No laughing matter: the unmaking of the greenhouse gas dinitrogen monoxide by nitrous oxide reductase

Met Ions Life Sci. 2014:14:177-210. doi: 10.1007/978-94-017-9269-1_8.

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Atmosphere / chemistry
  • Catalysis
  • Environment
  • Greenhouse Effect*
  • Nitric Oxide / chemistry
  • Nitric Oxide / metabolism*
  • Nitrogen / chemistry
  • Nitrogen / metabolism
  • Oxidation-Reduction
  • Oxidoreductases / chemistry
  • Oxidoreductases / metabolism*

Substances

  • Nitric Oxide
  • Oxidoreductases
  • nitrous oxide reductase
  • Nitrogen