Structural Basis of Biological N2O Generation by Bacterial Nitric Oxide Reductase

Dissecting Nitric Oxide Reductase Bacterial breakdown of nitrogen compounds in soil and the oceans provides the largest emission source of the greenhouse gas, nitrous oxide (N2O). A key enzyme in this process is nitric oxide reductase (NOR), which catalyzes the reduction of nitric oxide (NO) to N2O. Hino et al. (p. 1666, published online 25 November; see the Perspective by Moënne-Loccoz and Fee) now describe the crystal structure of NOR from Pseudomonas aeruginosa. Consistent with their evolutionary relatedness, the transmembrane region topology and arrangement of metal centers in NOR are similar to those in cytochrome oxidases, key enzymes in aerobic respiration. A structural comparison gives insight into the features that allow conversion between nitric oxide and oxygen reduction. Nitric oxide reductase (NOR) is an iron-containing enzyme that catalyzes the reduction of nitric oxide (NO) to generate a major greenhouse gas, nitrous oxide (N2O). Here, we report the crystal structure of NOR from Pseudomonas aeruginosa at 2.7 angstrom resolution. The structure reveals details of the catalytic binuclear center. The non-heme iron (FeB) is coordinated by three His and one Glu ligands, but a His-Tyr covalent linkage common in cytochrome oxidases (COX) is absent. This structural characteristic is crucial for NOR reaction. Although the overall structure of NOR is closely related to COX, neither the D- nor K-proton pathway, which connect the COX active center to the intracellular space, was observed. Protons required for the NOR reaction are probably provided from the extracellular side.