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 176

From here, the reductive ammonification pathway bridges a step of six elec-
trons with a single enzyme that exists in two distinct versions. Assimilatory nitrite
reductase is a siroheme-dependent metalloprotein that in bacteria and plants is
employed for the production of ammonium to be incorporated into biomolecules,
consequently using NADPH as an electron donor. In contrast, the dissimilatory
variant of nitrite reductase is a multiheme c enzyme that obtains electrons
from the pool of menaquinol in the membrane, coupling its oxidation to the
generation of a proton motive force for energy conservation. The enzyme,
NrfA, contains five covalently attached heme groups per monomer and forms
functional dimers [ 4 ]. The reversal of nitrate ammonification is realized in the
pathway of nitrification , the sole oxygen-dependent process in the nitrogen cycle.
Here, ammonium is oxidized to hydroxylamine by ammonia monooxygenase, in a
process with strong parallels to methane oxidation to methanol by methane
monooxygenase. Hydroxylamine is then converted to nitrite by an octaheme
hydroxylamine dehydrogenase, followed by the two-electron oxidation to
nitrate by a molybdenum-dependent nitrite oxidase. An alternative route for
ammonium oxidation exists as an anaerobic process, in which ammonium is
comproportionated with nitric oxide, NO, to yield hydrazine, N 2 H 4 , by hydrazine
hydrolase. The product is then oxidized to N 2 by an octaheme hydrazine
dehydrogenase, and this anammox process - while being the latest discovery in
the nitrogen cycle - is now known to predominate in many habitats, giving it a
significant influence on the global nitrogen balance [ 5 ]. Nevertheless, the highest
metabolic flux through the nitrogen cycle occurs along the remaining reductive
pathway, leading in four enzymatic steps from nitrate via nitrite, nitric oxide
and nitrous oxide, N 2 O, to the stable end product dinitrogen, N 2 .Eachstep
is energy-conserving through the generation of a proton motive force, and in
summary this denitrification pathway is the most energetically favorable process
known to operate in the absence of molecular oxygen [ 2 , 6 , 7 ]. Its final two
intermediates, N 2 OandN 2 , play a particular role in the nitrogen cycle. Both are
stable, inert gases, and for each nature seems to have evolved only a single
enzyme able to catalyze a reductive conversion. For N 2 , this is the assimilatory
enzyme nitrogenase, a complex iron-sulfur enzyme and the agent of biological
nitrogen fixation [ 8 ]. N 2 O, in contrast, is converted to N 2 and H 2 Obythe
copper enzyme nitrous oxide reductase (N 2 OR) (equation 1 ), the topic of the
present chapter.

2H þ þ 2e ! N 2 O þ N 2 þ H 2 O ð 1 Þ

N 2 OR is unusual in various ways, as it contains two non-canonical metal centers
that allow reacting a highly inert substrate molecule. Due to the limited stability of
this enzyme, N 2 O is a common byproduct of this microbial metabolism and gives
rise to substantial environmental problems (see Section 2.4 ).