Environmental Engineering Reference
In-Depth Information
one of the major constituents of bio-macromolecules (e.g., proteins and nucleic
acids), would eventually be converted to N
2
through denitrification, the conversion
of N
2
to NH
3
provides the necessary re-entry point for the inorganic nitrogen back
into the biosphere and, therefore, is essential for all life forms on Earth [
17
].
Biological nitrogen fixation has been estimated to reduce (or fix) approximately
90 million tons of nitrogen per year, which supports roughly half of the human
population [
18
,
19
]. The other half of the population relies on the Haber-Bosch
process, an industrial counterpart of biological nitrogen fixation, for the fixed
nitrogen supplies. Developed in 1908 by Fritz Haber, this process combines N
2
with hydrogen gas (H
2
) into NH
3
at high temperature (300-400
C) and pressure
(~300 atm) [
8
]. These conditions require approximately 1-2 % of annual power
production worldwide [
20
]. Such a high energy demand does not come as a
surprise, though, because the N, N triple bond has a bond dissociation energy of
225 kcal (945 kJ) per mole and, therefore, is one of the most stable bonds found in
Nature [
21
]. Given the energy crisis and population growth on our planet, it is
highly desirable to develop an energy-efficient alternative to the Haber-Bosch
process for NH
3
production.
Biological nitrogen fixation may just provide a suitable blueprint for future
development of such an alternative approach, as this process converts N
2
to NH
3
at ambient temperature and pressure while producing H
2
, a clean energy source, as
an abundant side product. The enzymes responsible for such a conversion are
nitrogenases, which can be found in a select group of microbes called diazotrophs.
These metal-containing enzymes couple the reduction of N
2
with the hydrolysis of
adenosine 5
0
-triphosphate (ATP), thereby overcoming the energy barrier of the
reaction and facilitating the cleavage of the N,N triple bond under ambient condi-
tions. Recently, it has been demonstrated that nitrogenases are also capable of
converting carbon monoxide (CO) to hydrocarbons [
22
]. As such, the reactions
catalyzed by nitrogenases can be appreciated not only from the perspective of
energy conservation, but also from the perspective of the useful products they
generate.
Four classes of nitrogenases have been identified to date: the molybdenum
(Mo)-nitrogenase, the vanadium (V)-nitrogenase, the iron (Fe)-only nitrogenase
and the superoxide dismutase (SOD)-dependent nitrogenase from
Streptomyces
thermoautotrophicus
[
5
,
8
,
10
,
23
,
24
]. The first three nitrogenases are highly
homologous in primary sequences and are believed to be structurally similar to
one another, namely, they all consist of two protein components, each of which
houses metal cluster(s) crucial for catalysis [
1
-
16
]. The three nitrogenases also
function similarly in that the two protein components act as redox partners,
allowing electrons to flow sequentially through their respective metal clusters for
the reduction of N
2
. The Mo-nitrogenase is the best characterized among these
nitrogenases, with more than 35 crystal structures deposited in the Protein
Data Bank (PDB) and the largest body of spectroscopic and catalytic data collected
on this protein. Consequently, most of our current knowledge regarding the
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