Environmental Engineering Reference
In-Depth Information
Abstract It has been estimated that 99 % of all organisms utilize dihydrogen (H
2
).
Most of these species are microbes and their ability to use H
2
as a metabolite arises
from the expression of H
2
metalloenzymes known as hydrogenases. These molecules
have been the focus of intense biological, biochemical, and chemical research
because hydrogenases are biotechnologically relevant enzymes.
Keywords dihydrogen [FeFe] hydrogenase hydrogen technology [NiFe]
Please cite as:
Met. Ions Life Sci
. 14 (2014) 99-124
1
Introduction
For humankind, dihydrogen (H
2
) is often discussed in terms of a 'fuel of the future'.
In the context of biogeochemistry this description could not be more inappropriate:
microbial H
2
metabolism is an ancient process catalyzed by enzymes known as
hydrogenases. In modern scientific research these H
2
-activating complexes are of
interest not only because of their biological importance but also because of their
technological relevance to the construction of a carbon-free energy economy.
Dihydrogen is an ideal replacement for fossil fuels: reaction with molecular
oxygen (O
2
) yields water as the only product, the energy per unit mass of fuel is
high (specific enthalpy of combustion 120 MJ (kg fuel)
1
for H
2
compared to 50 MJ
(kg fuel)
1
for natural gas [
1
]), and H
2
-powered motor vehicle technology already
exists. However, finding a sustainable and scalable method for H
2
production and a
replacement for the precious metal platinum in H
2
fuel applications is challenging.
Hydrogenases are therefore studied in the context of understanding how to
construct, tune, and utilize highly efficient and active H
2
catalysts in H
2
energy
devices, with the added inspiration that these biological molecules are built from
Earth abundant elements. A major issue is the O
2
reactivity of the enzymes, which
is being explored using a variety of complementary biochemical techniques. This
chapter will therefore introduce the H
2
enzymes and H
2
cycling in both a biological
and technological context, to fully overview our current understanding of how both
microbes and humans harness the chemical function of hydrogenases.
2 Dihydrogen Cycles and Hydrogenases
Hydrogenases are sub-classified into three different types based on the active site
metal content, yielding the terms iron-iron hydrogenase, nickel-iron hydrogenase,
and iron hydrogenase. All hydrogenases catalyze reversible H
2
uptake
in vitro
, but
while the [FeFe] and [NiFe] hydrogenases are true redox catalysts, driving H
2
oxidation and proton (H
+
) reduction (equation
1
), the [Fe] hydrogenases catalyze
the reversible heterolytic cleavage of H
2
shown by reaction (
2
).
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