Processive enzymes are proteins that catalyze consecutive reactions without releasing its substrate. An example of processive enzymes is RNA polymerase which binds to a DNA strand and repeatedly catalyzes nucleotide transfers, effectively synthesizing a corresponding RNA strand.
Nolte and coworkers developed an artificial processive enzyme in a form of manganese porphyrin rotaxane that treads along a long polymer of alkene and catalyze multiple rounds of alkene epoxidation. Manganese (III) ion in the porphyrin is the molecule's catalytic center, capable of epoxidation in the presence of an oxygen donor and an activating ligand. With a small ligand such pyridine that binds manganese from inside the cavity of the rotaxane, epoxidation happens outside the catalyst. With a large bulky ligand such as tert-butyl pyridine that does not fit inside the cavity however, epoxidation happens on the inside of the catalyst.