Crystal structure of nitrous oxide reductase from Paracoccus denitrificans at 1.6 A resolution

Abstract

N2O is generated by denitrifying bacteria as a product of NO reduction. In denitrification, N2O is metabolized further by the enzyme N2O reductase (N2OR), a multicopper protein which converts N2O into dinitrogen and water. The structure of N2OR remained unknown until the recent elucidation of the structure of the enzyme isolated from Pseudomonas nautica. In the present paper, we report the crystal structure of a blue form of the enzyme that was purified under aerobic conditions from Paracoccus denitrificans. N2OR is a head-to-tail homodimer stabilized by a multitude of interactions including two calcium sites located at the intermonomeric surface. Each monomer is composed of two domains: a C-terminal cupredoxin domain that carries the dinuclear electron entry site known as CuA, and an N-terminal seven-bladed β-propeller domain which hosts the active-site centre CuZ. The electrons are transferred from CuA to CuZ across the subunit interface. CuZ is a tetranuclear copper cluster in which the four copper ions (Cu1 to Cu4) are ligated by seven histidine imidazoles, a hydroxyl or water oxygen and a bridging inorganic sulphide. A bound chloride ion near the CuZ active site shares one of the ligand imidazoles of Cu1. This arrangement probably influences the redox potential of Cu1 so that this copper is stabilized in the cupric state. The treatment of N2OR with H2O2 or cyanide causes the disappearance of the optical band at 640nm, attributed to the CuZ centre. The crystal structure of the enzyme soaked with H2O2 or cyanide suggests that an average of one copper of the CuZ cluster has been lost. The lowest occupancy is observed for Cu3 and Cu4. A docking experiment suggests that N2O binds between Cu1 and Cu4 so that the oxygen of N2O replaces the oxygen ligand of Cu4. Certain ligand imidazoles of Cu1 and Cu2, as well as of Cu4, are located at the dimer interface. Particularly those of Cu2 and Cu4 are parts of a bonding network which couples these coppers to the CuA centre in the neighbouring monomer. This structure may provide an efficient electron transfer path for reduction of the bound N2O.