Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE PROTON PUMPS OF THE ELECTRON-TRANSPOrt CHAIN
767
O
H 3 C
Figure 14–17 Quinone electron
QH 2
O CH 3
e – + H + O CH 3
e – + H + O CH 3
O CH 3
HO
O CH 3
HO
O CH 3
Q QH •
O H 3 C
O
H 3 C
O H
hydrophobic
hydrocarbon tail
ubiquinone
ubisemiquinone
ubiquinol
(free radical)
into artificial lipid bilayer vesicles and shown to pump protons across the membrane
as electrons pass through them.
In the mitochondrion, the three complexes are linked in series, serving as electron-transport-driven
H + pumps that pump protons out of the matrix to acidify
the crista space (Figure 14–18):
1. The NADH dehydrogenase complex (often referred to as Complex I) is
the largest of these respiratory enzyme complexes. It accepts electrons
from NADH and passes MBoC6 them through m14.24/14.17 a flavin mononucleotide and eight
iron–sulfur clusters to the lipid-soluble electron carrier ubiquinone. The
reduced ubiquinol then transfers its electrons to cytochrome c reductase.
2. The cytochrome c reductase (also called the cytochrome b-c 1 complex) is a
large membrane protein assembly that functions as a dimer. Each monomer
contains three cytochrome hemes and an iron–sulfur cluster. The complex
accepts electrons from ubiquinol and passes them on to the small, soluble
protein cytochrome c, which is located in the crista space and carries
electrons one at a time to cytochrome c oxidase.
3. The cytochrome c oxidase complex contains two cytochrome hemes and
three copper atoms. The complex accepts electrons one at a time from
cytochrome c and passes them to molecular oxygen. In total, four electrons
and four protons are needed to convert one molecule of oxygen to water.
We have previously discussed how the redox potential reflects electron affinities.
Figure 14–19 presents an outline of the redox potentials measured along the
respiratory chain. These potentials change in three large steps, one across each
proton-translocating respiratory complex. The change in redox potential between
any two electron carriers is directly proportional to the free energy released when
an electron transfers between them. Each complex acts as an energy-conversion
device by harnessing some of this free-energy change to pump H + across the inner
membrane, thereby creating an electrochemical proton gradient as electrons pass
along the chain.
CRISTA SPACE
inner
mitochondrial
membrane
MATRIX
10 nm
2 e –
NADH
H + H +
H +
cytochrome c
ubiquinone
C
Q
succinate
H +
dehydrogenase
H + H 2 O
citric
H + acid cycle
2 H + + ½ O 2
NAD +
NADH
dehydrogenase
cytochrome
c reductase
cytochrome
c oxidase
carriers. Ubiquinone in the lipid bilayer
picks up one H + (red) from the aqueous
environment for each electron (blue) it
accepts, in two steps, from respiratorychain
complexes. The first step involves
the acquisition of a proton and an electron
and converts the ubiquinone into an
unstable ubisemiquinone radical. In the
second step, it becomes a fully reduced
ubiquinone (called ubiquinol), which is
freely mobile as an electron carrier in the
lipid bilayer of the membrane. When the
ubiquinol donates its electrons to the next
complex in the chain, the two protons
are released. The long hydrophobic tail
(green) that confines ubiquinone to the
membrane consists of 6–10 five-carbon
isoprene units, depending on the organism.
The corresponding electron carrier in the
photosynthetic membranes of chloroplasts
is plastoquinone, which has almost the
same structure and works in the same way.
For simplicity, we refer to both ubiquinone
and plastoquinone in this chapter as
quinone (abbreviated as Q).
Figure 14–18 The path of electrons
through the three respiratory-chain
proton pumps. (Movie 14.3) The
approximate size and shape of each
complex is shown. During the transfer
of electrons from NADH to oxygen (blue
arrows), ubiquinone and cytochrome c
serve as mobile carriers that ferry electrons
from one complex to the next. During the
electron-transfer reactions, protons are
pumped across the membrane by each
of the respiratory enzyme complexes, as
indicated (red arrows).
For historical reasons, the three
proton pumps in the respiratory chain
are sometimes denoted as Complex I,
Complex III, and Complex IV, according
to the order in which electrons pass
through them from NADH. Electrons from
the oxidation of succinate by succinate
dehydrogenase (designated as Complex II)
are fed into the electron-transport chain in
the form of reduced ubiquinone. Although
embedded in the crista membrane,
succinate dehydrogenase does not pump
protons and thus does not contribute to
the proton-motive force; it is therefore not
considered to be an integral part of the
respiratory chain.