  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 is higher than biologically relevant and accordingly exceeds kM,
the Michaelis-Menten constant. 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 kM dissociate into their smaller subunit fragments that diffuse
more rapidly, thus providing a simple physical mechanism of enhanced diffusion
in this regime of concentrations. Mindful that this conclusion may be
controversial, our findings are sup-ported by four independent analytical
techniques, static light scattering, dynamic light scattering (DLS),
size-exclusion chroma-tography (SEC), and fluorescence correlation spectroscopy
(FCS). Data for urease are presented in the main text and the con-clusion is
validated for hexokinase and acetylcholinesterase with data presented in
supplementary information. For substrate concentration regimes below kM at
which these enzymes do not dissociate, our findings from both FCS and DLS
validate that enzymatic catalysis does lead to the enhanced diffusion
phenomenon. INTRODUCT
