Single Molecule Imaging of Nanoscale Self-Propelled Active Matter

Active matter composed of ensembles of self-propelled particles display exciting emergent properties. When coupled to larger objects, enzymes can act as the propulsion of self-propelled particles, and recent studies have shown that these enzymes have exciting self-propelled properties, as well. Studies using fluorescence correlation spectroscopy (FCS) or macroscopic observation have revealed that enzymes can perform enhanced diffusion depending on the substrate concentration. The mechanism for enhanced diffusion remains unclear. We report new results using single particle tracking with total internal reflection (TIRF) microscopy to observe this emergent activity of active enzymes to test proposed mechanisms that could result in enhanced mobility of active enzymes. Using urease as a model enzyme, we observe that the diffusion of individual enzymes increases three fold as a function of substrate. This diffusion is unaffected by the background concentration of enzymes. Further, we find that the oligomerization state is unchanged by the presence of the substrate, implying that the enzyme does not significantly change size upon binding of substrate. This work effectively eliminates some previously-postulated theories of the mechanism of enzymatically enhanced diffusion.