The structure of zymogen FXI. Topology diagram of dimeric FXI viewed from 2 perspectives rotated 90 degrees. The catalytic domain is in white. Sites of residues implicated in ligand binding are red for thrombin (T), green for HK, black for GPIb, blue for heparin sites (H1 and H2), and orange for FIX. Positions for the activation loop (AL) cleavage site (Arg360-Ile370) residue Ile370 and active site (AS) residues Ser557, Asp462, and His413 are shown in purple.