occurs in a concerted manner to yield two molecules of
water:
4e~ + 0
2
+ 4H+ -+ 2H20
More than 90% of metabolic oxygen is consumed in the
cytochrome oxidase reaction. Oxygen contains an uncon-
ventional distribution of its two valence electrons. These
two electrons occupy different orbitals and are not spin
paired; thus oxygen is a diradical. The reduction of an
oxygen molecule with less than four electrons results in
the formation of an active oxygen species. One electron
transfer yields Superoxide radical (O2-) and the two elec-
tron transfer yields hydrogen peroxide (H
2
O
2
). Hydroxyl
free radical (HO') formation can take place from hydro-
gen peroxide in the presence of ferrous iron or cuprous
chelates. Both 0
2
and HO' free radicals are cytotoxic
oxidants. In the mitochondrial electron transport system,
leakage of electrons at any one of the redox-centers due
to aging or pathological conditions results in the forma-
tion of superoxide. Antioxidant enzymes, namely super-
oxide dismutases (SOD), catalase, and glutathione per-
oxidase participate in the elimination of toxic oxygen
metabolites. Three different SODs are present in human
cells; they are located in mitochondria, cytosol and ex-
tracellular fluid. The importance of mitochondrial SOD
(labeled as SOD2), which is a manganese containing
enzyme, is exemplified in the homozygous SOD2 knock-
out mice. These SOD2 knockout mice have low birth
weights and they die shortly after their birth from dilated
cardiomyopathy.
Redox
reactions
are
a
required
part
of
normal
metabolism. In neutrophils, for example, the killing of
the invading microorganisms requires reactive oxygen
metabolites (discussed later and also see Chapter 16).
Oxidants also are involved in gene expression (e.g., the
variety of protein kinases) and in the regulation of redox
homeostasis. Perturbation of redox homeostasis causes
oxidative stress and may contribute to chronic inflam-
matory diseases and malignancy. Cytochrome oxidase
in inhibited by cyanide (CN- ; see Chapter
6
), carbon
monoxide (CO), and azide (NJ).
Organization of the Electron Transport Chain
The arrangement of components of the electron trans-
port chain was deduced experimentally. Since electrons
pass only from electronegative systems to electropositive
systems, the carriers react according to their standard re-
dox potential (Table 14-2). Specific inhibitors and spectro-
scopic analysis of respiratory chain components are used
to identify the reduced and oxidized forms and also aid in
the determination of the sequence of carriers.
256
chapter 14
Electron Transport and Oxidative Phosphorylation
TABLE 14-2
Standard Oxidation-Reduction Potential (E° ) o f Compo-
nents of the Electron-Transport Chain
Redox Component
E°'(in volts)
NADH/NAD+
-0.32
FMNH2/FMN (of NADH dehydrogenase)
-
0 . 1 1
CoQH
2
/CoQ
+
0 . 1 0
Cyt b (red)/Cyt b (ox)
+
0 . 1 2
CytCj (red)/CytCj (ox)
+0.23
Cyt c (red)/Cyt c (ox)
+0.25
Cyt aa
3
(red)/Cyt aa
3
(ox)
+0.29
I
0
2 2
- / i °
2
(orH
2 0
/ i
0
2)
+0.82
Complexes I-IV of the respiratory chain are organized
asymmetrically in the inner membrane (Figure 14-11) as
follows:
1. The flavin prosthetic groups of NADH dehydrogenase
and succinate dehydrogenase face the
M
side of the
membrane;
2. CoQ and cytochrome b of complex III are probably
inaccessible from either side of the membrane;
3. Cytochrome c interacts with cytochrome c
1
and
cytochrome a, all located on the C side;
Inter-
Inner
Matrix
membrane
membrane
(M-side)
O
C
i X D
O
C
^ =
0
FIGURE 14-11
Orientation of the components of the electron transport complexes within
the inner mitochondrial membrane. Fe-S = Iron-sulfur center;
b
,
c, a , a,
and
a
3
= cytochromes; Cu = copper ion.
