  The effect of conformational fluctuations of modular macromolecules, such as
enzymes, on their diffusion properties is addressed using a simple generic
model of an asymmetric dumbbell made of two hydrodynamically coupled subunits.
It is shown that equilibrium fluctuations can lead to an interplay between the
internal and the external degrees of freedom and give rise to negative
contributions to the overall diffusion coefficient. Considering that this model
enzyme explores a mechanochemical cycle, we show how substrate binding and
unbinding affects its internal fluctuations, and how this can result in an
enhancement of the overall diffusion coefficient of the molecule. These
theoretical predictions are successfully confronted with recent measurements of
enzyme diffusion in dilute conditions using fluorescence correlation
spectroscopy.
