Figure 3.

Ribbon diagrams of (from left) I-antithrombin (15), pentasaccharide-complexed I-antithrombin, and α₁-antitrypsin (32). The pentasaccharide activation of I-antithrombin is seen to involve a closing of the A-sheet (magenta), an extension (blue) of helix D (yellow), and an expulsion of residues P₁₄ (green sphere) and P₁₅ (black sphere) of the reactive site loop (red). The reactive loop of both antithrombin molecules is constrained by the dimer contact (see Fig. 2a) of the β-pleated P₃–P₈ (ribboned arrow). An indication of the likely free conformation, with exposure of the P₁ reactive center (shown as a ball–stick model), is provided by the optimal inhibitory conformation of the reactive loop present in α₁-antitrypsin (32).