Chemistry Reference
In-Depth Information
between actin and the myosin head, which may include the power stroke length. The
interacting length is determined by the distance between the start and end positions of
the actomyosin interaction. Thus, the displacement (interacting length) caused by a
single event is not necessarily proportional to the neck length. In the case of other
reports, the actin
filament is suspended in solution by dual optical traps and the actin
filament is able rotate [67]. Therefore, a head with a short neck may bind to actin only
whentheorientationof theactinbindingsiterotates intoa favorablepositionforbinding
and thus the interacting length is small. A headwith a long neckmay bind to actin even
when the orientation of actin is unfavorable because the long neck would have the
elasticity to reach theunfavorable binding site, thus leading toa large interacting length.
Tsiavaliaris and his colleagues [68] have demonstrated, using protein engineering,
that the orientation ofmovement is reversedwhen the orientation of the neck domain
is reversed. They have argued that this result strongly supports the lever arm
swinging model, but this result is also consistent with our model because the
direction of the myosin binding sites relative to the actin helical pitch is reversed
and so the potential slope is also reversed.
2.4.3
Computer Simulation: from a Single Molecular Motor to Muscle
How does the stochastic nature of individual myosinmotors contribute to the
flexible
operation of muscle? To answer this question, we performed a computer simulation
of the cooperative behavior between multiple myosin motors. We assumed that
multiple myosins are bound to ADP and Pi during most of the ATPase cycle; are
tethered to a myosin
filament via elastic elements (neck domain and S2)
(Figure 2.10a,b); and move along the actin helical pitches due to a potential slope
generated by the steric compatibility (Figure 2.8a). At some point in time, one of these
heads releases Pi to forma rigor complex with actin (Figure 2.10b upper panel). It has
been demonstrated that the actin
71]
and during force generation in muscle [46, 47]. Since the actin
filament is rotated by
approximately 90
in muscle [47], we assume that the actin
filament is rotated by 90
due to the formation of a rigor complex (Figure 2.10b upper panel). Then, an ATP
molecule binds to the rigor head to dissociate it from actin and the actin
filament
rewinds to its original orientation because one end of it is
fixed to the z-line
(Figure 2.10b middle panel). ADP-Pi-bound myosin heads near the bottom end of
the potential slope dissociate from actin. The unbound myosin will bind to ATP,
which is then hydrolyzed into ADP-Pi. The new ADP-Pi myosin heads bind to new
actinmonomers at the top of the potential slope. Since the potential slope is shifted by
about three actin monomers
-
corresponding to a 90
o
rotation in the actin
filament
-
the heads previously located at the potential bottom canmove the actin
filament once
more now that they are at a higher position on the potential slope (Figure 2.10b
middle and lower panels, vertical arrows). The energy for rotating one end of a 1-
filament is rotated during sliding in vitro [69
-
m
long actin
filament by 90
when the other end is
fixed to the z-line is estimated to be
16
-
32 k
B
T based on its torsional rigidity (2.6
-
6.7
m
10
26
Nm
2
) [67]. This is similar to
the free energy (20 k
B
T) produced by the hydrolysis of one ATP molecule.
