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Copper-containing nitrous oxide reductase catalyzes a 2-electron reduction of the green-house gas N2O to yield N2. It contains two metal centers, the binuclear electron transfer site CuA, and the unique, tetranuclear CuZ center that is the site of substrate binding. Different forms of the enzyme were described previously, representing variations in oxidation state and composition of the metal sites. Hypothesizing that many reported discrepancies in the structural data may be due to radiation damage during data collection, we determined the structure of anoxically isolated Marinobacter nauticus N2OR from diffraction data obtained with low-intensity X-rays from an in-house rotating anode generator and an image plate detector. The data set was of exceptional quality and yielded a structure at 1.5 Å resolution in a new crystal form. The CuA site of the enzyme shows two distinct conformations with potential relevance for intramolecular electron transfer, and the CuZ cluster is present in a [4Cu:2S] configuration. In addition, the structure contains three additional types of ions, and an analysis of anomalous scattering contributions confirms them to be Ca2+, K+, and Cl–. The uniformity of the present structure supports the hypothesis that many earlier analyses showed inhomogeneities due to radiation effects. Adding to the earlier description of the same enzyme with a [4Cu:S] CuZ site, a mechanistic model is presented, with a structurally flexible CuZ center that does not require the complete dissociation of a sulfide prior to N2O binding. The [4Cu:2S] CuZ site in M. nauticus N 2O reductase. The electron density map shown is contoured at the 5 σ level, highlighting the presence of two sulfide ligands. 705x677mm (72 x 72 DPI) [ABSTRACT FROM AUTHOR]

Marinobacter hydrocarbonoclasticus nitric oxide reductase, c NOR, is an integral membrane protein composed of two subunits with different roles, NorC (electron transfer) and NorB (catalytic) that receives electrons from the soluble cytochrome c 552 and reduces nitric oxide to nitrous oxide in the denitrification pathway. The solvent-exposed domain of NorC, harboring a c -type heme was heterologously produced, along with its physiological electron donor, cytochrome c 552 . These two proteins were spectroscopically characterized and shown to be similar to the native proteins, both being low-spin and Met-His coordinated, with the soluble domain of NorC presenting some additional features of a high-spin heme, which is consistent with the higher solvent accessibility of its heme and weaker coordination of the methionine axial ligand. The electron transfer complex between the two proteins has a 1:1 stoichiometry, and an upper limit for the dissociation constant was estimated by 1 H NMR titration to be 1.2 ± 0.4 μM. Electrochemical techniques were used to characterize the interaction between the proteins, and a model structure of the complex was obtained by molecular docking. The electrochemical observations point to the modulation of the NorC reduction potential by the presence of NorB, tuning its ability to receive electrons from cytochrome c 552 . [ABSTRACT FROM AUTHOR]

Nitrous oxide reductase (N[sub2]OR) catalyzes the two-electron reduction of N[sub2]O to N[sub2] and H[sub2]O in the last step of the bacterial denitrification process. It is a dimeric protein. The recently solved crystal structure of N[sub2]OR indicates that in each subunit there is a Cu[subA] center, which is the electron-transfer site, and a Cu[subZ] center, which is the catalytic site. The neighboring Cu[subA] and Cu[subZ] centers are from different subunits. The whole Cu[subZ] cluster is coordinated by seven His ligands with a weakly bound water at the Cu[subI]/Cu[subIV] edge, which is the substrate access site.