Structure of factor XI (FXI). A) Cartoon structure of FXI. Filled circles represent amino acids, the catalytic triad is in green and cys-cys bonds in red. A1–4 are the apple domains. (Figure provided by David Gailani and modeled after an image in McMullen et al.43) B) Crystal structure of FXI. (Figures provided by S.L. Perkins, UCL, adapted by permission from Macmillan Publishers Ltd10 copyright 2006.) The FXI monomer is displayed in the “cup and saucer” view. The serine protease domain (SP) is shown at the top with red α-helices and yellow β-strands. The four apple (Ap1–Ap4) domains are shown at the bottom with cyan α-helices and violet β-strands. The artwork was prepared using PYMOL from the Protein Data Base entry code 2F83. C) Crystal structure of the FXI dimer is displayed to show the interaction of the two Ap4 domains with each other. The SP and Ap1–Ap4 domains follow the same coloring as in Figure 1B. The two monomers are shown on either side of the dashed line.
