Biomedical Engineering Reference
In-Depth Information
3. The Presence of Inhibitory Compounds
Many inhibitory compounds of electron transport and oxidative phosphorylation have
been employed to demonstrate the pathway of electron flow through the electron transport
system, the properties of the PMF, as well as to identify aspects of an enzyme mechanism. So
far, the used inhibitory compounds basically can be classified into the following two groups:
(i) inhibitors of electron transport at specific sites in the electron transport system, and (ii)
chemical uncouplers leading to reduce proton gradient across cellular membrane [65].
Theoretically, inhibitors of electron transport may stop the proton pumping by blocking
electron transfer, and uncouplers will minimize PMF through dissipating the proton gradient,
leading to stop of ATP synthesis. The specific effects of some inhibitory compounds on
electron transport and oxidative phosphorylation are summarized in Table 2.
Uncouplers of oxidative phosphorylation are typically weak acids with substantial lipid
solubility and high hydrophobicity to carry proton across the cellular membrane into the
matrix (Figure 6) [68,69]. Uncouplers are thought to have the ability to act as lipophilic
proton conductors which interact nonspecifically with membranes to dissipate the proton
gradient that energizes many membrane bioenergetic functions [57]. These agents cause
maximum respiratory rates, electron transport continues, but no ATP synthesis occurs, since
the translocated protons do not return to the interior through ATP synthase [65]. As the result,
the reduction of ATP production caused by uncoupling oxidative phosphorylation occurs. In
such a condition, biomass cells still continue to satisfy their maintenance energy requirement
prior to that for anabolism so that the amount of ATP available for biosynthesis would be
reduced dramatically which in turn reduce biomass production. Replicating these uncoupling
processes in wastewater treatment would therefore be admirable. Oxidative phosphorylation
can be effectively uncoupled by the addition of organic protonophores, such as 2,4-
dinitrophenol (DNP), para-nitrophenol (PNP), 2,4-dichlorophenol (DCP), 2,4,6-
trichlorophenol (TCP), pentachlorophenol (PCP), dicumarol, 3,3',4',5-
tetrachlorosalicylanilide (TCS), fluorocarbonyl-cyanide phenylhydrazone (FCCP), etc. The
structures of some uncouplers are presented in Figure 7.
3.3. Application of Uncoupling of Oxidative Phosphorylation
Mayhew et al. [7] tested 8 different chemical uncouplers at concentration of 8 μg L
-1
to
18 mg L
-1
, and found that erythromycin, 2,4-dNP, rotenone and trypan blue had the highest
levels of metabolic uncoupling without the significant effect on microorganisms (Table 3).
The mechanism of the action of 2,4-dinitrophenol (DNP) has been investigated for
several decades. It has been shown that this substance acts mainly by inhibiting the uptake of
inorganic orthophosphate from the reaction medium, whereas the oxygen uptake is
unmodified or increased [70]. Chen et al. [71] studied the response of activated sludge to the
presence of DNP in batch cultures, and found that effect of DNP on the substrate removal and
growth of activated sludge heavily depended upon its concentration. When DNP
concentration was lower than 10 mg L
-1
, sludge yield and relative specific growth rates (μ/μ
0
)
were significantly reduced, but its relative specific chemical oxygen demand removal rate
(q/q
0
) was not substantially affected. DNP addition at 1-20 mg L
-1
increased the specific
oxygen uptake rate of activated sludge, and slightly changed its hydrophobicity.
