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ATP Synthase's Two-Step
MAUREEN ROUHI
By observing a gold bead attached to the rotor of the energy-converting enzyme ATP synthase, Japanese researchers have found that the rotor takes two steps per hydrolysis cycle [Nature, 410, 898 (2001)].
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IMAGE COURTESY OF RYOHEI YASUDA |
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The results will help "solve the mechanism of this highly efficient, reversible motor," says Ryohei Yasuda, a physicist at Teikyo University Biotechnology Center. Previously, Yasuda and coworkers had shown that the rotor (orange in figure) moves in 120š steps, coinciding with sequential ATP hydrolysis on three of the enzyme's so-called b-subunits (blue). They observed that motion by attaching an actin filament to the rotor. However, the friction created by the filament blurred the picture.
"We refined the system by using a smaller marker," says Yasuda, who is now at Cold Spring Harbor Laboratory. With a 40-nm gold bead attached through streptavidin (red) linked to biotin, friction drops to one ten-thousandth of that caused by actin, allowing a resolution on the order of 0.1 millisecond.
The motion of the gold bead, observed through laser dark-field microscopy, reveals that the 120š step is composed of a 90š substep driven by ATP binding and a 30š substep due to product release. These steps likely correspond to different conformations already revealed by structural studies.
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