Location of mutations in the URO-D structure.

(A) View of a URO-D monomer approximately along the axis of the β-barrel, looking directly at the active site cleft. Secondary structural elements are labeled. β-Strands (cyan) forming the core of the barrel are labeled S1 to S8. Corresponding helices (purple) are labeled H1 to H8. Accessory helices are labeled HA to HJ (pink), and the accessory β-strands (cyan) are labeled SA and SB. The loop that became disordered in mutant structures (residues 56-62) between HB and H1 is shown in red. (B) Electron density (1ς 2F_o-F_c) covering the refined structure of Gly156Asp URO-D. The Gly156Asp structure is shown in yellow, and the wild-type URO-D structure is shown in green. The Asp side chain at position 156 causes the movement in nearby residues, 85 to 87, toward the active site cleft (right side of figure). Two water molecules (as depicted by the red difference density −3ς F_o-F_c) fill the gap left behind and form hydrogen bonds to the Asp. (C) Stereo diagram of the structure of a URO-D monomer, from blue to green to yellow, along the amino acid sequence. The disordered loop is shown as a thin blue line with spheres on the Cα positions. Numbers indicate the positions of mutant residues. Amino acids that were identified as mutant in this study are depicted as yellow ball-and-stick, and they are depicted as pink spheres for previously reported mutations.