Angew.Chem.Int.Ed.http://doi.org/f2nv64(2013)

Credit: © 2013 WILEY

Eukaryotic cells contain small compartments that enable different reactions to be separated in space. This has advantages — for instance in supporting the spatiotemporal control of reactions, the formation of high local concentrations or concentration gradients, or the separation of incompatible reaction components. In an effort to mimic this multicompartment arrangement, a team of researchers led by Jan van Hest from Radboud University Nijmegen and Sébastien Lecommandoux from the University of Bordeaux have created a cell mimic formed from a series of smaller polymersomes encapsulated within a larger polymersome. This multicompartment system was used to separate the individual steps of a sequential enzymatic cascade reaction.

The outer compartment is a micrometre-sized polymersome made from the block copolymer polybutadiene-b-poly(ethylene oxide). The smaller polymersomes — formed from polystyrene-b-poly(3-(isocyano-L-alanyl-aminoethyl)-thiophene) — reside inside this outer compartment. The team encapsulated different enzymes within different smaller polymersomes and also inside the main cavity. For the catalytic cascade to be successful, the reactants and products of the individual steps need to diffuse across the polymersome membranes while the enzyme catalysts remain within their designated compartment.

In the first step of the catalytic cascade, which takes place inside the main cavity of the large polymersome, a pro-fluorescent substrate is oxidized to an ester by a Baeyer–Villiger monooxygenase and a cofactor. The second step occurs within one of the sub-compartments and involves the hydrolysis of the ester to an alcohol by a lipase. In the third step, which occurs in another polymersome sub-compartment, the alcohol is oxidized to an aldehyde by alcohol dehydrogenase. Finally, the aldehyde undergoes spontaneous β-elimination to give a fluorescent dye — which could be easily detected to confirm completion of the reaction cascade. To demonstrate that the polymersomes-inside-polymersome arrangement is capable of separating incompatible components, the lipase was replaced with a protease that shows esterase activity but also causes the degradation of other enzymes. The dye was also produced in this arrangement, showing that the other enzymatic catalysts were not significantly degraded by the compartmentalized protease.