of a phosphate -- releases from the
ATP hydrolysis makes the switch mechanism irreversible. Though ATP normally
provides energy for macromolecular synthesis, Fox argues that in motor proteins
ATP performs a switching role, changing the protein conformation and its binding
The unbound head -- just 5-7 nanometers in diameter -- is moved about randomly
by Brownian motion in the cellular fluid until it encounters a new site where
it can bind. Reported in the early 1800s by biologist Robert Brown, Brownian
motion is the irregular activity of tiny particles suspended in a fluid. It
results from the thermally driven movement of molecules in a fluid, the velocity
of the particles depending on the temperature temperature.
Because of structural limits in the kinesin and spacing of binding sites
on the microtubules, the moving head can reach only one possible binding site
-- 8 nanometers past the bound head, which temporarily remains attached to
The head binds to the new site, moving the kinesin and its cargo about
8 nanometers along the microtubule.
The process quickly starts anew with the original two heads in interchanged
"Normally, Brownian motion cannot do anything concerted or with directionality,
because it is random," Fox explained. "But what happens here is a random process
in a system that has asymmetric boundary conditions created by the ATP switching.
That makes it possible to get a net directed motion along the microtubule."
The model described by Fox and post-doctoral colleague Mee Hyang Choi depends
on two unique properties of structures at the nanometer-scale: thermal energy
can be a robust source of power, and random motion occurs very rapidly.
"Normally, we would tPage: 1 2 3 4 Related biology news :1
Contact: John Toon
Georgia Institute of Technology Research News
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