The need for efficient, tailor-made catalysts has inspired chemists to fuse the design principles of natural enzymes with synthetic macromolecular architectures. A highly interesting pathway mimics a metallo-enzyme's tertiary structure via the target placement of metal-ions in a tailor-made polymeric framework, resulting in catalytically active single-chain nanoparticles. Initial studies reveal unusual and promising effects, regarding both new catalyst characteristics and a high impact on product formation. These multifunctional nanoreactors, constructed from simple folded polymer chains, will lead to advanced bioinspired catalytic systems. As found in enzymes, their impact lies specifically within the defined construction of a polymeric pocket around the catalytic active cores for substrate recognition.