Chemistry Reference
In-Depth Information
10
Two Rotary Motors of ATP Synthase
Ryota Iino and Hiroyuki Noji
10.1
Introduction
10.1.1
ATP Synthase: a Significant and Ubiquitous Enzyme in the Cell
Adenosine triphosphate (ATP) is one of the most important compounds for living
organisms and serves as an energy carrier to drive many biological reactions that
support the activity of life. For example, in many enzymes, an energetically unfavor-
able chemical reaction is coupled to the energetically favorable ATP hydrolysis
reaction. ATP also powers many biological molecular machines such as molecular
motors, ion pumps, molecular chaperones, and proteasomes. ATP synthase is
responsible for the generation of ATP. It synthesizes ATP from adenosine diphos-
phate (ADP) and inorganic phosphate (Pi). ATP synthase is one of themost abundant
proteins found in nature from bacteria to plants and animals; however, its reaction
mechanism is unique and different from those of the other enzymes. ATP synthase
uses physical rotation of its own subunits for catalysis. Prior to the discovery of ATP
synthase as a rotary motor, the bacterial
flagella motor
-
a large complex of many
proteins
-
was the only known rotary motor.
ATP synthase is composed of two rotary motors, namely, F
1
and F
o
, and it is also
termed F
o
F
1
-ATP synthase (Figure 10.1, left) [1
-
3]. F
1
is a water-soluble component
and rotates upon ATP hydrolysis. When isolated, F
1
catalyzes only the ATP hydrolysis
reaction and is often termed F
1
-ATPase (Figure 10.1, lower right). Bacterial F
1
has the
subunit structure,
subunits have noncatalytic and catalytic
nucleotide-binding sites, respectively, and forma ring-shape structure. The
a
b
gde
. The
a
and
b
3
3
g
subunit
rotates in the
to F
o
,
respectively. Bacterial F
o
has the subunit structure, ab
2
c
10
-
15
, and is embedded in
the cell membrane (Figure 10.1, upper right). Although direct observation has not
yet been possible, it is widely accepted that the central c
10
-
15
ring rotates against
the peripheral ab
2
complex, with the proton
flow across the membrane driven by
a
b
3
ring, and the
d
and
e
subunits connect the
a
b
3
ring and
g
3
3
