The second fundamental feature for a cell-like system is the presence of a metabolism, that is the ability to perform localized chemical reactions, as both real and synthetic cells are effectively microreactors. This generally requires catalysts, where enzymes play this role in biological settings, able to conduct the required reactions in the cells. Even without the encapsulation of an additional enzyme, the bioPISA-derived GUVs are microreactor themselves as they host Mb that self-encapsulates during the PISA process. The capability of encapsulated Mb to transform the substrate Amplex Red (AR) into the fluorescent resorufin in the presence of H2O2 was monitored (Figure ). Fluorescence increased over time, indicating that the Mb is active inside of the polymersomes and that the polymersome membrane is permeable for the small molecule substrates. The latter observation is in line with previous studies that showed that PISA-derived mPEG-b-PHPMA vesicle membranes are permeable for small water-soluble compounds, but impermeable for macromolecules. At the same enzyme concentration, encapsulated Mb exhibited a lower activity than free Mb (246 RFU/minute vs 18000 RFU/minute) (Figure ). The decrease in specific activity might be a consequence of the slow diffusion of the substrate across the membrane, 62 although it cannot be ruled out that some of the enzyme is irreversibly deactivated. The barrier effect of the membrane was also confirmed in CLSM, where fluorescence was visible only within the lumen of GUVs (Figure ).