3.8 Inverse Rotation Compelled by External Torque Imposed …
55
PS3 that even under the solution condition that the ATP hydrolysis reaction should
occur, the application of sufficiently strong external torque to the γ subunit using an
advanced technique induces the inverse rotation accompanied by the occurrence of
ATP synthesis reaction. This intriguing behavior can be explicated by the competition
of the entropic force by water driving the normal rotation and the external force
driving the inverse rotation.
As discussed in Sects. 3.3.2–3.3.4, for the packing structure of the α 3 β 3 complex
given, the water entropy is strongly dependent on the orientation of the γ subunit.
Without the application of the external torque, the γ subunit changes its orientation
in accordance with the change in packing structure of the α 3 β 3 complex, which is
induced by the ATP hydrolysis cycle. In the case where the inverse rotation is forcibly
executed, on the other hand, the α 3 β 3 complex changes its packing structure in
accordance with the orientational change of the γ-subunit. Importantly, the packingstructure change of the α 3 β 3 complex is accomplished by changing the chemical
compounds bound to the three β subunits.
When the inverse rotation of the γ subunit is forcibly executed, state change
(a)→(b) (states (a) and (b) are the same) illustrated in Fig. 3.22 occurs for the
following reason. The hydration entropies of the α 3 β 3 γ complex in states (a) and
(b) are the same, and state change (a)→(b) does not give rise to a water-entropy
loss (factor 1). It should be emphasized that the solution is under the condition that
the ATP hydrolysis reaction should occur. Hence, during state change (a)→(b), an
increase in system free energy is caused by the ADP and Pi binding to, ATP synthesis
in, and ATP dissociation from the α 3 β 3 γ complex (factor 2). In state change (a)→(c)
or (a)→(d) illustrated in Fig. 3.23, on the other hand, an acceptably large loss of
water entropy is unavoidable (factor 1). A decrease in system free energy is caused
by the ATP binding to, ATP hydrolysis in, and ADP and Pi dissociation from the
α 3 β 3 γ complex during state change (a)→(c), and no change in system free energy
Closely packed
Closely packed
Moderately packed
Moderately packed
Loosely packed
Loosely packed
State (a)
State (b)
: Closely packed interface
Fig. 3.22 Inverse rotation of γ subunit forcibly executed: state change (a)→(b) during which the
ADP and Pi binding to, ATP synthesis in, and ATP dissociation from the α 3 β 3 γ complex take place
(the ATP synthesis occurs). The water entropy remains constant (i.e., the water entropy already
maximized is retained)
55
PS3 that even under the solution condition that the ATP hydrolysis reaction should
occur, the application of sufficiently strong external torque to the γ subunit using an
advanced technique induces the inverse rotation accompanied by the occurrence of
ATP synthesis reaction. This intriguing behavior can be explicated by the competition
of the entropic force by water driving the normal rotation and the external force
driving the inverse rotation.
As discussed in Sects. 3.3.2–3.3.4, for the packing structure of the α 3 β 3 complex
given, the water entropy is strongly dependent on the orientation of the γ subunit.
Without the application of the external torque, the γ subunit changes its orientation
in accordance with the change in packing structure of the α 3 β 3 complex, which is
induced by the ATP hydrolysis cycle. In the case where the inverse rotation is forcibly
executed, on the other hand, the α 3 β 3 complex changes its packing structure in
accordance with the orientational change of the γ-subunit. Importantly, the packingstructure change of the α 3 β 3 complex is accomplished by changing the chemical
compounds bound to the three β subunits.
When the inverse rotation of the γ subunit is forcibly executed, state change
(a)→(b) (states (a) and (b) are the same) illustrated in Fig. 3.22 occurs for the
following reason. The hydration entropies of the α 3 β 3 γ complex in states (a) and
(b) are the same, and state change (a)→(b) does not give rise to a water-entropy
loss (factor 1). It should be emphasized that the solution is under the condition that
the ATP hydrolysis reaction should occur. Hence, during state change (a)→(b), an
increase in system free energy is caused by the ADP and Pi binding to, ATP synthesis
in, and ATP dissociation from the α 3 β 3 γ complex (factor 2). In state change (a)→(c)
or (a)→(d) illustrated in Fig. 3.23, on the other hand, an acceptably large loss of
water entropy is unavoidable (factor 1). A decrease in system free energy is caused
by the ATP binding to, ATP hydrolysis in, and ADP and Pi dissociation from the
α 3 β 3 γ complex during state change (a)→(c), and no change in system free energy
Closely packed
Closely packed
Moderately packed
Moderately packed
Loosely packed
Loosely packed
State (a)
State (b)
: Closely packed interface
Fig. 3.22 Inverse rotation of γ subunit forcibly executed: state change (a)→(b) during which the
ADP and Pi binding to, ATP synthesis in, and ATP dissociation from the α 3 β 3 γ complex take place
(the ATP synthesis occurs). The water entropy remains constant (i.e., the water entropy already
maximized is retained)