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

780 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
ADP
ATP
CYTOSOL
inhibitor in ATPor
ADP-binding site
ADP
ATP
ADP/ATP
carrier
protein
INTERMEMBRANE
SPACE
ADP
ATP
MATRIX
(A)
ATP synthase
ADP
+
P i
ATP
inner
mitochondrial
membrane
(B)
Figure 14–34 The ADP/ATP carrier protein. (A) The ADP/ATP carrier protein is a small
membrane protein that carries the ATP produced on the matrix side of the inner membrane to
the intermembrane space, and the ADP that is needed for ATP synthesis into the matrix. (B) In
the ADP/ATP carrier, six transmembrane α helices define a cavity that binds either ADP or ATP. In
this x-ray structure, the substrate is replaced by a tightly bound inhibitor instead (colored). When
ADP binds from outside the inner membrane, it triggers a conformational change and is released
into the matrix. In exchange, a molecule of ATP quickly binds to the matrix side of the carrier
and is transported to the intermembrane space. From there the ATP diffuses through the outer
mitochondrial membrane to the cytoplasm, where it powers the energy-requiring processes in the
cell. (B, PDB code: 1OKC.)
MBoC6 n14.317/14.34
mitochondrial membrane contains about 20 related carrier proteins exchanging
various other metabolites, including the phosphate that is required along with
ADP for ATP synthesis.
In some specialized fat cells, mitochondrial respiration is uncoupled from ATP
synthesis by the uncoupling protein, another member of the mitochondrial carrier
family. In these cells, known as brown fat cells, most of the energy of oxidation is
dissipated as heat rather than being converted into ATP. In the inner membranes
of the large mitochondria in these cells, the uncoupling protein allows protons
to move down their electrochemical gradient without passing through ATP synthase.
This process is switched on when heat generation is required, causing the
cells to oxidize their fat stores at a rapid rate and produce heat rather than ATP.
Tissues containing brown fat serve as “heating pads,” helping to revive hibernating
animals and to protect newborn human babies from the cold.
Chemiosmotic Mechanisms First Arose in Bacteria
Bacteria use enormously diverse energy sources. Some, like animal cells, are aerobic;
they synthesize ATP from sugars they oxidize to CO 2 and H 2 O by glycolysis,
the citric acid cycle, and a respiratory chain in their plasma membrane that is
similar to the one in the inner mitochondrial membrane. Others are strict anaerobes,
deriving their energy either from glycolysis alone (by fermentation, see Figure
2–47) or from an electron-transport chain that employs a molecule other than
oxygen as the final electron acceptor. The alternative electron acceptor can be a
nitrogen compound (nitrate or nitrite), a sulfur compound (sulfate or sulfite), or
a carbon compound (fumarate or carbonate), for example. A series of electron
carriers in the plasma membrane that are comparable to those in mitochondrial
respiratory chains transfers the electrons to these acceptors.
Despite this diversity, the plasma membrane of the vast majority of bacteria
contains an ATP synthase that is very similar to the one in mitochondria. In bacteria
that use an electron-transport chain to harvest energy, the electron-transport
chain pumps H + out of the cell and thereby establishes a proton-motive force
across the plasma membrane that drives the ATP synthase to make ATP. In other