FXI-binding site for HK. (A) The relative positions of Gly104 and Gly155 in the A2 domain are shown. Gly155 is involved in a tight β-turn forming a hydrogen bond with Thr152. The compact hydrogen bond network extends through Thr152 and Thr158 to form contacts to the main chain through residues Lys103 and Ile105. Changes at Gly104 or Gly155 will probably disrupt this network. (B) Charged surface (blue represents positive; red, negative) representation of the underside of the FXI apple domain disk, showing positions of Gly104 and Gly155. These residues are required for proper formation of a charged channel that is a probable binding site for HK. The dotted line represents the predicted binding site of HK that terminates in a pocket at the base of the A2 domain.