Ferrochelatase produces . It can also . However, the ability to insert other metal ions is species specific. In this way Bacillus subtilis can insert copper,. In contrast, the human and Saccharomyces cerevisiae prefer cobalt over copper. . A third residue, Tyr in B. subtilis ferrochelatase and is a third ligand via a water molecule. In the structures of the ferrochelatases the Tyr/Met occupies the same position. We also know that the Tyr residue of the is a . By site directed mutagenesis we changed the Tyr to a Met residue and showed that the metal specificity changed so that the modified B. subtilis ferrochelatase preferred cobalt over copper. Two crystal structures are presented. how . The how a in the B. subtilis enzyme.