title: No laughing matter: the unmaking of the greenhouse gas dinitrogen monoxide by nitrous oxide reductase.
abstract: The gas nitrous oxide (N₂O) is generated in a variety of abiotic, biotic, and anthropogenic processes and it has recently been under scrutiny for its role as a greenhouse gas. A single enzyme, nitrous oxide reductase, is known to reduce N₂O to uncritical N₂, in a two-electron reduction process that is catalyzed at two unusual metal centers containing copper. Nitrous oxide reductase is a bacterial metalloprotein from the metabolic pathway of denitrification, and it forms a 130 kDa homodimer in which the two metal sites CuA and CuZ from opposing monomers are brought into close contact to form the active site of the enzyme. CuA is a binuclear, valence-delocalized cluster that accepts and transfers a single electron. The CuA site of nitrous oxide reductase is highly similar to that of respiratory heme-copper oxidases, but in the denitrification enzyme the site additionally undergoes a conformational change on a ligand that is suggested to function as a gate for electron transfer from an external donor protein. CuZ, the tetranuclear active center of nitrous oxide reductase, is isolated under mild and anoxic conditions as a unique [4Cu:2S] cluster. It is easily desulfurylated to yield a [4Cu:S] state termed CuZ (*) that is functionally distinct. The CuZ form of the cluster is catalytically active, while CuZ (*) is inactive as isolated in the [3Cu(1+):1Cu(2+)] state. However, only CuZ (*) can be reduced to an all-cuprous state by sodium dithionite, yielding a form that shows higher activities than CuZ. As the possibility of a similar reductive activation in the periplasm is unconfirmed, the mechanism and the actual functional state of the enzyme remain under debate. Using enzyme from anoxic preparations with CuZ in the [4Cu:2S] state, N2O was shown to bind between the CuA and CuZ sites, suggesting direct electron transfer from CuA to the substrate after its activation by CuZ.
authors.affiliation: Institute for Biochemistry, Albert-Ludwigs-Universität Freiburg, D-79104, Freiburg im Breisgau, Germany.
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Schneider
authors.forename: Anja
authors.initials: A
authors.lastname: Wüst
authors.forename: Anja
authors.initials: A
authors.lastname: Pomowski
authors.forename: Lin
authors.initials: L
authors.lastname: Zhang
authors.forename: Oliver
authors.initials: O
authors.lastname: Einsle
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D002384:Catalysis; D004777:Environment; D017752:Greenhouse Effect; D009569:Nitric Oxide / Q000737:chemistry / Q000378:metabolism*; D009584:Nitrogen / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism*
chemical_list: D009569:Nitric Oxide; D010088:Oxidoreductases; C022763:nitrous oxide reductase; D009584:Nitrogen