Chemistry Reference
In-Depth Information
communication). The
subunit in the F
o
F
1
complex is also likely to bind ATP since
the binding site is outside the contact interfaces between the
e
and other subunits.
Considering the above-mentioned results, the conformational dynamics and states
of the
e
subunit are likely to affect the coupling ratio of ATP synthesis. The rotational
direction of the
e
subunit,
may affect its conformation and function under non-equilibriumconditions. Further
insights will be gained by single-molecule FRETmeasurement of the conformations
of the carboxyl-terminal helices of the
ge
c
10
-
15
complex, i.e. the direction of force applied to the
e
e
subunit during forced rotation of F
1
.With
regard to the amino-terminal
subunit in F
o
F
1
from
Escherichia coli, no conformational change was detected between ATP synthesis and
hydrolysis conditions [48].
b
-sandwich moiety of the
e
10.2.3
Rotation of F
o
F
1
or F
o
10.2.3.1 Steps in the Rotation of F
o
F
1
driven by ATP hydrolysis
When the rotation of F
o
F
1
from thermophilic Bacillus PS3 solubilized with a
detergent was driven by ATP hydrolysis, 80
and 40
substeps were observed [49].
F
o
from thermophilic Bacillus PS3 has a subunit structure of ab
2
c
10
. The rotor c-ring
consists of 10 c subunits and has 10 times symmetry [50]. However, the smaller
substeps re
ecting the structure of the c-ring (36
for c
10
) were not observed. This
indicates that the friction between the rotor and stator in F
o
is not rate-limiting and
does not slow down the ATP-driven rotation.
The number of c subunits in a ring varies among species, and a total of 10, 11, 14,
and 15 subunits have been reported [50
-
54]. Each monomer has a single proton (or
sodium ion)-binding site, and the number of c subunits in a ring determines the
number of ions transported per turn. Since three ATPmolecules are synthesized per
turn in the F
1
component, the stoichiometry of ions/ATP becomes 3.3, 3.7, 4.7, and
5.0. Therefore, except for one example, most species exhibit non-integer stoichiome-
try between ions and ATP. This non-integer ratio or symmetry mismatch between F
1
and F
o
may have advantage for the smooth rotation because such mismatch would
avoid the formation of deep potential minima. Furthermore, the importance of elastic
power transmission between F
1
and F
o
was suggested for the smooth rotation under
the symmetry mismatch condition [55, 56].
10.2.3.2 Ratchet versus Power Stroke as the Driving Force of F
o
Rotation
In the case of F
1
, it is widely accepted that the conformational changes in the
b
subunits coupled to the ATP hydrolysis reaction generate the torque for rotation of
the
subunit. With regard to the rotational mechanism of F
o
, there are two
controversial models, namely, power stroke and Brownian ratchet [57
g
59]. The
power stroke model assumes the conformational change of the c subunit in the rotor
c-ring. This model was proposed based on the result of structural analysis by
NMR [60, 61]. In the c subunit monomer from E. coli dissolved in a water/
methanol/chloroform mixed solvent, one of the two
-
-helices of the c subunit
swiveled relative to the other at a different pH. Based on this result, Rastogi and
a
