  Routinely navigating through an ever-changing and unsteady environment, and
utilizing chemical energy, molecular motors transport the cell's crucial
components, such as neurotransmitters and organelles. They generate force and
pull cargo, as they literally walk along the polymeric tracts, e.g.
microtubules. However, using experimental data one may derive that the energy
needed for this pulling would take the most part of the 22 kT that ATP
hydrolysis makes available. In such a case there would not be sufficient energy
left to drive the conformational changes in the catalytic cycle of the protein.
Furthermore, the medium inside living cell is viscoelastic. Pulling cargo in
such an environment takes more energy than in aqueous buffer solution. Here we
propose a mechanism for the motor to more efficiently utilize chemical energy.
In our model the energy is used to ratchet the cargo forward. The motor no
longer pulls, but only holds a bead or a vesicle, allowing for Brownian motion
in a range limited by the elasticity of the motor-cargo-track system. The
consequence of such a mechanism is the dependency of motion not only on the
motor, but also on the cargo (especially it's size) and on the environment
(i.e. it's viscosity and structure). However, current experimental works rarely
provide this type of information for in vivo studies. We suggest that even
small differences between assays can impact the outcome. Our results agree with
those obtained in wet laboratories and provide novel insight in the mechanism
of a molecular motor's functioning.
