The modularity of the second metal binding site in OxsA permits multifunctionality. The modularity of the second metal binding site in OxsA permits multifunctionality. (A) An overlay of the OxsAOXT-PPP (dark teal), OxsAOXT-PP (aquamarine), and OxsAOXT-P (light cyan) active sites reveal that the second metal site and the sidechain of the bridging Asp132 clash with the α–phosphate group of the bound OXT-P molecule. (B) Loss of the second metal binding site could be facilitated by the solvent accessibility of the site. Jennifer Bridwell-Rabb et al. PNAS 2016;113:48:13750-13755 ©2016 by National Academy of Sciences