Chemistry Reference
In-Depth Information
Figure 10.10 Structure of the c-ring of F
o
from Ilyobacter tartaricus.
Among the 11 monomers that form the ring, four are shown
in red, orange, yellow, and green. Sodium ions (Na
รพ
) are shown
in blue.
Girvin proposed that (de)protonation of the critical asparagine acid residue of the c
subunit induces the swiveling of the helix at the interface with the a subunit and
generates the torque for rotation of the entire c-ring [60].
In contrast, the Brownian ratchet model does not assume the swiveling of the c
subunit as a driving force of rotation. Instead, this model assumes the presence of a
barrier for rotation at the interface between the a subunit and c-ring. The c-ring can
overcome this barrier and rotate only when the critical asparagine acid residue of the c
subunit is deprotonated. In the absence of the proton motive force, the c-ring
undergoes rotational Brownian movement [62], and the proton motive force biases
the direction of rotation by modulating the frequency of binding and release of
protons from the two aqueous phases separated by the membrane.
Recently, the power stroke model that assumes the rotation of the helix in the c
subunit has been challenged. The helix of the c subunit monomer from thermophilic
Bacillus PS3 did not swivel at a different pH [63]. Furthermore, the crystal structure of
the c-ring fromIlyobacter tartaricus (Figure 10.10) [64] did not support the swiveling of
the carboxyl-terminal helix of the c subunit since the ion-binding site was located at
the interface of the two monomers and near the outer surface of the ring, and ions
would be transferred to and from the a subunit without swiveling. In addition, the
crystal structure of the K-ring of V-ATPase from E. hirae did not support swiveling of
the helix [65]. At present, the Brownian ratchet model is more plausible, although
critical evidence is yet to be provided.
10.2.3.3 Rotation of F
o
F
1
Driven by the Proton Motive Force
In 2004, the rotation of F
o
F
1
driven by the proton motive force was
rst detected by
Diez et al.; they measured FRET between single donor and acceptor molecules
introduced into the
subunit of F
1
and the b subunit of F
o
[66]. They reconstituted
F
o
F
1
fromE. coli into a liposome and applied the protonmotive force generated by pH
transition and additional electric potential difference. This method revealed a
g
