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 202

derives from this mode of N 2 O binding is that the substrate is located in between
the two metal sites, making it unlikely that in this arrangement electron transfer
from Cu A would proceed first into Cu Z and then to the substrate, but rather directly
from Cu A to an activated state of N 2 O that is coordinated to Cu Z .
5.4 The Fate of the Products
The substrate N 2 O is a weakly polar molecule with a positive partial charge on the
central nitrogen atom and according negative partial charges on the terminal N and
O (Figure 2 ). It is well suited to access the active site of N 2 O reductase via a largely
hydrophobic access channel. After reduction by two electrons, the gaseous, apolar
product N 2 will be able to exit the enzyme along the same route. However, the
oxygen atom of N 2 O remains as a polar water molecule, and the hydrophobic
substrate channel seems to disfavor the passage of water.
In the structures of N 2 O reductase, water molecules are found in close proximity
to the metal sites, and indeed the position held by the oxygen atom of N 2 O in the
substrate complex structure (Figure 9 ) is commonly occupied by water if substrate
is absent. From here, a network of coordinated water molecules, interacting through
hydrogen bonds, stretches through the protein matrix. This arrangement suggests
that after the reduction of N 2 O, the fate of the two product molecules differs, and
while N 2 will exit through the original substrate channel, the water molecule will
leave the active site in a different way, to move through a hydrophilic cavity and
exit the protein at a remote site. In many enzymes, such optimizations serve to
prevent product inhibition and accelerate the reaction rate, but in the present case it
seems more likely that this is a mechanism to accommodate the very different
physicochemical properties of the two reaction products.
6 General Conclusions
Nitrous oxide reductase is a complex and fascinating metalloenzyme that catalyzes
the conversion of an inert gas of high chemical interest and ecological relevance.
Its copper sites have features that so far are unique in bioinorganic chemistry, and
the seemingly straightforward two-electron redox process it catalyzes is of suffi-
cient complexity to defy a final elucidation.
At present, the model of the action of N 2 O reductase brought forward by
Solomon, Moura, and coworkers integrates the majority of the available experi-
mental data and presents the outlines of a mechanism in which the simultaneous
coordination of N 2 O to two copper ions within Cu Z * is required to overcome the
activation energy barrier for substrate reduction [ 34 , 81 ]. It places the purple form I
of the enzyme on a catalytic sideline, but this in particular raises the question why
all biochemical evidence then points to Cu Z * being a degradation product of the