Chemistry Reference
In-Depth Information
Then two or three heads, dissociated from actin by its rotation, interact with the actin
filament and exert force on the actin
filament in the forward direction (Figure 2.10b
lower panel). Since the potential energy produced by two or three heads is suf
cient to
move the
first head at the potential bottom to the next forward helical pitch, the actin
filament is moved further. After that, if more than four heads interact with the actin
filament, the
filament is moved again. Thus, the actin
filament can potentially be
moved more than
60 nm per ATP by the cooperative action of multiple heads.
In order to test this model quantitatively, we performed computer simulations
describing the cooperative action betweenmultiplemyosin heads undergoing biased
Brownian motion along the actin helical pitches. Analogously, it has been demon-
strated that collective myosin motors connected by rigid bonds to a backbone can
conduct dynamic phase transitions, such as hysteretic behavior in sliding velocity
against external loads or spontaneous oscillations in a collective manner [75, 76]. In
our model, the long (
>
60 nm) sliding distance per ATP at zero load was success-
fully simulated by connecting each head to a backbone via a
flexible spring
(Figure 2.11) [25]. This means that muscle can modulate the sliding distance
(interacting distance) per ATP from nearly zero to
>
60 nm to economize energy
depending on loads. Thus, the stochastic nature of individual actomyosin motors is
important for the dynamic and adaptive operation of muscle because it can adapt to
the ever-changing demands of the muscle.
>
2.5
Conclusion for the Unique Mechanism of Biological Molecular Machines
Themyosinmotor does not overcome thermal
fluctuation (noise) but rather utilizes it
to operate. The origin of motion is Brownianmotion and the chemical energy released
from ATP hydrolysis is used to bias the Brownian motion or repeat the mechanical
cycle. Molecularmotors can thus operate at an energy level as small as average thermal
energy (
ΒΌ
k
B
T)withhighef
ciency of energy conversion. This is in sharp contrast to
3
When the actin filament is rotated by 90
[47], the
relative position between the actin helical
pitches and the myosin heads shifts by
approximately three actin monomers. The actin
slopes along the actin helical pitches are
represented by a color gradient. (b) Qualitative
explanation of the cooperative action of myosin
heads on a thick filament. Themyosin filament is
equivalently represented by a row of myosin
heads connected with springs at intervals of
43 nm. The actin filament is represented by
periodic, saw-tooth shape potentials along the
half helical pitches. Cooperative action of the
myosin heads leads to a long actin filament
sliding distance (>60 nm) per ATP (see text
for details).
Figure 2.10 Cooperative action of multiple
heads undergoing stochastic steps [25].
(a) Schematic diagrams of actin and myosin
filaments in skeletal muscle [80]. The actin
filament has a helical structure with a half pitch of
36 nm. The myosin filament also has a helical
structure with a pitch of 43 nm and a subunit
repeat of 14.3 nm. Myosin heads on a myosin
filament project toward an actin filament at
43-nm intervals. In skeletal muscle, the actin and
myosin filaments are arranged in a hexagonal
lattice with one actin surrounded by threemyosin
filaments. Therefore, the number of myosin
molecules that project toward one actin filament
0.7 mm long (length when fully overlapped with
myosin filaments) is approximately 50 [80].
