1 Introduction
3
F-actin [3, 4], and before the substrate insertion into ATP-binding cassette (ABC)
transporter [5, 6] and after the substrate release from it, respectively, the structures of
the α 3 β 3 γ complex, S1, and transporter are the same. (In this book, ABC transporter
is also considered as an ATP-driven molecular motor.) Therefore, it appears that the
stabilized structure (or the equilibrium state) must be destructed for the functional
expression. One might be inclined to think that energy or free energy is required
for the destruction. In the prevailing view, the following explanations are made: The
chemical energy stored in an ATP molecule or the free energy of ATP hydrolysis reaction (i.e., energy or free-energy supply by ATP) is utilized for the destruction; myosin
or the γ subunit must perform mechanical work against the viscous resistance force by
water for achieving the unidirectional movement or rotation [7, 8], and the energy or
free-energy supplied by ATP is converted to this work; and a molecular motor is characterized by its ability of reversible chemo-mechanical coupling implying that it can
transduce the chemical energy or free energy to the mechanical work and vice versa.
In the prevailing view, the molecular motor and the aqueous solution in which it is
immersed are regarded as the system of interest and the external system, respectively.
In this book, by pointing out that the prevailing view is physically problematic and
conflicting with some of recent experimental observations, we suggest a completely
new view. Its outline is as follows. The ATP hydrolysis reaction, an irreversible
process, is catalyzed by the molecular motor. The molecular motor is thus involved
in the ATP hydrolysis cycle referred to above. The functional expression of the molecular motor is accompanied by a decrease in system free energy and a spontaneously
occurring process. The system of interest comprises not only the molecular motor but
also the aqueous solution, and it performs essentially no mechanical work because
the system volume does not change much and the system pressure is only 1 atm
during the ATP hydrolysis cycle (see Sect. 2.10 for more details). The force which
moves myosin or rotates the γ subunit is generated by nothing but water. Especially,
the entropic force ascribed to the translational displacement of water molecules in
the whole system plays a pivotal role. The concept of reversible chemo-mechanical
coupling is physically irrelevant. Interestingly, the functional expression of a molecular motor is hindered by water in the prevailing view, whereas it is driven by water
in the new view. Choosing the unidirectional rotation of the γ subunit in F 1 -ATPase
as a paradigmatic example, we discuss its mechanism in detail.
The molecular motors were already considered in our earlier books [9, 10]. In the
first book [9], we summarized the water roles in functional expression of the molecular motors as an extension of biological self-assembly and ordering processes with
which we had been dealing (refer to our review articles [11–14]). In the second
book [10], by choosing actomyosin (i.e., myosin and F-actin), we raised questions
about the prevailing view on the mechanism of functional expression of the molecular motors. In this book, we explain how the prevailing view is problematic and
inconsistent with some of recent experimental observations in much more detail,
and construct a new view in a more complete and convincing form. Moreover, taking
the unidirectional rotation of the γ subunit in F 1 -ATPase as a paradigmatic example,
we show, for the first time, that the following four scenarios can be elucidated in a
unified manner within the same theoretical framework: (A) rotation in the normal
3
F-actin [3, 4], and before the substrate insertion into ATP-binding cassette (ABC)
transporter [5, 6] and after the substrate release from it, respectively, the structures of
the α 3 β 3 γ complex, S1, and transporter are the same. (In this book, ABC transporter
is also considered as an ATP-driven molecular motor.) Therefore, it appears that the
stabilized structure (or the equilibrium state) must be destructed for the functional
expression. One might be inclined to think that energy or free energy is required
for the destruction. In the prevailing view, the following explanations are made: The
chemical energy stored in an ATP molecule or the free energy of ATP hydrolysis reaction (i.e., energy or free-energy supply by ATP) is utilized for the destruction; myosin
or the γ subunit must perform mechanical work against the viscous resistance force by
water for achieving the unidirectional movement or rotation [7, 8], and the energy or
free-energy supplied by ATP is converted to this work; and a molecular motor is characterized by its ability of reversible chemo-mechanical coupling implying that it can
transduce the chemical energy or free energy to the mechanical work and vice versa.
In the prevailing view, the molecular motor and the aqueous solution in which it is
immersed are regarded as the system of interest and the external system, respectively.
In this book, by pointing out that the prevailing view is physically problematic and
conflicting with some of recent experimental observations, we suggest a completely
new view. Its outline is as follows. The ATP hydrolysis reaction, an irreversible
process, is catalyzed by the molecular motor. The molecular motor is thus involved
in the ATP hydrolysis cycle referred to above. The functional expression of the molecular motor is accompanied by a decrease in system free energy and a spontaneously
occurring process. The system of interest comprises not only the molecular motor but
also the aqueous solution, and it performs essentially no mechanical work because
the system volume does not change much and the system pressure is only 1 atm
during the ATP hydrolysis cycle (see Sect. 2.10 for more details). The force which
moves myosin or rotates the γ subunit is generated by nothing but water. Especially,
the entropic force ascribed to the translational displacement of water molecules in
the whole system plays a pivotal role. The concept of reversible chemo-mechanical
coupling is physically irrelevant. Interestingly, the functional expression of a molecular motor is hindered by water in the prevailing view, whereas it is driven by water
in the new view. Choosing the unidirectional rotation of the γ subunit in F 1 -ATPase
as a paradigmatic example, we discuss its mechanism in detail.
The molecular motors were already considered in our earlier books [9, 10]. In the
first book [9], we summarized the water roles in functional expression of the molecular motors as an extension of biological self-assembly and ordering processes with
which we had been dealing (refer to our review articles [11–14]). In the second
book [10], by choosing actomyosin (i.e., myosin and F-actin), we raised questions
about the prevailing view on the mechanism of functional expression of the molecular motors. In this book, we explain how the prevailing view is problematic and
inconsistent with some of recent experimental observations in much more detail,
and construct a new view in a more complete and convincing form. Moreover, taking
the unidirectional rotation of the γ subunit in F 1 -ATPase as a paradigmatic example,
we show, for the first time, that the following four scenarios can be elucidated in a
unified manner within the same theoretical framework: (A) rotation in the normal