  Mechanochemical coupling was studied for two different types of myosin motors
in cells: myosin V, which carries cargo over long distances by as a single
molecule; and myosin II, which generates a contracting force in cooperation
with other myosin II molecules. Both mean and variance of myosin V velocity at
various [ATP] obeyed Michaelis-Menten mechanics, consistent with tight
mechanochemical coupling. Myosin II, working in an ensemble, however, was
explained by a loose coupling mechanism, generating variable step sizes
depending on the ATP concentration and realizing a much larger step (200 nm)
per ATP hydrolysis than myosin V through its cooperative nature at zero load.
These different mechanics are ideal for the respective myosin's physiological
functions.
