(a) The AbRibD subunit structure. Secondary-structure elements are coloured as follows: blue β-strands and purple helices for the reductase domain and cyan β-strands and gold helices for the deaminase domain. The Zn2+ ion is indicated by a grey sphere and the phosphate ion by orange (P) and red (O) spheres. (b) The AbRibD dimer. The subunits are coloured green and blue and the active sites are indicated by the phosphate and Zn2+ ions.