Structural characterization of the two copper proteins nitrous oxide reductase from Pseudomonas stutzeri and laccase Lcc5 from Coprinopsis cinerea

Although, the reduction of nitrous oxide (N2O) is highly exergonic, a high activation barrier hinders a spontaneous reaction. With regard to inertness, N2O is second only to molecular nitrogen and in both cases a complex metal center is required for activation. The only known enzyme, which catalyzes the reduction of N2O to N2 is the oxygen sensitive copper-protein nitrous oxide reductase (N2OR). Crystallographic studies on this enzyme from Paracoccus denitrificans, Marinobacter hydrocarbonoclasticus and Achromobacter cycloclastes provided insight into its structure: The protein forms a head-to-tail homodimer, which was shown to be obligatory for enzyme reaction. Each monomer comprises of two distinct domains, an N-terminal β-propeller with the tetranuclear CuZ site and a C-terminal cupredoxin-like domain carrying the mixed valent CuA center, similar to the one found in cytochrome c oxidase. However, these structures represent the aerobically isolated protein, which is only active upon extended incubation with reducing agents. In contrast, the purple form of nitrous oxide reductase from Pseudomonas stutzeri shows physiological activity without the necessity of reductive activation. This work presents the first structure of the purple form of nitrous oxide reductase and as well the first structure of a metal-N2O complex, providing new insights into the binding mode of N2O to the catalytic site. In pressurized crystals N2O binds between CuZ and CuA site, which is as previously described a mixed-valent center alternating between the oxidized mixed-valent [Cu+1.5:Cu+1.5] and the reduced [Cu+:Cu+] state thereby providing one electron per cycle. In contrast to previous structures, the histidine ligand of CuA1 is flexible and rotates to form hydrogen bonds with a near-by serine and aspartate residue in dependence of substrate binding. Additionally, a major structural difference could be observed for the CuZ site. A second sulfur ion is found at the edge of CuZ1 and CuZ4 replacing the earlier described water molecule. This observation offers an explanation for several spectroscopic features of the different enzyme forms. The CuZ of purple nitrous oxide reductase from Pseudomonas stutzeri is therefore a [4Cu:2S] site, whereas the [4Cu:1S] center observed in previously structures represents the CuZ* state. The second sulfur might stabilize the CuZ geometry by binding to CuZ1 and CuZ4, which might be a prerequisite for successful substrate binding. The observed binding mode of N2O indicates that both copper center act in concert to reduce the substrate.