Physical properties of biomolecular condensates modulate enzymatic activity

Biomolecular condensates are membraneless organelles in the cytoplasm that form due to liquid-liquid phase separation. They orchestrate cellular biochemical activities on a wide range of scales. A model condensate system that can be used to study these droplets consist of the protein Bovine Serum Albumin (BSA) and Polyethylene glycol (PEG) 4k, with the former crowded by the latter through depletion interactions. The addition of different molecules show that large molecules like Lactate Dehydrogenase and Urease partition inside of the BSA-dense phase, while smaller substrates are free to diffuse in and out. This effectively makes the droplets microreactors of enzymatic activity. Using Isothermal Calorimetry and NADH assays, we demonstrate that enzymatic activity can be suppressed or enhanced by changing the viscosity of these droplets. LDH and Urease show different responses to these changes, suggesting condensate-enzyme interactions that are specific to each enzyme.