The structure of the ternary complex of human a-thrombin with a covalently bound analogue of fibrinopeptide A and a C-terminal hirudin peptide has been determined by X-ray diffraction methods at 0.25 nm resolution. Fibrinopeptide A folds in a compact manner, bringing together hydrophobic residues that slot into the apolar binding site of human a-thrombin. Fibrinogen residue Phe8 occupies the aryl-binding site of thrombin, adjacent to fibrinogen residues Leu9 and Val15 in the S2 subsite. The species diversity of fibrinopeptide A is analysed with respect to its conformation and its interaction with thrombin. The non-covalently attached peptide fragment hirudin(54 -65) exhibits an identical conformation to that observed in the hirudin-thrombin complex. The occupancy of the secondary fibrinogen-recognition exosite by this peptide imposes restrictions on the manner of fibrinogen binding. The surface topology of the thrombin molecule indicates positions PI' -P3', differ from those of the canonical serine-proteinase inhibitors, suggesting a mechanical model for the switching of thrombin activity from fibrinogen cleavage to protein-C activation on thrombomodulin complex formation. The multiple interactions between thrombin and fibrinogen provide an explanation for the narrow specificity of thrombin. Structural grounds can be put forward for certain congenital clotting disorders.The specific cleavage of fibrinogen by the serine proteinase thrombin initiates the polymerisation of fibrin monomers, a primary event in blood clot formation [4]. Fibrinogen (340 kDa) is a covalently linked dimer of three peptide chains, with stoichiometry (Aa, BP,y), [5]. The cleavage releases two peptides, fibrinopeptides A and B, from the N-termini of chains Aa and Bfl respectively, thereby revealing recognition sites for aggregation with the y chain.Thrombin exhibits primarily a trypsin-like specificity, i.e. a preference for P1 arginine residues [6]. The cleavage of fibrinogen by thrombin represents a very specific reaction however; of the 376 Arg/Lys-Xaa bonds in the fibrinogen molecule, thrombin cleaves only four, releasing the fibrinopeptides [6]. Residues of fibrinogen contributing to this exceptional specificity have been localised to the first 51 amino acids of the Acc chain [7, 81. This region has been further dissected to explore subsites, assigning roles (in order of im-Enzyrnr. Thrombin (EC 3.4
.21 .S).Nomencluture. The peptide and subsite nomenclature is that suggested by Schechter and Berger 111: amino acid residues of substrates are numbered P1, P2, P3 etc. towards the amino terminus, and P1 ', P2', P3' etc. towards the carboxy terminus from the reactive-sitc bond; the complementary subsites of the enzyme are numbered S1, S2, S3 etc. and SI', S2', S3' etc., respectively. Thrombin residues are numbered according to the chymotrypsinogen system [2], with inserted residues marked by a lower-case suffix [ 3 ] . Fibrinogen residues are prefixed by the letter F, hirudin residues by the letter H and uPhe-Pro-Arg-MeC1 residues by the le...