The catalytic properties of an enzyme result from the three-dimensional folding of a single protein chain, which brings together a well-defined set of amino-acid residues to form the enzyme's active site. This pocket is a highly organized domain that binds tightly and selectively to the enzyme's substrate, which becomes trapped and polarized in a network of supramolecular interactions. In this way, active sites lower the energy of transition states for reactions, so that products form up to billions of times faster than in the uncatalysed reactions. A challenge for chemists has been to devise systems that mimic enzyme activity, and a breakthrough has now been reported by Terashima et al.1 in the Journal of the American Chemical Society. They have synthesized a polymer, single chains of which fold in water to form an inner compartment that acts, through its supramolecular structure, as an 'active site' for a catalytic reaction.