Site-directed mutagenesis of alanine-382 of human antithrombin III

Structural basis for serpin inhibitor activity

The mechanism of formation and the structures of serpin-inhibitor complexes are not completely understood, despite detailed knowledge of the structures of a number of cleaved and uncleaved inhibitor, noninhibitor, and latent serpins. It has been proposed from comparison of inhibitor and noninhibitor serpins in the cleaved and uncleaved forms that insertion of strand s4A into preexisting beta-sheet A is a requirement for serpin inhibitor activity. We have investigated the role of this strand in formation of serpin-proteinase complexes and in serpin inhibitor activity through homology modeling of wild type inhibitor, mutant substrate, and latent serpins, and of putative serpin-proteinase complexes. These models explain the high stability of the complexes and provide an understanding of substrate behavior in serpins with point mutations in s4A and of latency in plasminogen activator inhibitor I.

Proteolytically cleaved mutant antithrombin-Hamilton has high stability to denaturation characteristic of wild type inhibitor serpins

The serpin family of proteins consists primarily of proteinase inhibitors which form tight complexes with target proteinases. Inhibitor serpins are cleaved by proteinase and undergo a large conformational change in which the polypeptide segment terminating at the target reactive site, at which cleavage takes place, inserts itself as an additional strand, s4A, in the center of a preexisting beta-sheet. This change in conformation increases the stability towards denaturation of the cleaved serpin relative to the native uncleaved form. Mutant serpins with single amino acid changes in the s4A strand have been identified, and in most cases these are proteinase substrates but not inhibitors. We have measured the stability to denaturation of one of these non-inhibitor substrate mutants, antithrombin-Hamilton, which has an Ala-->Thr change at position P12 in strand s4A. We find that it undergoes the transformation to the more stable form which is observed for inhibitor serpins, from which we conclude that the Ala-->Thr change in antithrombin-Hamilton does not prevent insertion of s4A into beta-sheet A in the cleaved form.

Crystal structure of cleaved equine leucocyte elastase inhibitor determined at 1.95 A resolution

The crystal structure of active-site cleaved equine leucocyte elastase inhibitor, a member of the serpin superfamily, has been solved and refined to a crystallographic R-factor of 17.6% at 1.95 A resolution. Despite being an intracellular inhibitor with rather low sequence homology of 30% to human alpha 1-antichymotrypsin and alpha 1-proteinase inhibitor, the three-dimensional structures are very similar, with deviations only at the sites of insertions and few mobile secondary structure elements. The better resolution in comparison with the structures of other cleaved serpins allows a more precise description of the so-called R-state of the serpins.

The N-terminal domain of antithrombin-III is essential for heparin binding and complex-formation with, but not cleavage by, alpha-thrombin

Normal and mutant forms of human antithrombin-III (AT-III) were synthesized in a cell-free system in order to identify putative functional domains required for heparin binding and complex-formation with alpha-thrombin. Heparin-Sepharose chromatography resulted in the elution of approx. 70% of cell-free-derived normal AT-III-(1-432)-polypeptide as a peak between 0.2 M- and 0.7 M-NaCl. The cell-free-derived normal AT-III also reacted with alpha-thrombin. Approx. 15% of this AT-III formed covalent complexes with alpha-thrombin in 2 min. Unfractionated heparin accelerated the rate of formation of such complexes. Two truncated forms of AT-III (amino acid residues 219-432 and 251-432), containing only the putative thrombin-binding domain, were synthesized independently in this cell-free system. These truncated AT-III polypeptides did not bind heparin and were unable to form stable covalent complexes with alpha-thrombin. However, both of these AT-III polypeptides were cleaved by alpha-thrombin, presumably at the reactive centre Arg-393-Ser-394. The formation of the disulphide bond between Cys-247 and Cys-430 in AT-III-(219-432)-polypeptide had no effect on the results obtained. Mutations in full-length AT-III at Cys-430 had no effect on the ability of AT-III to bind heparin. There was, however, a slight decrease in the formation of stable inhibitory complexes with alpha-thrombin. A cell-free-derived AT-III mutant, devoid of amino acid residues 41-49, which comprise heparin-binding region 1 of AT-III, had slightly decreased heparin binding compared with cell-free-derived normal AT-III-(1-432)-polypeptide. This mutant AT-III polypeptide was unable, however, to form a stable complex with alpha-thrombin. We conclude therefore that the N-terminal domain of AT-III is essential for both heparin binding and complex-formation with alpha-thrombin, but not for the cleavage of AT-III at its reactive centre by alpha-thrombin.

Mobile reactive centre of serpins and the control of thrombosis

Two protease inhibitors in human plasma play a key part in the control of thrombosis: antithrombin inhibits coagulation and the plasminogen activator inhibitor PAI-1 inhibits fibrinolysis, the dissolving of clots. Both inhibitors are members of the serpin family and both exist in the plasma in latent or inactive forms. We show here that the reactive centre of the serpins can adopt varying conformations and that mobility of the reactive centre is necessary for the function of antithrombin and its binding and activation by heparin; the identification of a new locked conformation explains the latent inactive state of PAI-1. This ability to vary conformation not only allows the modulation of inhibitory activity but also protects the circulating inhibitor against proteolytic attack. Together these findings explain the retention by the serpins of a large and unconstrained reactive centre as compared to the small fixed peptide loop of other families of serine protease inhibitors.