The 1.2 Å crystal structure of hirustasin reveals the intrinsic flexibility of a family of highly disulphide-bridged inhibitors

Background: Leech-derived inhibitors have a prominent role in the development of new antithrombotic drugs, because some of them are able to block the blood coagulation cascade. Hirustasin, a serine protease inhibitor from the leech Hirudo medicinalis, binds specifically to tissue kallikrein and possesses structural similarity with antistasin, a potent factor Xa inhibitor from Haementeria officinalis, Although the 2.4 Angstrom structure of the hirustasin-kallivrein complex is known, classical methods such as molecular replacement were not successful in solving the structure of free hirustasin. Results: Ab initio real/reciprocal space iteration has been used to solve the structure of free hirustasin using either 1.4 Angstrom room temperature data or 1.2 Angstrom low temperature diffraction data. The structure was also solved independently from a single pseudo-symmetric gold derivative using maximum likelihood methods, A comparison of the free and complexed structures reveals that binding to kallikrein causes a hinge-bending motion between the two hirustasin subdomains, This movement is accompanied by the isomerisation of a cis proline to the irans conformation and a movement of the P3, P4 and P5 residues so that they can interact with the cognate protease. Conclusions: The inhibitors from this protein family are fairly flexible despite being highly cross-linked by disulphide bridges. This intrinsic flexibility is necessary to adopt a conformation that is recognised by the protease and to achieve an optimal fit, such observations illustrate the pitfalls of designing inhibitors based on static lock-and-key models. This work illustrates the potential of new methods of structure solution that require less or even no prior phase information.