X-ray structure of the A. vinelandii nitrogenase in complex with ADP•AlF₄^–. The enzyme, which is initially viewed along its molecular 2-fold axis, is an (αβγ₂)₂ heterooctamer in which the β–α–α–β assembly, the MoFe-protein, is flanked by two γ₂ Fe-proteins whose 289-residue subunits are related by local 2-fold symmetry. The homologous α subunits (cyan and red; 491 residues) and β subunits (brown and light blue; 522 residues) are related by pseudo-2-fold symmetry. The two γ subunits forming each Fe-protein (pink and green with their Switch I and Switch II segments red and blue) bind to the MoFe-protein with the 2-fold axis relating them coincident with the pseudo-2-fold axis relating the MoFe-protein's α and β subunits. The ADP•AlF₄^–, [4Fe–4S] cluster, FeMo-cofactor, and P-cluster are drawn in space-filling form with C green, N blue, O red, S yellow, Fe orange, Mo pink, and the AlF₄^– ion purple. Click on the "Hide" button to see only the prosthetic groups. [Based on an X-ray structure by Douglas Rees, California Institute of Technology. PDBid 1N2C.]