Biomedical Engineering Reference
In-Depth Information
point of view of environmental impact because this would be the perfect way
to produce energy, using only two renewable resources, solar radiation and
water.
The
photosynthetic
microorganisms
able
to
produce
hydrogen
are
cyanobacteria and green algae.
2.3.1 Cyanobacteria
Cyanobacteria can use two different enzymes to generate hydrogen gas [7].
The first is
nitrogenase
, which catalyzes the fixation of nitrogen into ammo-
nia, producing hydrogen as a by-product. Hydrogen photo-evolution catalyzed
by nitrogenases requires anaerobic conditions, as hydrogenases. As oxygen is
a by-product of photosynthesis, cyanobacteria have developed two different
strategies to achieve anaerobiosis:
1. Some evolved the capacity of building structures with reduced oxygen-
permeability, called heterocysts, thus separating physically oxygen evolu-
tion from nitrogenase activity [8].
2. Nonheterocystous cyanobacteria instead separate temporally oxygen evo-
lution from nitrogenase activity activating the latter only during dark
periods.
Whatever is the case, hydrogen production by nitrogenase enzyme is in-
teresting but presents an important limitation. As the reaction is coupled to
the nitrogen fixation, it is energetically demanding, making the whole process
poorly e
cient.
The other hydrogen-metabolizing enzymes in cyanobacteria belong to the
class of hydrogenases. One class,
uptake hydrogenase
(encoded by
hupSL
gene
), however, catalyzes the oxidation of hydrogen molecules to produce pro-
tons and electrons [9]. Uptake hydrogenase enzymes are found in the thylakoid
membrane of heterocystis from filamentous cyanobacteria, and they transfer
the electrons from hydrogen for to the respiratory chain. Nitrogenases of this
type are not responsible for net hydrogen production in vivo, rather, allow for
partial recuperation of energy invested in N
2
fixation and employed for hydro-
gen production. In the perspective of the production of hydrogen molecules,
their activity is not desirable and should be inhibited.
The second type of hydrogenases are
reversible, or bi-directional hydroge-
nases
(encoded by
hoxFUYH
) that can either consume or produce hydrogen.
The biological role of these bidirectional or reversible hydrogenases is poorly
understood and thought to be involved in the ions level control of the organ-
ism. Reversible hydrogenases are associated with the cytoplasmic membrane
and likely function as electron acceptor from both NADH and H
2
[10]. If op-
portunely regulated, these enzymes can be exploited for hydrogen production.
