Role of Calcium in Secondary Structure Stabilization during Maturation of Nitrous Oxide Reductase

The copper enzyme nitrous oxide reductase catalyzes the two-electron reduction of nitrous. oxide (N2O) to dinitrogen (N-2). Its maturation largely occurs in.. the periplasm and includes the insertion of at least one Ca2+ ion per monomer. Here we have investigated the role of this structural cation in recombinantly produced apo-N2OR from Shewanella, denitrificans-and have determined the three-dimensional. structure of the protein by X-ray crystallography. In the absence of Ca2+, substantial parts of the enzyme surrounding the binding sites for the copper ions show structural disorder. Reconstitution of the binuclear Cu-A site was possible in vitro but required the presence of Ca2+ ions for a stable insertion of the Center. In contrast, an,excess of Ca2+ prevented copper insertion, and the structural analysis of the Ca2+ apo form revealed that the cation is sufficient to structure the disordered regions of the protein even in the absence of Cu ions, indicating that the geometry of the two noncanonical copper centers is largely predetermined by the protein structure.