As indicated in Fig. 5 the [NiFe]-H 2 ase family has two variants; the much more
abundant, all cysteinyl form is better referred to as [NiFeS]-H 2 ase while the selenium variant is known as [NiFeSe]-H 2 ase. The impact of selenium on the activity of
[NiFe]-H 2 ase is to make it better, both in rates and in oxygen tolerance [39–41].
However the irregular biodistribution and scarcity of selenium, and the cost of
biosynthetic pathways to the installation of Selenocysteine in the specific point
required, leads to much smaller abundance of [NiFeSe]-H 2 ase. For biology, it is
something to learn from; for synthetic analogues as more robust artificial electrocatalysts, it should be considered to be a promising target [42, 43].
The currently accepted mechanisms for the two major hydrogenases are
described in Fig. 6. While these biocatalysts evolved via separate phylogenetic
paths, the convergent features required for proton/electron uptake, and H 2 formation
via an H
+
/H
− coupling process (or heterolytic H 2 cleavage) are readily seen [44–
46]. Each are bimetallic with thiolate sulfur bridges between Ni and Fe or between
the two irons in [FeFe]-H 2 ase. Such thiolate sulfur bridges place the metals within
bonding or semi-bonding distances so as to suggest the possibility of M-M bonding
and electron delocalization. Both [NiFe]- and [FeFe]-H 2 ase contain CO and CN
−
ligands for maintaining low-valence, electron-rich iron via p-delocalization; the
cyanide, both a good donor and p-acceptor ligand, additionally provides H-bonding
connections to the protein. Both H 2 ase active sites contain an open site on iron and
a well-positioned “pendant” base for proton relay in the last step of proton delivery.
For the [NiFe]-H 2 ase duo, the cysteinyl sulfur is shown in Ni-R by crystallography
to bind a proton in proximity to the hydride that is an asymmetric NiFe bridge, and
largely on Ni [29]. Giving credence to the significance of the thiolate sulfur as a
proton shuttle/pendant base indicated in Fig. 6, Nature positions selenium in the
Se analogue in precisely that spot (Fig. 5) [40, 41] and accounts for the superior
activity [NiFeSe]-H 2 ase.
The pendant base in the [FeFe]-H 2 ase is obviously the amine in the unique
azadithiolate unit that creates two metallocyclohexane-type rings in the HN
(CH 2 ) 2 S 2 Fe arrangement. Its efficacious design has been proven in hybrid enzyme
studies where the NH has been replaced by O or CH 2 in synthetic diiron units
[47–49]. Only the amine base works, achieving wild-type activity.
4 The Mono-iron Hydrogenase: From the Historical
Conundrum of “Metal-Free” Hydrogenase
4.1 A Bit of History
The third hydrogenase class, the [Fe]-H 2 ase, was recognized from its discovery, in
1992, to be distinctly different from both the [NiFe]- and [FeFe]-H 2 ases [50].
Isolated from methanogenic archaea grown under nickel limitation in order to
suppress the expression of the [NiFe]-H 2 ase, the enzyme clearly did not contain any
iron-sulfur clusters [50–52]. While preparations were found to contain up to 1 mol
282
M. Y. Darensbourg et al.
abundant, all cysteinyl form is better referred to as [NiFeS]-H 2 ase while the selenium variant is known as [NiFeSe]-H 2 ase. The impact of selenium on the activity of
[NiFe]-H 2 ase is to make it better, both in rates and in oxygen tolerance [39–41].
However the irregular biodistribution and scarcity of selenium, and the cost of
biosynthetic pathways to the installation of Selenocysteine in the specific point
required, leads to much smaller abundance of [NiFeSe]-H 2 ase. For biology, it is
something to learn from; for synthetic analogues as more robust artificial electrocatalysts, it should be considered to be a promising target [42, 43].
The currently accepted mechanisms for the two major hydrogenases are
described in Fig. 6. While these biocatalysts evolved via separate phylogenetic
paths, the convergent features required for proton/electron uptake, and H 2 formation
via an H
+
/H
− coupling process (or heterolytic H 2 cleavage) are readily seen [44–
46]. Each are bimetallic with thiolate sulfur bridges between Ni and Fe or between
the two irons in [FeFe]-H 2 ase. Such thiolate sulfur bridges place the metals within
bonding or semi-bonding distances so as to suggest the possibility of M-M bonding
and electron delocalization. Both [NiFe]- and [FeFe]-H 2 ase contain CO and CN
−
ligands for maintaining low-valence, electron-rich iron via p-delocalization; the
cyanide, both a good donor and p-acceptor ligand, additionally provides H-bonding
connections to the protein. Both H 2 ase active sites contain an open site on iron and
a well-positioned “pendant” base for proton relay in the last step of proton delivery.
For the [NiFe]-H 2 ase duo, the cysteinyl sulfur is shown in Ni-R by crystallography
to bind a proton in proximity to the hydride that is an asymmetric NiFe bridge, and
largely on Ni [29]. Giving credence to the significance of the thiolate sulfur as a
proton shuttle/pendant base indicated in Fig. 6, Nature positions selenium in the
Se analogue in precisely that spot (Fig. 5) [40, 41] and accounts for the superior
activity [NiFeSe]-H 2 ase.
The pendant base in the [FeFe]-H 2 ase is obviously the amine in the unique
azadithiolate unit that creates two metallocyclohexane-type rings in the HN
(CH 2 ) 2 S 2 Fe arrangement. Its efficacious design has been proven in hybrid enzyme
studies where the NH has been replaced by O or CH 2 in synthetic diiron units
[47–49]. Only the amine base works, achieving wild-type activity.
4 The Mono-iron Hydrogenase: From the Historical
Conundrum of “Metal-Free” Hydrogenase
4.1 A Bit of History
The third hydrogenase class, the [Fe]-H 2 ase, was recognized from its discovery, in
1992, to be distinctly different from both the [NiFe]- and [FeFe]-H 2 ases [50].
Isolated from methanogenic archaea grown under nickel limitation in order to
suppress the expression of the [NiFe]-H 2 ase, the enzyme clearly did not contain any
iron-sulfur clusters [50–52]. While preparations were found to contain up to 1 mol
282
M. Y. Darensbourg et al.