Space-filling model of thrombin residues important for recombinant human FVIII activation.

The RasMol V2.5 software package was used to depict thrombin (E.C.3.4.21.5, Brookhaven entry 1PPB) as a space-filling model with solvent removed. Residues substituted with alanine with less than 50% WT human FVIII activation are colored and shown in the left panel, and they are compared with thrombin residues important for the activation of FV in the right panel. The upper thrombin molecule is the classic front view of thrombin,51 with the active-site cleft running horizontally left to right and the 50-insertion loop on the top of the cleft occluding the active site Ser205 (orange). The active site is shown with the bound active-site inhibitor, PPACK, as a brown stick model. The 50-insertion loop residues are cyan, and the Na⁺-binding loop residues are green. ABE-I runs to the right of the active site cleft, and the residues are lavender. The bottom thrombin molecule is rotated 90° to the right and shows ABE-II residues in yellow and the residues Lys196 and Asp193 in magenta. Residues of interest are labeled using the single-letter amino acid code.