Chemistry Reference
In-Depth Information
Figure 10.7 Mechanical activation of pausing F
1
by external force. (A) Schematic representation
of the experimental set-up. Orientation of the
of mechanical activation by pulling in the
backward direction. F
1
was not activated when
pulled and stalled at
20
,
40
, and
80
but
g
80
.
subunit was manipulated by rotating the
magnetic field. (B) Mechanical activation by
pushing in the forward direction. F
1
that lapsed
into the MgADP-inhibition state at 280
was
pushed at
was activated when pushed and stalled at
þ
(D) Angle dependence of the probability of
activation. MgADP-inhibited F
1
was pushed or
pulled and stalled for 3 s in the presence of
0.2
40
and
stalled for 3 s (red). After being pushed and
stalled at
10
,
20
,
30
, and
þ
þ
þ
þ
m
M ATP (blue); 0.2
m
M ATP and 30mM Pi
(green); 0.2
m
M ATP and 1mM ADP (red); and
40
,F
1
restarted its rotation. (C) Trial
þ
0.2
m
M ATP, 30mM Pi, and 1mM ADP (purple).
the tight mechanochemical coupling between the rotation of the
g
subunit and the
chemical reaction on the catalytic
subunits.
The most important result obtained from this experiment is that the angle of the
b
g
subunit determines the rates of ADP dissociation from the catalytic
subunit. This
angle dependency is expected to be applicable to ADP release during the rotation of
active F
1
. Our preliminary results indicate that not only the rate of ADP release but
also that of ATP binding and hydrolysis are also angle dependent. The angle
dependency of the af
nity of ATP is also suggested by the constant torque generation
(40 pNnm) by ATP binding, thereby indicating a linear downhill potential with a slope
of 1.7 k
B
T/10
which corresponds to the af
nity being increased by a factor of 5.5 per
10
. The angle dependency of the rate of each chemical reaction step appears to be the
fundamental mechanism that supports the unidirectional rotation of F
1
. This
b
