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

768 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
free energy per electron (kJ/mole)
150
100
50
0
ubiquinone
Q
cytochrome c
cytochrome c
reductase complex
X-ray crystallography has elucidated the structure of each of the three respiratory-chain
complexes in great detail, and we next examine each of them in turn to
see how they work.
The NADH Dehydrogenase Complex Contains Separate Modules
for Electron Transport and Proton Pumping
The NADH dehydrogenase complex is a massive assembly of membrane and
nonmembrane proteins that receives electrons from NADH and passes them to
ubiquinone. In animal mitochondria, it consists of more than 40 different protein
subunits, with a molecular mass of nearly a million daltons. The x-ray structures
of the NADH dehydrogenase complex from fungi and bacteria show that it
is L-shaped, with both a hydrophobic membrane arm and a hydrophilic arm that
projects into the mitochondrial matrix (Figure 14–20).
Electron transfer and proton pumping are physically separated in the NADH
dehydrogenase complex, with electron transfer occurring in the matrix arm and
proton pumping in the membrane arm. The NADH docks near the tip of the matrix
arm, where it transfers its electrons via a bound flavin mononucleotide to a string
of iron–sulfur clusters that runs down the arm, acting like a wire to carry electrons
to a protein-bound molecule of ubiquinone. Electron transfer to the quinone
is thought to trigger proton translocation in a set of proton pumps in the membrane
arm, and for this to happen the two processes must be energetically and
mechanically linked. A mechanical link is thought to be provided by a 6-nm long,
amphipathic α helix that runs parallel to the membrane surface on the matrix
side of the membrane arm. This helix may act like the connecting rod in a steam
engine to generate a mechanical, energy-transducing power stroke that links the
quinone-binding site to the proton-translocating modules in the membrane (see
Figure 14–20).
The reduction of each quinone by the transfer of two electrons can cause four
protons to be pumped out of the matrix into the crista space. In this way, NADH
dehydrogenase generates roughly half of the total proton-motive force in mitochondria.
Cytochrome c Reductase Takes Up and Releases Protons on the
Opposite Side of the Crista Membrane, Thereby Pumping Protons
As described previously, when a quinone molecule (Q) accepts its two electrons,
it also takes up two protons to form a quinol (QH 2 ; see Figure 14–17). In
C
cytochrome c
oxidase
complex
2 H + + ½ O 2 H 2 O
direction of electron flow
MBoC6 m14.29/14.19
_ 100
0
100
200
300
400
500
600
700
800
redox potential (mV)
NADH
Figure 14–19 Redox potential changes
NAD + _ along the mitochondrial electron-
H + 400
_ 300
200
NADH
dehydrogenase
_ 200
complex
H + H +
transport chain. The redox potential
(designated Eʹ0) increases as electrons flow
down the respiratory chain to oxygen. The
standard free-energy change in kilojoules,
∆G°, for the transfer of each of the two
electrons donated by an NADH molecule
can be obtained from the left-hand
ordinate [∆G° = –n(0.096) ∆Eʹ0, where
n is the number of electrons transferred
across a redox potential change of ∆Eʹ0
mV]. Electrons flow through a respiratory
enzyme complex by passing in sequence
through the multiple electron carriers in
each complex (blue arrows). As indicated,
part of the favorable free-energy change
is harnessed by each enzyme complex to
pump H + across the inner mitochondrial
membrane (red arrows). The NADH
dehydrogenase pumps up to four H + per
electron, the cytochrome c reductase
complex pumps two, whereas the
cytochrome c oxidase complex pumps one
per electron.
Note that NADH is not the only source
of electrons for the respiratory chain.
The flavin FADH 2 , which is generated
by fatty acid oxidation (see Figure 2–56)
and by succinate dehydrogenase in the
citric acid cycle (see Figure 2–57), also
contributes. Its two electrons are passed
directly to ubiquinone, bypassing NADH
dehydrogenase.