Fig. 2: Crystal structure of MefH. Global view of the MefH crystal structure.

MefH is a dimer of two heterotrimers. Green spheres depict the Ni²⁺ ions in the active site of the MefHα subunits. A The heterotrimer is formed by the active MefHα subunit (red) and two inactive MefHβ subunits (blue). B The MefHα subunits interact tightly whereas between the loosely interacting MefHβ subunits a cleft is formed. C Overlay of a MefHα (red) and a MefHβ (purple) subunit with bound Ni²⁺ (green spheres) and urea (beige). D Close-up of the active site of MefHα with the side chains of the nickel-coordinating amino acids and the corresponding amino acids of the degenerated binding site of MefHβ, where no metal was bound. For visual clarity, biuret bound at the N-termini of the MefHα subunits is not displayed. Ligand binding is further illustrated in Supplementary Figs. 10, 11. Model coordinates and experimental diffraction data have been deposited with the Protein Data Bank under accession code 8RYI.