44
3 Mechanism of Unidirectional Rotation of γ Subunit in F 1 -ATPase
Subcomplex III−γ possesses the highest packing efficiency. During the ATP
hydrolysis cycle, the packing of subcomplex III−γ becomes less efficient because
the chemical compound bound to the β subunit in this subcomplex changes from
ATP• • • H 2 O to ADP + Pi due to the ATP hydrolysis. Subcomplex I−γ is characterized by the lowest packing efficiency. During the ATP hydrolysis cycle, the
packing of subcomplex I becomes more efficient because the chemical compound
bound to the β subunit in this subcomplex changes from Pi to nothing due to the
dissociation of Pi. The chemical compound bound to the β subunit in subcomplex II
changes from ATP toward ATP• • • H 2 O, leading to its higher packing efficiency. As
explained in Sect. 3.4.2, in one ATP hydrolysis cycle, subcomplexes III−γ, II−γ,
and I−γ become loosely, closely, and moderately packed, respectively, resulting in
a structural rotation of the α 3 β 3 complex by 120° in the counterclockwise direction.
In response to the structural rotation of the α 3 β 3 complex, the γ subunit rotates by
120° in the same direction. Especially, it is crucial to recover the closely packed
interfaces between the γ subunit and the β subunit to which ATP• • •H 2 O is bound,
the β subunit named β DP , and between the γ subunit and the α subunit adjacent to
β DP in the counterclockwise direction, the α subunit named α DP .
3.4.2 Details of Rotation Mechanism
First, we summarize the experimentally available information on the rotational
behavior [9, 22–25] (see Fig. 3.15).
(1) The ATP hydrolysis occurs in β DP and Pi dissociates from β E , with the result that
the structure of β DP becomes half-open [9]. This structural change of β DP leads
to a 40° rotation of the γ subunit [22–25]. The β subunit with this half-open
structure is denoted by β
HO
DP . β TP and β E are renamed β’ TP and β’ E , respectively.
ADP + Pi are bound to β
HO
DP and nothing is bound to β’ E . The chemical compound
bound to β’ TP is ATP(ATP• • •H 2 O). Here, ATP(ATP• • •H 2 O) represents an
intermediate between ATP and ATP• • •H 2 O.
(2) ADP dissociates from β
HO
DP and ATP binds to β’ E , inducing an 80° rotation of the
γ subunit [22–25]. Changes of β
HO
DP →β E , β’ TP →β DP , and β’ E →β TP occur. The
α 3 β 3 γ complex now takes the structure that is the same as the structure before
the 40° rotation of the γ subunit.
We then discuss the rotation mechanism on the basis of the information summarized above and the basic concept explained in Sect. 3.4.1 by starting from the
catalytic dwell state shown as state (a) in Fig. 3.16. The ATP concentration is
sufficiently high and the ADP and Pi concentrations are sufficiently low (see
Sect. 2.1.1).
(1) The events which spontaneously occur in β DP , β E , β TP are the ATP hydrolysis,
dissociation of Pi, and change of ATP toward ATP• • •H 2 O (ATP→ATP(ATP• •
•H 2 O)), respectively. Each event leads to a decrease in system free energy under
3 Mechanism of Unidirectional Rotation of γ Subunit in F 1 -ATPase
Subcomplex III−γ possesses the highest packing efficiency. During the ATP
hydrolysis cycle, the packing of subcomplex III−γ becomes less efficient because
the chemical compound bound to the β subunit in this subcomplex changes from
ATP• • • H 2 O to ADP + Pi due to the ATP hydrolysis. Subcomplex I−γ is characterized by the lowest packing efficiency. During the ATP hydrolysis cycle, the
packing of subcomplex I becomes more efficient because the chemical compound
bound to the β subunit in this subcomplex changes from Pi to nothing due to the
dissociation of Pi. The chemical compound bound to the β subunit in subcomplex II
changes from ATP toward ATP• • • H 2 O, leading to its higher packing efficiency. As
explained in Sect. 3.4.2, in one ATP hydrolysis cycle, subcomplexes III−γ, II−γ,
and I−γ become loosely, closely, and moderately packed, respectively, resulting in
a structural rotation of the α 3 β 3 complex by 120° in the counterclockwise direction.
In response to the structural rotation of the α 3 β 3 complex, the γ subunit rotates by
120° in the same direction. Especially, it is crucial to recover the closely packed
interfaces between the γ subunit and the β subunit to which ATP• • •H 2 O is bound,
the β subunit named β DP , and between the γ subunit and the α subunit adjacent to
β DP in the counterclockwise direction, the α subunit named α DP .
3.4.2 Details of Rotation Mechanism
First, we summarize the experimentally available information on the rotational
behavior [9, 22–25] (see Fig. 3.15).
(1) The ATP hydrolysis occurs in β DP and Pi dissociates from β E , with the result that
the structure of β DP becomes half-open [9]. This structural change of β DP leads
to a 40° rotation of the γ subunit [22–25]. The β subunit with this half-open
structure is denoted by β
HO
DP . β TP and β E are renamed β’ TP and β’ E , respectively.
ADP + Pi are bound to β
HO
DP and nothing is bound to β’ E . The chemical compound
bound to β’ TP is ATP(ATP• • •H 2 O). Here, ATP(ATP• • •H 2 O) represents an
intermediate between ATP and ATP• • •H 2 O.
(2) ADP dissociates from β
HO
DP and ATP binds to β’ E , inducing an 80° rotation of the
γ subunit [22–25]. Changes of β
HO
DP →β E , β’ TP →β DP , and β’ E →β TP occur. The
α 3 β 3 γ complex now takes the structure that is the same as the structure before
the 40° rotation of the γ subunit.
We then discuss the rotation mechanism on the basis of the information summarized above and the basic concept explained in Sect. 3.4.1 by starting from the
catalytic dwell state shown as state (a) in Fig. 3.16. The ATP concentration is
sufficiently high and the ADP and Pi concentrations are sufficiently low (see
Sect. 2.1.1).
(1) The events which spontaneously occur in β DP , β E , β TP are the ATP hydrolysis,
dissociation of Pi, and change of ATP toward ATP• • •H 2 O (ATP→ATP(ATP• •
•H 2 O)), respectively. Each event leads to a decrease in system free energy under