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

778 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
through the second half-channel into the matrix. Thus proton flow causes the
rotor ring to spin against the stator like a proton-driven turbine.
The mitochondrial ATP synthase is of ancient origin: essentially the same
enzyme occurs in plant chloroplasts and in the plasma membrane of bacteria or
archaea. The main difference between them is the number of c subunits in the
rotor ring. In mammalian mitochondria, the ring has 8 subunits. In yeast mitochondria,
the number is 10; in bacteria and archaea, it ranges from 11 to 13; in
plant chloroplasts, there are 14; and the rings of some cyanobacteria contain 15 c
subunits.
The c subunits in the rotor ring can be thought of as cogs in the gears of a bicycle.
A high gear, with a small number of cogs, is advantageous when the supply
of protons is limited, as in mitochondria, but a low gear, with a large number of
cogs in the wheel, is preferable when the proton gradient is high. This is the case
in chloroplasts and cyanobacteria, where protons produced through the action of
sunlight are plentiful. Because each rotation produces three molecules of ATP in
the head, the synthesis of one ATP requires around three protons in mitochondria
but up to five in photosynthetic organisms. It is the number of c subunits in the
ring that defines how many protons need to pass through this marvelous device
to make each molecule of ATP, and thereby how high a ratio of ATP to ADP can be
maintained by the ATP synthase.
In principle, ATP synthase can also run in reverse as an ATP-powered proton
pump that converts the energy of ATP back into a proton gradient across the membrane.
In many bacteria, the rotor of the ATP synthase in the plasma membrane
changes direction routinely, from ATP synthesis mode in aerobic respiration, to
ATP hydrolysis mode in anaerobic metabolism. In this latter case, ATP hydrolysis
serves to maintain the proton gradient across the plasma membrane, which
is used to power many other essential cell functions including nutrient transport
and the rotation of bacterial flagella. The V-type ATPases that acidify certain cellular
organelles are architecturally similar to the F-type ATP synthases, but they
normally function in reverse (see Figure 13–37).
Mitochondrial Cristae Help to Make ATP Synthesis Efficient
In the electron microscope, the mitochondrial ATP synthase complexes can be
seen to project like lollipops on the matrix side of cristae membranes. Recent
studies by cryoelectron microscopy and tomography have shown that this large
complex is not distributed randomly in the membrane, but forms long rows of
dimers along the cristae ridges (Figure 14–32). The dimer rows induce or stabilize
these regions of high membrane curvature, which are otherwise energetically
unfavorable. Indeed, the formation of ATP synthase dimers and their assembly
into rows are required for cristae formation and have far-reaching consequences
for cellular fitness. By contrast with bacterial or chloroplast ATP synthases, which
do not form dimers, the mitochondrial complex contains additional subunits,
located mostly near the membrane end of the stator stalk. Several of these subunits
are found to be dimer-specific. If these subunits are mutated in yeast, the
ATP synthase in the membrane remains monomeric, the mitochondria have no
cristae, cellular respiration drops by half, and the cells grow more slowly.
(A)
(B)
Figure 14–31 F o ATP synthase rotor rings. (A) Atomic force microscopy
image of ATP synthase rotors from the cyanobacterium Synechococcus
elongatus in a lipid bilayer. Whereas 8 c subunits form the rotor in Figure
14–30, there are 13 c subunits in this ring. (B) The x-ray structure of the F o
ring of the ATP synthase from Spirulina platensis, another cyanobacterium,
shows that this rotor has 15 c subunits. In all ATP synthases, the c subunits
are hairpins of two membrane-spanning α helices (one subunit is highlighted
in gray). The helices are highly hydrophobic, except for two glutamine
and glutamate side chains (yellow) that create proton-binding sites in the
membrane. (A, courtesy of Thomas Meier and Denys Pogoryelov; B, PDB
code: 2WIE.)
c subunit