In situ, reversible coacervate formation within lipid vesicles represents a key step in the development of responsive synthetic cellular models. Herein, we exploit the pH responsiveness of a polycation above and below its pKa, to drive liquid–liquid phase separation, to form single coacervate droplets within lipid vesicles. The process is completely reversible as coacervate droplets can be disassembled by increasing the pH above the pKa. We further show that pH‐triggered coacervation in the presence of low concentrations of enzymes activates dormant enzyme reactions by increasing the local concentration within the coacervate droplets and changing the local environment around the enzyme. In conclusion, this work establishes a tunable, pH responsive, enzymatically active multi‐compartment synthetic cell. The system is readily transferred into microfluidics, making it a robust model for addressing general questions in biology, such as the role of phase separation and its effect on enzymatic reactions using a bottom‐up synthetic biology approach.

To understand how membrane-free subcompartmentalization can modulate biochemical reactions by coupled spatial enzyme localization with substrate and product partitioning, we use microfluidic strategies to synthesize, stabilize and characterize micron-sized functional coacervates in waterÀ oil emulsions. Our methodologies have allowed for the first time to quantitatively characterize partition coefficients of a broad range of different molecules with different coacervate chemistries and to measure reaction rates of individual subcompartments and their surrounding aqueous environment at the single coacervate level. Our results show that sub-compartmentalisation increases the overall rates of reactions. This bottom-up synthetic strategy for the production of synthetic organelles offers a physical model for membrane-free compartmentalization in biology and provides insights into the role of sub-compartmentalisation in regulating out-of-equilibrium behaviours in biological systems.

In situ, reversible coacervate formation within lipid vesicles represents a key step in the development of responsive synthetic cellular models. Herein, we exploit the pH responsiveness of a polycation above and below its pKa, to drive liquid–liquid phase separation, to form single coacervate droplets within lipid vesicles. The process is completely reversible as coacervate droplets can be disassembled by increasing the pH above the pKa. We further show that pH‐triggered coacervation in the presence of low concentrations of enzymes activates dormant enzyme reactions by increasing the local concentration within the coacervate droplets and changing the local environment around the enzyme. In conclusion, this work establishes a tunable, pH responsive, enzymatically active multi‐compartment synthetic cell. The system is readily transferred into microfluidics, making it a robust model for addressing general questions in biology, such as the role of phase separation and its effect on enzymatic reactions using a bottom‐up synthetic biology approach.