  Molecular agitation more rapid than thermal Brownian motion is reported for
cellular environments, motor proteins, synthetic molecular motors, enzymes, and
common chemical reactions, yet that chemical activity couples to molecular
motion contrasts with generations of accumulated knowledge about diffusion at
equilibrium. To test the limits of this idea, a critical testbed is mobility of
catalytically active enzymes. Sentiment is divided about reality of enhanced
enzyme diffusion with evidence for and against. Here a master curve shows that
enzyme diffusion coefficient increases in proportion to the energy release
rate, the product of Michaelis-Menten reaction rate and Gibbs free energy
change with the highly satisfactory correlation coefficient of 0.97. For ten
catalytic enzymes (urease, acetylcholinesterase, seven enzymes from the glucose
cascade cycle, and another), our measurements span from roughly 40% enhanced
diffusion coefficient at high turnover rate and negative Gibbs free energy to
no enhancement at slow turnover rate and positive Gibbs free energy. Moreover,
two independent measures of mobility show consistency, provided that one avoids
undesirable fluorescence photophysics. The master curve presented here
quantifies the limits of both ideas, that enzymes display enhanced diffusion
and that they do not within instrumental resolution, and has possible
implications for understanding enzyme mobility in cellular environments. The
striking linear dependence for the exergonic enzymes (negative Gibbs free
energy) together with the vanishing effect for endergonic enzyme (positive
Gibbs free energy) are consistent with a physical picture where the mechanism
boosting the diffusion is an active one, utilizing the available work from the
chemical reaction.
