No Laughing Matter: The Unmaking of the Greenhouse Gas Dinitrogen Monoxide by Nitrous Oxide Reductase - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 174

5 ACTIVATION OF NITROUS OXIDE: THE WORKINGS
OF NITROUS OXIDE REDUCTASE ................................................... 203

5.1 Substrate Access ..................................................................... 203

5.2 Gated Electron Transfer ............................................................. 205

5.3 Activation of Nitrous Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

5.4 The Fate of the Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

6 GENERAL CONCLUSIONS ............................................................. 206

ABBREVIATIONS AND DEFINITIONS . . . . . .............................................. 207

ACKNOWLEDGMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

Abstract The gas nitrous oxide (N 2 O) 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 N 2 O
to uncritical N 2 , in a two-electron reduction process that is catalyzed at two unusual
metal centers containing copper. Nitrous oxide reductase is a bacterial metallo-
protein from the metabolic pathway of denitrification, and it forms a 130 kDa
homodimer in which the two metal sites Cu A and Cu Z from opposing monomers
are brought into close contact to form the active site of the enzyme. Cu A is a
binuclear, valence-delocalized cluster that accepts and transfers a single electron.
The Cu A 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. Cu Z , 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 Cu Z *
that is functionally distinct. The Cu Z form of the cluster is catalytically active,
while Cu Z * is inactive as isolated in the [3Cu 1+ :1Cu 2+ ] state. However, only Cu Z *
can be reduced to an all-cuprous state by sodium dithionite, yielding a form that
shows higher activities than Cu Z . 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 Cu Z in the [4Cu:2S] state, N 2 O was shown to bind between the Cu A and
Cu Z sites, suggesting direct electron transfer from Cu A to the substrate after its
activation by Cu Z .

Keywords copper enzymes • global warming • nitrogen cycle • nitrous oxide
• X-ray crystallography

Please cite as: Met. Ions Life Sci . 14 (2014) 177-210