Enhanced diffusion and enzyme dissociation at high substrate concentration

The concept that catalytic enzymes can act as molecular machines transducing chemical activity into motion has conceptual and experimental support, but much of the claimed support comes from experimental conditions where the substrate concentration exceeds k_M (the Michaelis-Menten constant), meaning that it exceeds concentrations that are biologically relevant. Moreover, many of the enzymes studied experimentally to date are oligomeric. Urease, a hexamer of subunits, has been considered to be the gold standard demonstrating enhanced diffusion. Here we show that urease and certain other oligomeric enzymes of high catalytic activity above k_M dissociate into their smaller subunit fragments that diffuse more rapidly, thus providing a simple physical mechanism of enhanced diffusion in this regime of concentrations. Data for urease are presented in the main text and the conclusion is validated for hexokinase and acetylcholinesterase with data presented in supplementary information. For substrate concentration regimes below k_M at which these enzymes do not dissociate, we validate that enzymatic catalysis does lead to the enhanced diffusion phenomenon.