Antithrombin conformation and the catalytic role of heparin. II. Is the heparin-induced conformational change in antithrombin required for rapid inactivation of thrombin?

The role of antithrombin conformation in heparin-catalyzed inhibition of thrombin was investigated using antithrombins modified with the tryptophan reagent dimethyl (2-hydroxy-5-nitrobenzyl) sulfonium bromide (HNB). Affinity fractionation of HNB-labeled antithrombin (0.6-0.7 mol of HNB/mol of protein) on heparin-Sepharose using a linear salt gradient allowed separation of three singly labeled protein species and a fourth HNB-antithrombin species which co-eluted with unlabeled protein. Conformational alterations induced by heparin binding to each of the labeled antithrombins were assessed by spectroscopic techniques, including protein fluorescence, difference spectroscopy in the ultraviolet-visible range, and circular dichroism. Comparison of spectra of the labeled proteins in the presence and absence of added heparin indicated changes to occur in protein conformation at the sites of the bound HNB moieties and at aromatic amino acid residues within the protein matrix. These spectroscopic alterations mimicked changes induced by heparin in the native protein, but were reduced in magnitude. Rates of thrombin inactivation by the labeled antithrombins were measured over a wide range in both heparin concentration and inhibitor concentration to determine maximal rates of protease inactivation. The kinetic analysis indicated that each of these HNB-antithrombin derivatives, which undergo the heparin-induced changes to varying extents, can react with thrombin at the same maximal rate. Thus, this series of chemically modified antithrombin species demonstrated that the conformational change which is induced in antithrombin by heparin does not render the protein intrinsically more reactive toward thrombin.