Chemistry Reference
In-Depth Information
[ATP], and the
subunit from
the chloroplast or cyanobacteria inhibits ATP hydrolysis activity regardless of [ATP]
and inhibited the ATP-driven rotation [37]. In the crystal structure, the
e
subunit rotates with the
g
subunit [19]. In contrast, the
e
e
subunit
stabilizes the residues of the
g
subunit protruding from the
a
b
3
ring and disordered
3
in the
subcomplex [38]. Since these stabilized residues include the contact
interfaces with the
a
b
g
3
3
b
subunits, this may support the highly ef
cient mechanical
coupling between the
g
subunit rotation and the conformational change of the
b
subunits, which results in a highly coupled ATP synthesis reaction.
Furthermore, recent studies on the
subunit suggested another possibility. Crystal
structures of the F
1
subcomplex from different species revealed that at the carboxyl-
terminus two
e
subunit assume different conformations, namely,
retracted (Figure 10.9, left) and partly-extended (Figure 10.9, right) [38
-
40]. These
results suggested that the
a
-helices of the
e
e
subunit was able to change its conformation. Conforma-
tional change of the
subunit was supported by chemical cross-linking studies, and
the alternative fully-extended and extended-hairpin conformations were also
suggested [41
-
44]. Cross-linking studies further revealed that F
1
and F
o
F
1
with the
e
e
subunit
fixed in partly- and fully-extended forms exhibited a signi
cant suppression
of ATP hydrolysis activity, but those with the retracted form of the
e
subunit did not.
In contrast, the extended forms of the
subunit did not affect the ATP synthesis
activity of F
o
F
1
driven by the proton motive force. Based on these results, it is
suggested that the
e
subunit acts as a switch that changes the F
o
F
1
activity between
ATP hydrolysis and synthesis mode [43]. In addition to the cross-linking experiment,
the measurement of FRET between the dye molecules introduced into the
e
g
and
e
subunits directly revealed that the
subunit in F
1
actually undergoes nucleotide-
dependent, large, reversible conformational change [45]. Furthermore, the isolated
e
e
subunit can bind ATP with relatively low af
nity [46, 47], and the bound ATP
stabilizes the retracted form of the isolated
e
subunit (H. Yagi et al., personal
Figure 10.9 Structural models of the
subunit in F
1
.F
1
from
Escherichia coli (right) and bovinemitochondria (left) are shown in
blue. Here, the
e
d
subunit of mitochondrial F
1
is referred to as
e
because it is equivalent to the bacterial
e
subunit. The positions of
carboxyl-terminal residues of the
e
subunit are indicated by
arrowheads.
