Environmental Engineering Reference
In-Depth Information
to weak reductants, quinols, and ferrocytochrome c. Oxidation of these weakest
biological reductants requires a terminal (environmental) oxidant, such as O
2
. The
stronger this terminal oxidant, the larger the amount of energy an organism would
extract from a given amount of food (Fig. 18.2). Molecular oxygen is by far the stron-
gest bioavailable oxidant on Earth; it has high permeation rates across biological mem-
branes [Ligeza et al., 1998; Wittenberg and Wittenberg, 2003] and its fully reduced
form, H
2
O, is nontoxic. (However, partially reduced oxygen species, such as superox-
ide (O
2
2
), hydroperoxyl radical (HO
2
), hydrogen peroxide (H
2
O
2
), and hydroxyl
radical (HO
) are highly toxic.) Hence, O
2
is an attractive terminal oxidant, and
indeed all multicellular organisms are obligatory aerobes, whereas anaerobic respiration
is limited to unicellular organisms.
Although respiration is a sequence of electron transfer steps, not unlike reduction of
O
2
in a fuel cell, living organisms employ fundamentally different schemes to couple
the free energy of O
2
reduction to their energy-dissipative processes. Living organisms
cannot utilize electron currents; instead, a portion of the free energy of each electron
transfer step in the respiratory chain is captured in the form of a transmembrane elec-
trochemical proton (H
þ
) gradient [Nicholls, 1982]. This proton gradient powers the
operation of ATP synthase that converts ADP (adenosine diphosphate) and phosphate
ion into ATP (adenosine triphosphate) [Dimroth et al., 2003; Senios et al., 2002;
Yoshida, 2001]. Exergonic hydrolysis of ATP powers most energy-consuming biologi-
cal processes, from biosynthesis to motility [Alberts et al., 2002]. To effect the conver-
sion of the free energy of redox processes into an electrochemical gradient during
respiration, the suite of respiratory enzymes (e.g., NADH dehydrogenase, cytochmore
bc
1
, and cytochrome c oxidase; Fig. 18.3) is embedded in a relatively H
þ
-impermeable
membrane [Schults and Chan, 2001; Hosler et al., 2006]. The respiratory enzymes
maintain the membrane-encapsulated space at a pH different from that on the outside
by two mechanisms. One involves active translocation of H
þ
from the less acidic to
the more acidic environment (proton pumps). The other relies on carrying out
proton-consuming and proton-releasing redox processes at the opposite site of the
membrane thanks to the proper location of catalytic moieties. As a result of relying
on the proton-motive force (PMF) rather than electron-motive force (EMF) as do bat-
teries and fuel cells, organisms use fairly reducing reductants for O
2
without losing
much free energy of the ORR. For example, at pH 7, the standard redox potential of
cytochrome c, which is the natural electron donor for cytochrome c oxidase, is
.500 mV more reducing that that of the O
2
/H
2
O couple; however, only 100 mV of
this difference is dissipated (see below).
Although it is highly exergonic, four-electron, four-proton reduction of O
2
to H
2
Ois
slow without catalysis. The inertness of O
2
arises from it being a very weak base,
having a low affinity for H atoms and an unfavorable one-electron reduction potential
in neutral aqueous media, and being a ground-state triplet [Taube, 1986]. This elec-
tronic configuration disfavors direct reactions between O
2
and most organic com-
pounds, which are ground-state singlets. This inertness of O
2
allows the existence of
reduced organic matter in the oxidizing atmosphere of modern Earth. However, it
also means that reduction of O
2
at rates and potentials that make the ORR useful
for energy metabolism requires catalysis. This catalysis is affected by heme-containing
enzymes generically called terminal oxidases, which are divided into two large
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