Essential Cell Biology 5th edition

Molecular Mechanisms of Electron Transport and Proton Pumping
473
O CH e – + H + e – + H +
3
O CH Figure 14–21 Quinones carry electrons
3
within the lipid bilayer. The quinone in the
mitochondrial electron-transport chain is
O
O CH 3
H O
O CH 3 called ubiquinone. It picks up one H + from
the aqueous environment for every electron
it accepts, and it can carry two electrons as
H 3 C
O
H 3 C
O H part of its hydrogen atoms (red). When this
reduced ubiquinone donates its electrons to
the next carrier in the chain, the protons are
hydrophobic
released. Its long, hydrophobic hydrocarbon
hydrocarbon tail
tail confines ubiquinone to the inner
mitochondrial membrane.
oxidized
reduced
ubiquinone
ubiquinone
to a respiratory complex, it can move within the complex by skipping
from one embedded metal ion to another ion with an even greater affinity
for electrons.
When electrons pass from one respiratory complex to the next, in contrast,
they are ferried by electron carriers that can diffuse freely within or
along the lipid bilayer. These mobile molecules pick up electrons from
one complex and deliver them to the next in line. In the mitochondrial
respiratory chain, for example, a small, hydrophobic molecule called
ECB5 e14.23/14.23
ubiquinone picks up electrons from the NADH dehydrogenase complex
and delivers them to the cytochrome c reductase complex (see Figure
14–14). A related quinone functions similarly during electron transport in
photosynthesis. A ubiquinone molecule can accept or donate either one
or two electrons, and it picks up one H + from water with each electron
that it carries (Figure 14–21). Its redox potential of +30 mV places ubiquinone
between the NADH dehydrogenase complex and the cytochrome QUESTION 14–6
c reductase complex in terms of its tendency to gain or lose electrons—
which explains why ubiquinone receives electrons from the former and At many steps in the electrontransport
chain, Fe ions are used
donates them to the latter (Figure 14–22). Ubiquinone also serves as the
entry point for electrons donated by the FADH 2 that is generated both as part of heme or FeS clusters to
during the citric acid cycle and from fatty acid oxidation (see Figures bind the electrons in transit. Why
13−11 and 13−12).
do these functional groups that
carry out the chemistry of electron
The redox potentials of different metal complexes influence where they transfer need to be bound to
are used along the electron-transport chain. Iron–sulfur centers have relatively
low affinities for electrons and thus are prominent in the electron why this is necessary.
proteins? Provide several reasons
carriers that operate in the early part of the chain. An iron–sulfur center
_ 400
_
NADH NAD +
300
redox potential (mV)
_ 200
_ 100
0
100
200
300
400
500
600
Q
NADH
dehydrogenase
complex
H + H +
ubiquinone
c
cytochrome c
reductase complex
H + H +
cytochrome c
oxidase complex
cytochrome c
100
80
60
40
20
free energy per electron (kJ/mole)
700
800
½ O 2 + 2
direction of electron flow
H 2 O
0
Figure 14–22 Redox potential increases
along the mitochondrial electrontransport
chain. The biggest increases in
redox potential occur across each of the
three respiratory enzyme complexes, which
allows each of them to pump protons.