Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 98

2H þ þ 2e

H 2 Ð ð 1 Þ

H þ þ H H 2 Ð ð 2 Þ

The biochemical and chemical details of this activity are described in more detail in
Sections 3 , 4 , and 5 , while this section sets into context the biological and techno-
logical significance of enzymatic H 2 catalysis.

2.1 The Global Dihydrogen Cycle

It has been proposed that biological H 2 uptake is of such fundamental importance that
it could have been a key reaction in the origin of life on Earth [ 2 ]. The inorganic
elements required to construct the reaction centers of the H 2 -oxidizing hydrogenases:
sulfur, Ni, and Fe, would have been readily available in the oceanic hydrothermal
vents where life may first have arisen [ 2 ]. The evolutionary relationship between the
essential respiratory enzyme Complex I and [NiFe] hydrogenase also suggests that
biological H 2 catalysis is an ancient process [ 3 ]. There is the possibility that H 2
availability in the early Earth atmosphere was as high as 30 % [ 4 , 5 ]. This has large
implications on the possible scale of biological carbon fixation in Earth's
pre-photosynthetic era, because anaerobic methanogens, a major component of life
on the planet during the Archaean period (2.5 billion years ago), produce methane by
coupling hydrogenase-catalyzed H 2 splitting with carbon dioxide (CO 2 )reduction
(equation 3 )[ 6 , 7 ]. Indeed the same biochemistry may even be responsible for life
elsewhere in the solar system [ 8 ].

CO 2 þ 4H 2 ! CH 4 þ 2H 2 O ð 3 Þ

In the modern era, H 2 is not a major constituent of the atmosphere of Earth,
with an average abundance of only approximately 0.5 parts-per-million in dry air
(0.5 μ mole H 2 per mole of air) [ 9 ]. This level varies with seasonality and geographical
location, and comparing different studies is complex because the amount of
atmospheric H 2 is so low that accurate measurement using gas chromatography is
sensitive to even the metallic material of the air calibration cylinder [ 10 ].
Regardless of precise H 2 levels, across different atmospheric H 2 studies there is a
consensus that both biological and abiological processes contribute significantly to
the global H 2 cycle (Table 1 ). The largest source of environmental H 2 is the
atmospheric photochemical process of hydrocarbon dissociation, and microbial
H 2 production only ranks as the fourth largest contributor [ 9 ]. However, biological
processes are the dominant sink for atmospheric H 2 illustrating that overall the
most important physiological role of H 2 is as a biological fuel. Within microbial
environments cellular processes which result in H 2 production are nearly always
linked with either inter or intracellular H 2 uptake (Section 2.2 ).