Chapter 1
Introduction
Abstract In the literature, we frequently come across such routine expressions as
“an ATP-driven molecular motor converts chemical energy to mechanical work”
and “a molecular motor realizes the reversible chemo-mechanical coupling”. In the
prevailing view, it is postulated that the molecular motor must perform mechanical
work against the viscous resistance force by water during its functional expression
such as the unidirectional movement and rotation. Namely, the molecular motor (a
protein or protein complex) and the aqueous solution in which it is immersed are
regarded as the system of interest and the external system, respectively. However,
this regard is misleading. The system of interest comprises not only the molecular
motor but also the aqueous solution, and the molecular motor performs essentially no
mechanical work because the system volume does not change much and the system
pressure is only 1 atm during its functional expression. We propose a new view
pointing out that the force moving the protein or rotating a protein in the complex is
generated by water. The entropic force originating from the translational displacement of water molecules in the whole system plays a pivotal role. As an example,
we discuss the mechanism of unidirectional rotation of the central shaft (i.e., the γ
subunit) in F 1 -ATPase on the basis of the new view.
Keywords Molecular motor · ATP-driven protein · Chemo-mechanical coupling ·
Linear motor · Unidirectional movement · Rotary motor · Unidirectional rotation
There is an interesting class of molecular motors which functions by utilizing the ATP
hydrolysis cycle comprising the ATP binding to the molecular motor, ATP hydrolysis
(ATP + H 2 O → ADP + Pi), and dissociation of ADP and Pi from the molecular
motor. Typical examples are a linear molecular motor, myosin, which performs unidirectional movement along filamentous actin (F-actin) and a rotary molecular motor,
F 1 -ATPase, in which its central shaft (i.e., the γ subunit) performs unidirectional
rotation. The molecular motor is often referred to as the “ATP-driven protein or
protein complex”. Unfortunately, the mechanism of its functional expression (e.g.,
the unidirectional movement and rotation) remains quite elusive.
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
M. Kinoshita, Mechanism of Functional Expression of F 1 -ATPase,
SpringerBriefs in Molecular Science,
https://doi.org/10.1007/978-981-33-6232-1_1
1
Introduction
Abstract In the literature, we frequently come across such routine expressions as
“an ATP-driven molecular motor converts chemical energy to mechanical work”
and “a molecular motor realizes the reversible chemo-mechanical coupling”. In the
prevailing view, it is postulated that the molecular motor must perform mechanical
work against the viscous resistance force by water during its functional expression
such as the unidirectional movement and rotation. Namely, the molecular motor (a
protein or protein complex) and the aqueous solution in which it is immersed are
regarded as the system of interest and the external system, respectively. However,
this regard is misleading. The system of interest comprises not only the molecular
motor but also the aqueous solution, and the molecular motor performs essentially no
mechanical work because the system volume does not change much and the system
pressure is only 1 atm during its functional expression. We propose a new view
pointing out that the force moving the protein or rotating a protein in the complex is
generated by water. The entropic force originating from the translational displacement of water molecules in the whole system plays a pivotal role. As an example,
we discuss the mechanism of unidirectional rotation of the central shaft (i.e., the γ
subunit) in F 1 -ATPase on the basis of the new view.
Keywords Molecular motor · ATP-driven protein · Chemo-mechanical coupling ·
Linear motor · Unidirectional movement · Rotary motor · Unidirectional rotation
There is an interesting class of molecular motors which functions by utilizing the ATP
hydrolysis cycle comprising the ATP binding to the molecular motor, ATP hydrolysis
(ATP + H 2 O → ADP + Pi), and dissociation of ADP and Pi from the molecular
motor. Typical examples are a linear molecular motor, myosin, which performs unidirectional movement along filamentous actin (F-actin) and a rotary molecular motor,
F 1 -ATPase, in which its central shaft (i.e., the γ subunit) performs unidirectional
rotation. The molecular motor is often referred to as the “ATP-driven protein or
protein complex”. Unfortunately, the mechanism of its functional expression (e.g.,
the unidirectional movement and rotation) remains quite elusive.
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
M. Kinoshita, Mechanism of Functional Expression of F 1 -ATPase,
SpringerBriefs in Molecular Science,
https://doi.org/10.1007/978-981-33-6232-1_1
1