The active site of RrQR. a Stereo view of the active site of RrQR and its superimposition with those of TR-II and 3β/17β-hydroxysteroid dehydrogenase. The catalytic residues, serine, asparagine, tyrosine, and lysine, are shown by a stick model. RrQR, TR-II, and 3β/17β-hydroxysteroid dehydrogenase are shown in light blue, pink, and pale yellow, respectively. The residues of RrQR are labeled. b Stereo view of the substrate-binding site of RrQR. A water molecule in the active site is shown as a sphere, and hydrogen bonds are shown by dotted lines. The hydrophobic wall is comprised of I167 and F212.