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Figure 10.6 Models of chemo-mechanical coupling of F 1 . Top,
bi-site scheme. Middle, tri-site scheme. Bottom, an alternative
scheme. The nucleotide states of F 1 that may correspond to the
1994 and 2001 crystal structures are enclosed in squares.
10.2.2
Single-molecule Manipulation of F 1 Rotation
10.2.2.1 Mechanical Activation of Pausing F 1
As described above, the rotating F 1 occasionally lapses into the MgADP-inhibited
state at the position after an 80 substep [25]. Usually, this inhibition lasts for several
tens of seconds, and presumably, after the dissociation of the inhibitoryMgADP from
the catalytic
subunit of
MgADP-inhibited F 1 is pushed and stalled in the forward (rotational) direction by
using magnetic tweezers (Figure 10.7A and B), the time constant for activation
became shorter [33]. However, the same result was not observed when the
b
subunit, F 1 spontaneously restarts its rotation. When the
g
subunit
was pushed and stalled in the backward direction (Figure 10.7C). The probability of
activation after stalling was strongly dependent on the angle (Figure 10.7D), and the
rate of activation was increased exponentially. The Arrehenius activation energy of
the mechanical activation decreased to
g
1.3 k B T/10 . This strongly suggests that the
mechanical energy input through the
subunit shifted the energy state of the
MgADP-inhibited F 1 to a higher state and lowered the activation energy. The potential
of the MgADP-inhibited F 1 , which was estimated from the Brownian motion of the
beads attached to the
g
20 and had a slope of 1.5 k B T/10 . This
energy increment with the angle coincides well with the decrement in activation
energy, suggesting that 85% of the mechanical energy input was transferred to the
catalytic site to weaken the binding energy of MgADP. This result is consistent with
g
subunit, was linear at
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