Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 118

6 General Conclusions

Biological H 2 catalysis is a widespread and important microbial reaction and the
action of hydrogenases can be harnessed in biotechnological devices. In the past
10 years, applied hydrogenase research has moved away from enzymatic fuel cells,
which utilize hydrogenases as H 2 uptake catalysts, to focusing on solar H 2 produc-
tion. Of the three active site classes of hydrogenases the [NiFe] enzymes are the
most widely studied but the [FeFe] enzymes have been best characterised in terms
of small molecule mimicry. The [Fe] hydrogenases are the most recently discovered
hydrogenase class and the least understood. It has been important to use a comple-
mentary biological and biochemical experimental toolkit to unravel the structure,
function and reactivity of hydrogenases, as well as details of how these enzymes are
assembled in vivo .
Although the [NiFe], [FeFe], and [Fe] hydrogenases are phylogenetically
unrelated enzymes, they are examples of convergent evolution because all three
enzyme classes contain H 2 -activating sites based around a minimal chemical
unit of Fe coordinated by CO and sulfur ligands. The presence of CO (and CN )
as a structural element of an enzymatic active site is a unique feature of
hydrogenases, and biosynthesis of the catalytic center requires metal transport,
ligand synthesis, pre-assembly and transport. In addition to this complex
process, both [NiFe] and [FeFe] hydrogenases also require FeS cluster assem-
bly. Mapping the mechanism of hydrogenase biosynthesis is important in
enabling the design of heterologous expression systems which may ultimately
make sustained photo-biological H 2 O splitting a feasible method for large scale
H 2 production.

Abbreviations and Definitions

adenosine 5 0 -triphosphate ATP

Cys cysteine

E 298

standard reduction potential at 298 K

E. coli Escherichia coli

EPR electron paramagnetic resonance

Fe D distal iron

Fe p proximal iron

FeGP Fe guanylylpyridinol

FeS iron sulfur

FTIR Fourier transform infrared

IR infrared

MBH membrane bound hydrogenase

MJ megaJoule

NADH nicotinamide adenine dinucleotide (reduced)

NADPH nicotinamide adenine dinucleotide phosphate (reduced)