Biomedical Engineering Reference
In-Depth Information
3. Uncoupled Energy Metabolism
Bacteria have a complete set of metabolic maps forming the metabolic network which are
striking features of metabolism and important to their life (Figure 3). The catabolic pathways
are those in which complicated molecules are taken in and broken down into simple ones and
free energy is taken out, gathered and stored in energy carriers (ATP). Anabolic pathways are
the construct biochemical processes involving the use of free energy to increase complexity
of molecules from simple ones and build up the structures required by the cell [42]. To reduce
the production of excess sludge, wastewater treatment processes must be well adjusted to
divert from biosynthesis via substrate assimilation to energy requiring functions associated
with non-growth activities, e.g. maintenance, mobility, and so on, which is called uncoupling,
spillage or overflow metabolism [44]. Russel et al. [51] defined “uncoupling” as being the
inability of chemosmotic oxidative phosphorylation to generate the maximum theoretical
quantity of metabolic energy in the form of ATP, which was also redefined by Low et al. [34]
as “uncoupled oxidative phosphorylation” to differentiate it from other mechanisms of
uncoupling metabolism.
3.1. Mechanism of Energy Coupling
For better understanding uncoupled energy metabolism, attention is given to oxidative
phosphorylation first. Cells generate ATP during cellular respiration through oxidative steps
in the degradation of carbohydrates, fats, and proteins. In eukaryotes, it occurs in
mitochondria, involving the reduction of O
2
to H
2
O using electrons donated by NADH and
FADH
2
. The light-dependent photophosphorylation occurs in chloroplasts involving the
conversion of H
2
O to O
2
, using NADP
+
as ultimate electron acceptor. Bacteria are a simple
life form whose cells typically consist of a single compartment surrounded by a plasma
membrane that separates the cell from its environment. In such a system, the energy
production by electron transport and oxidative phosporylation through electron flow from
NADH and FADH
2
to O
2
is carried out at (and across) the plasma membrane.
Correspondingly, there are three types of membranes taking part in coupling the electron
transport and phosphorylation reactions including mitochondrial membrane in eukaryotes,
chloroplast membrane in photosynthetic organisms and plasma membrane in prokaryotes
[52]. Though they capture different kinds of energy, all these three types of membranes allow
proton move through them from specific protein channels down proton concentration
gradient, and then provides the free energy for ATP synthesis. So far, three different
mechanisms of energy coupling have been proposed for energy transfer between electron
transport and ATP synthesis as specified in the following sections [53].
1.The Chemical Coupling Hypothesis
According to this hypothesis, electron transport is coupled to ATP synthesis by a
sequence of consecutive reactions and free energy is converted to the assumed
nonphosphorylated high energy intermediate and is used finally for binding ADP and
inorganic phosphate [54]. However, this hypothesis has been argued for decades because the
proposed nonphosphorylated high energy intermediate has never been identified. On the other
