The Structure of a Complex of Recombinant Hirudin and Human α-Thrombin

The crystallographic structure of a recombinant hirudin-thrombin complex has been solved at 2.3 angstrom (Å) resolution. Hirudin consists of an NH 2 -terminal globular domain and a long (39 Å) COOH-terminal extended domain. Residues Ile 1 to Tyr 3 of hirudin form a parallel β-strand with Ser 214 to Glu 217 of thrombin with the nitrogen atom of Ile 1 making a hydrogen bond with Ser 195 Oγ atom of the catalytic site, but the specificity pocket of thrombin is not involved in the interaction. The COOH-terminal segment makes numerous electrostatic interactions with an anion-binding exosite of thrombin, whereas the last five residues are in a helical loop that forms many hydrophobic contacts. In all, 27 of the 65 residues of hirudin have contacts less than 4.0 Å with thrombin (10 ion pairs and 23 hydrogen bonds). Such abundant interactions may account for the high affinity and specificity of hirudin.