Structure of thrombin showing location of mutated ionic cluster. Ribbon structure for thrombin is shown as produced by the RasMol modeling program with coordinates obtained from the Brookhaven database. Side chains are shown at the bottom left for residues Arg^517(187) marked by the arrow head, Asp^552(221), and Asp^554(222). In this view the active site serine residue is located at the center, anion binding exosite I extends across the center right, and anion binding exosite II is along the upper left edge.