Environmental Engineering Reference
In-Depth Information
2.1 Biochemistry
Fermentation is the process of deriving energy from the oxidation of organic com-
pounds using an endogenous electron (e
-
) acceptor, which is usually an organic
compound [14]. This is in contrast to cellular respiration, where e
-
are donated to
an exogenous e
-
acceptor, such as oxygen, via an electron transport chain (ETC).
Considering glucose as substrate, fermentative H
2
production starts with the con-
version of glucose to pyruvate through glycolysis by both obligate and facultative
anaerobic bacteria. In facultative anaerobes, pyruvate is converted to acetyl-CoA
and formate, which is catalysed by pyruvate formate lyase (PFL) [3] and H
2
is
produced from formate by the formate hydrogen lyase (FHL) complex. In obligate
anaerobes, pyruvate is converted to acetyl-CoA and CO
2
through pyruvate ferre-
doxin oxidoreductase (PFOR) and this oxidation requires reduction of ferredoxin
(Fd) [3, 15]. The fate of pyruvate in the case of anaerobic operation depends on
the operating pH. Under acidic condition pyruvate is converted into volatile fatty
acids along with H
2
by acidogenic bacteria. Neutral operation leads to the formation
of CH
4
and CO
2
by methanogenic bacteria. Under basic pH, anaerobic digestion
leads to solventogenesis. At all the pH conditions, H
+
shuttling takes place between
metabolic intermediates with the help of various redox mediators under anaerobic
operation. The H
+
from the redox mediator is detached by a specific dehydrogenase
(NADH-dehydrogenase) and combined with the e
-
from oxidized ferredoxin to gen-
erate H
2
in presence of the hydrogenase enzyme (Fig. 1b). Hydrogenase activity is
higher at acidic pH but with increase in pH, metabolic pathway might proceed to the
next step of anaerobic digestion where H
+
get reduced to CH
4
(methanogenesis) or
ethanol (solventogenesis).
Biodegradation of substrate is always accompanied by the release of protons
(H
+
) and electrons (e
-
) associated with various redox reactions and enzymes.
Dehydrogenase is one of the important enzymes involved in the inter-conversion
of metabolites and the transfer of protons (H
+
) between metabolic intermediates
through redox reactions using several mediators (NAD
+
,FAD
+
, etc.). Redox medi-
ators are capable of carrying H
+
and e
-
, otherwise known as energy carriers as they
are involved in biological energy (ATP) generation [16]. Generally, in the anaerobic
microenvironment, inter-conversion of substrates takes place through degradation
that increases the availability of H
+
in the cell. The protons associated with redox
mediators are the main source of fermentative H
2
production. The protons from
redox mediators are detached in presence of NADH-dehydrogenase and reduced to
H
2
in presence of the hydrogenase enzyme with the help of e
-
donated by oxidized
ferredoxin (co-factor) [3]. Hydrogenases are complex metalloenzymes that can be
classified into three groups based on the number and identity of the metals in their
active sites: [NiFe]-, [FeFe]- and [Fe]-hydrogenases [17]. These enzymes are also
responsible for the reversible conversion of molecular H
2
into two H
+
and two e
-
[H
2
↔
2H
+
+2e
-
] [3]. The dehydrogenase activity is crucial along with the hydro-
genase activity as it maintains H
+
equilibrium in the cell through redox reactions and
inter-conversion of metabolic intermediates. Nitrogenase enzymes are also involved
in H
2
production along with nitrogen-fixation. Nitrogenases irreversibly catalyze
Search WWH ::
Custom Search