Essential Cell Biology 5th edition

Mitochondria and Oxidative Phosphorylation
467
P
H + H + H + H + H + H + H +
H +
H + H +
H +
H + H +
H + H +
H + H + H + H +
INTERMEMBRANE SPACE
H + MATRIX
H + H +
F 0 rotor
H + H +
+ ADP
ATP
H + H +
H
ATP
+
P + ADP
stator
F 1 ATPase
(A) ATP SYNTHESIS
(B) ATP HYDROLYSIS
Figure 14–17 ATP synthase is a reversible
coupling device. The protein can either
(A) synthesize ATP by harnessing the
electrochemical H + gradient or (B) pump
protons against this gradient by hydrolyzing
ATP. The direction of operation (and of
rotation) at any given instant depends on
the net free-energy change (ΔG, discussed
in Chapter 3) for the coupled processes
of H + translocation across the membrane
and the synthesis of ATP from ADP and P i .
For example, if the electrochemical proton
gradient falls below a certain level, the
ΔG for H + transport into the matrix will
no longer be large enough to drive ATP
production; instead, ATP will be hydrolyzed
by the ATP synthase to rebuild the proton
gradient. A tribute to the activity of this
all-important protein complex is shown in
Movie 14.7.
mitochondrial matrix from the cytosol, while others, such as ATP, must be
transported in the opposite direction. Carrier proteins that bind some of
these molecules couple their transport ECB5 e14.17/14.17 to the energetically favorable flow
of H + into the matrix (see the “coupled transporters” in Figure 12−15).
Pyruvate and P i , for example, are each co-transported inward, along with
protons, as the protons move down their electrochemical gradient into
the matrix.
Other transporters take advantage of the membrane potential generated
by the electrochemical proton gradient, which makes the matrix side of
the inner mitochondrial membrane more negatively charged than the
side that faces the intermembrane space (see Figure 14−15). A special
antiport carrier protein exploits this voltage gradient to export ATP from
the mitochondrial matrix and to bring ADP in. This exchange allows the
ATP synthesized in the mitochondrion to be exported rapidly, which is
important for energizing the rest of the cell (Figure 14–18).
The electrochemical proton gradient is also required for the translocation
of proteins across the inner mitochondrial membrane and into the
matrix. As mentioned earlier, although mitochondria retain their own
genome—and synthesize some of their own proteins—most of the proteins
required for mitochondrial function are made in the cytosol and
must be actively imported into the organelle. We discuss this transport
process—which requires energy supplied by the electrochemical proton
gradient as well as ATP hydrolysis—in Chapter 15.
In eukaryotic cells, therefore, the electrochemical proton gradient is
used to drive both the generation of ATP and the transport of selected
metabolites and proteins across the inner mitochondrial membrane. In
bacteria, the proton gradient across the plasma membrane is similarly
used to drive ATP synthesis and metabolite transport. But it also serves
as an important source of directly usable energy: in motile bacteria, for
instance, the flow of protons into the cell drives the rapid rotation of the
bacterial flagellum, which propels the bacterium along (Movie 14.8).
QUESTION 14–4
The remarkable properties that
allow ATP synthase to run in either
direction allow the interconversion
of energy stored in the H + gradient
and energy stored in ATP to proceed
in either direction. (A) If ATP
synthase making ATP can be likened
to a water-driven turbine producing
electricity, what would be an
appropriate analogy when it works
in the opposite direction? (B) Under
what conditions would one expect
the ATP synthase to stall, running
neither forward nor backward?
(C) What determines the direction in
which the ATP synthase operates?
The Rapid Conversion of ADP to ATP in Mitochondria
Maintains a High ATP/ADP Ratio in Cells
As a result of the nucleotide exchange shown in Figure 14−18, ADP molecules—produced
by hydrolysis of ATP in the cytosol—are rapidly drawn
back into mitochondria for recharging, while the bulk of the ATP molecules