Stereo views of the crystal structures of two separate encounter complexes between PN1 and thrombin. (A) Complex 1 is shown with PN1 colored as described in Figure 1 and thrombin colored magenta. The P1 arginine and P1′ serine side chains are shown as sticks, and the heparin-binding residues of thrombin's exosite II are depicted as blue balls. In complex 1 the heparin-binding sites of PN1 and thrombin are in close proximity and aligned. (B) Complex 2 is shown, colored as in panel A. Thrombin has rotated by 130° so that its heparin-binding site is now oriented toward the back, away from the heparin-binding helix of PN1 (cyan).