(A) The structure of the SorT homodimer. Molecule A in blue/yellow; molecule B in gray (with transparent surface). For molecule A, the ‘SUOX-fold domain’ and ‘dimerization domain’ are represented in blue and yellow, respectively. The molybdopterin cofactor is shown as sticks within the SUOX-fold domain. The corresponding domains of the opposing protomer (shown in molecular surface representation), which constitute the dimer interface are colored to highlight the ‘head-to-tail’ dimer arrangement. INSET: a closer view of the molybdenum binding-site: the molybdenum atom (green sphere) is coordinated by two dithioline ligands from the molybdopterin (yellow spheres), residue Cys 127, an axial oxo ligand and an equatorial hydroxo or water ligand (red spheres). (B) The structure of SorU. The main three helices are labeled and the heme cofactor is shown in red. INSET: the heme binding site with the heme cofactor, coordinating residues, covalent links to Cys 50 and 57 and hydrophobic residues lining binding site: Phe 39, Val 62, Val 76, Val 80, Val 101 highlighted.