by promoting the initial encounter of proteinase and inhibitor in a ternary thrombin-AT-heparin complex. The aim of the present work was to evaluate the relative contributions of an AT conformational change induced by heparin and of a thrombin-heparin interaction to the promotion by heparin of the thrombin-AT interaction in this ternary complex. This was achieved by comparing the ionic and nonionic contributions to the binary and ternary complex interactions involved in ternary complex assembly at pH 7.4, 25 degrees C, and 0.1-0.35 M NaCl. Equilibrium binding and kinetic studies of the binary complex interactions as a function of salt concentration indicated a similar large ionic component for thrombin-heparin and AT-heparin interactions, but a predominantly nonionic contribution to the thrombin-AT interaction. Stopped-flow kinetic studies of ternary complex formation under conditions where heparin was always saturated with AT demonstrated that the ternary complex was assembled primarily from free thrombin and AT-heparin binary complex at all salt concentrations. Moreover, the ternary complex interaction of thrombin with AT bound to heparin exhibited a substantial ionic component similar to that of the thrombin-heparin binary complex interaction. Comparison of the ionic and nonionic components of thrombin binary and ternary complex interactions indicated that: 1) additive contributions of ionic thrombin-heparin and nonionic thrombin-AT binary complex interactions completely accounted for the binding energy of the thrombin ternary complex interaction, and 2) the heparin-induced AT conformational change made a relatively insignificant contribution to this binding energy. The results thus suggest that heparin promotes the encounter of thrombin and AT primarily by approximating the proteinase and inhibitor on the polysaccharide surface. Evidence was further obtained for alternative modes of thrombin binding to the AT-heparin complex, either with or without the active site of the enzyme complexed with AT. This finding is consistent with the ternary complex encounter of thrombin and AT being mediated by thrombin binding to nonspecific heparin sites, followed by diffusion along the heparin surface to a unique site adjacent to the bound inhibitor.