The structure of iron superoxide dismutase from Pseudomonas ovalis complexed with the inhibitor azide

The 2.9 A resolution structure of iron superoxide dismutase (FeSOD) (EC 1.15.1.1) from Pseudomonas ovalis complexed with the inhibitor azide was solved. Comparison of this structure with free enzyme shows that the inhibitor is bound at the open coordination position of the iron, with a bond length of 2.0 A. The metal moves by 0.4 A into the trigonal plane to produce an orthogonal geometry at the iron. Binding of the inhibitor also causes a movement of the axial ligand (histidine 26) away from the metal, a lengthening of the iron-histidine bond, and a rotation of the histidine 74 ring. The inhibitor possesses contacts in the binding pocket with a pair of conserved tryptophan residues and with the side chains of tyrosine 34 and glutamine 70. This glutamine is conserved between all FeSODs, but is absent in MnSOD. Comparisons with MnSOD show that a different glutamine which possesses the same interactions in the active site as Gln70 in FeSOD is conserved at position 154 in the overall SOD sequence, implying that while manganese and FeSODs are structural homologues in a global sense, their functional and evolutionary relationship is that of second-site mutation revertants.