Environmental Engineering Reference
In-Depth Information
coordinated nickel, iron, sulfur, and in some cases selenium [21], while Fe-hydrogenases
contain only Fe-S centers in their active sites [23]. Metal-free hydrogenases also exist [24].
Many of the known bidirectional hydrogenases are Fe-hydrogenases [23], while the majority
of uptake hydrogenases are NiFe-hydrogenases; Fe-hydrogenases functioning to produce H2
are thus of greatest interest in microbial H2 production. Nevertheless, uptake hydrogenases
are of equal or perhaps greater concern in some systems, because they frequently co-exist
with H
2
-producing hydrogenases and can recycle H2 within the microbe, greatly diminishing
overall H
2
yield [12].
The physiological role of hydrogenase-based H2 production appears to be the discharge
of excess reducing power, necessary when other suitable electron acceptors such as O2 are
absent [25-27]. It is not surprising, therefore, that Fe-hydrogenases are rapidly inhibited both
transcriptionally and post-translationally by molecular oxygen [28, 23]. This feature has
significant implications for biological H
2
production, requiring anoxic cellular environments
to be maintained for both induction of hydrogenase synthesis as well as for continued
hydrogenase activity. For fermentative H
2
production, this is no obstacle, as the entire
microbial metabolism takes place anaerobically. However, oxygenic phototrophs provide the
majority of activated electrons to Fe-hydrogenases through the O
2
-generating photosynthetic
process [29]. Because Fe-hydrogenases are inhibited both transcriptionally and post-
translationally by O
2
, photosynthetic electron production and hydrogenase-based H
2
production cannot occur simultaneously in a wild-type organism. These processes therefore
must be separated either temporally or spatially, and much research in biohydrogen
production is directed toward the accomplishment of these goals.
Fe-hydrogenases exist in monomeric, dimeric, and at least one trimeric form [30, 23].
The two C.
reinhardtii
Fe-hydrogenase enzymes cloned and sequenced to date [31] encode
enzymes that are among the smallest hydrogenases known. Consistent with other algal
hydrogenases, they contain only the single catalytic Fe-S center, or H-cluster, and ferredoxin
is the only putative electron donor. In contrast, bacterial hydrogenases typically contain
several additional iron-sulfur centers and accept electrons from a variety of donors.
Clostridial hydrogenases, involved in hydrogen evolution during the fermentation of
carbohydrates, can accept electrons from flavodoxins, for example [24].
2.2.2. Nitrogenases
. Nitrogenases are tetrameric organometallic enzymes that catalyze
reductive cleavage of the second-strongest chemical bond known:the triple bond of dinitrogen
gas, N
2
. To supply the tremendous energy needed for this process, the enzyme uses ATP;
additional activated electrons are also required, however, which may be delivered through
NADH, pyruvate, photoactivation, or H2 itself (Figure 18). Nitrogenases share many
characteristics with hydrogenases: they employ metallic catalytic centers to facilitate the
redox reaction (Fe-Mo-Co and Fe-S clusters); they are rapidly inactivated by exposure to
molecular oxygen; and their synthesis is tightly regulated, requiring N deficiency as well as
anaerobiosis. Nitrogenases are found in many cyanobacteria, most of the purple non-sulfur
bacteria, and numerous symbiotic and free-living eubacteria. They evolve H
2
rapidly during
active nitrogen fixation; however, this H2 is produced at the cost of 16 ATP per H
2
[32].
Nitrogenases appear to produce the majority of H2 in nitrogen-fixing cyanobacteria (indirect
photolysis) and purple non- sulfur bacteria (photofermentation) [19].
