Crystallographic Studies on the Short-lived Respiratory Enzyme-Substrate Complex

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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 (N_2O). In this study, we report the crystal structures of cNOR from Pseudomonas aeruginosa at 2.7 angstrom resolution and qNOR from Geobacillus stearothermophilus at 2.5 angstrom resolution. Although the overall structure of these NORs are closely related to cytochrome oxidase (COX), neither the D- nor K-proton pathway, which connect the COX active center to the intracellular space, was observed. Protons required for the cNOR reaction are probably provided from the extracellular side. On the contrary, qNOR has potential proton channel from the cytoplasmic side to the active center.