2.10 Problems in Prevailing View on Functional Expression of a Molecular Motor
25
myosin and F-actin plays essential roles in their behavior. During the movement of
myosin along F-actin, the PMVs of myosin and F-actin do not change much and PV
~ 0 (P = 1 atm), with the result that the system performs essentially no mechanical
work. The concept, “myosin must perform mechanical work against the viscous
resistance force by water during the movement”, is incorrect. It is water that drives
myosin to move along F-actin [14, 44, 47]. The unidirectional movement of myosin
along F-actin, which is coupled with the ATP hydrolysis reaction (an irreversible
process accompanied by a decrease in system free energy), spontaneously occurs.
Taken together, the prevailing view, which distinguishes the functional expression of a
molecular motor from self-assembly processes such as protein folding, is problematic
and must be reconsidered.
2.11 Inconsistency of Prevailing View with Some of Recent
Experimental Facts
The prevailing view on the functional expression of a molecular motor is problematic
as theoretically pointed out in Sect. 2.10. What is worse, it does not coincide with
some of recent experimental observations. Iwaki et al. [48] experimentally studied
the unidirectional movement of myosin along F-actin in aqueous solution to which
sucrose was added. First, a sucrose concentration of 1 mol/L was tested. By this
sucrose addition, the viscosity of aqueous solution became about six times higher and
the viscous resistance force by water became about six times stronger. If the prevailing
view was correct, the movement of myosin would be stopped or remarkably affected.
The result observed was that the sucrose addition has virtually no effects on the
myosin movement. When the sucrose concentration was increased to 2 mol/L, the
myosin movement was stopped. However, this stop was shown to be attributable
to significantly stronger binding of myosin to F-actin [48]. In water, myosin gets
detached from F-actin upon the ATP binding to myosin. In aqueous solution of
sucrose at 2 mol/L, on the other hand, the ATP binding does not lead to the detachment
with the result that the myosin movement is stopped.
Addition of a highly hydrophilic cosolvent such as sucrose increases the packing
fraction of aqueous solution. Therefore, the addition of sucrose enlarges the waterentropy effect: It enhances the thermostability of the folded state of a protein [49]
but never hinders protein folding. In the case of actomyosin, the entropic potential
field acting on myosin along the y-axis (see Fig. 2.11) due to the presence of Factin, Φ F-actin (y), is qualitatively similar to Φ Wall (h) shown in Fig. 2.6b. In water
containing no sucrose, myosin without ATP bound cannot overcome the free-energy
barrier by the thermal fluctuation for getting detached from F-actin [47]. Upon the
ATP binding to myosin, the structure of myosin and Φ F-actin (y) change with the result
of the reduction in the barrier, and myosin can readily get detached from F-actin [47].
However, the amplitudes of Φ F-actin (y) and the barrier are made larger by the sucrose
25
myosin and F-actin plays essential roles in their behavior. During the movement of
myosin along F-actin, the PMVs of myosin and F-actin do not change much and PV
~ 0 (P = 1 atm), with the result that the system performs essentially no mechanical
work. The concept, “myosin must perform mechanical work against the viscous
resistance force by water during the movement”, is incorrect. It is water that drives
myosin to move along F-actin [14, 44, 47]. The unidirectional movement of myosin
along F-actin, which is coupled with the ATP hydrolysis reaction (an irreversible
process accompanied by a decrease in system free energy), spontaneously occurs.
Taken together, the prevailing view, which distinguishes the functional expression of a
molecular motor from self-assembly processes such as protein folding, is problematic
and must be reconsidered.
2.11 Inconsistency of Prevailing View with Some of Recent
Experimental Facts
The prevailing view on the functional expression of a molecular motor is problematic
as theoretically pointed out in Sect. 2.10. What is worse, it does not coincide with
some of recent experimental observations. Iwaki et al. [48] experimentally studied
the unidirectional movement of myosin along F-actin in aqueous solution to which
sucrose was added. First, a sucrose concentration of 1 mol/L was tested. By this
sucrose addition, the viscosity of aqueous solution became about six times higher and
the viscous resistance force by water became about six times stronger. If the prevailing
view was correct, the movement of myosin would be stopped or remarkably affected.
The result observed was that the sucrose addition has virtually no effects on the
myosin movement. When the sucrose concentration was increased to 2 mol/L, the
myosin movement was stopped. However, this stop was shown to be attributable
to significantly stronger binding of myosin to F-actin [48]. In water, myosin gets
detached from F-actin upon the ATP binding to myosin. In aqueous solution of
sucrose at 2 mol/L, on the other hand, the ATP binding does not lead to the detachment
with the result that the myosin movement is stopped.
Addition of a highly hydrophilic cosolvent such as sucrose increases the packing
fraction of aqueous solution. Therefore, the addition of sucrose enlarges the waterentropy effect: It enhances the thermostability of the folded state of a protein [49]
but never hinders protein folding. In the case of actomyosin, the entropic potential
field acting on myosin along the y-axis (see Fig. 2.11) due to the presence of Factin, Φ F-actin (y), is qualitatively similar to Φ Wall (h) shown in Fig. 2.6b. In water
containing no sucrose, myosin without ATP bound cannot overcome the free-energy
barrier by the thermal fluctuation for getting detached from F-actin [47]. Upon the
ATP binding to myosin, the structure of myosin and Φ F-actin (y) change with the result
of the reduction in the barrier, and myosin can readily get detached from F-actin [47].
However, the amplitudes of Φ F-actin (y) and the barrier are made larger by the sucrose