FIGURE 5.

Active site structures of SDH^R55M and SDH^H57A. A and B show a transparent surface at the substrate binding sites of SDH^WT and SDH^R55M, respectively. The water-filled cavity adjacent to the equatorial oxygen is larger in the mutated enzyme and is occupied by a sulfate ion. C, hydrogen bonding network around the active site of SDH^H57A in a similar view as SDH^WT in Fig. 1D. The two alternative positions for Arg-55 and the heme propionate group are shown. An additional water molecule occupies a site close to the pterin, and fulfills potential hydrogen bond contacts.