Crystal structure of human ADO.A, the structure is composed of three α-helices, ten β-strands, and eight long connecting loops (L1–L8). Residues 22–40 and 230–239 are disordered. The left and right panels show the front and back ends of the β-barrel, respectively. The rainbow color begins with blue at the N-terminus to red at the C-terminus. Loop6 (β7-β8) and Loop8 (β9-β10) and extended β10 are proposed substrate recognizing regions. B, the nickel ion in this structure is hexacoordinated by a 3-His facial triad and three water molecules. The 2Fo– Fc map is contoured at 1 σ, colored in gray. The metal–ligand distances (Å) are shown in parentheses.