"Artificial metalloenzymes are expected to bring together the best of the two worlds of homogeneous and enzyme catalysis, combining broad substrate scope with high activity and reaction selectivity under mild conditions. Herein we propose to construct an artificial metalloenzyme based on a dirhodium active site in the capsid scaffold of the protein ferritin. Catalytically competent dirhodium compounds are derived from dirhodium tetraacetate, which possesses fourfold symmetry, with the ligands symmetrically arranged around the equator of the rhodium dimer. The transition metal core is active in a wide range of reactions including cyclopropanation, C-H activation and O-H insertion. These transformations play an important role in the synthesis of natural products, pharmaceuticals, and other industrially relevant targets. We will exploit the fourfold-symmetric pores of the capsidic protein ferritin to construct a dirhodium binding site. A ferritin mutant will be produced with four glutamate residues pointing in the channel lumen, suitable as ligands for the rhodium dimer. After derivatization with dirhodium the artificial capsid will be employed as a catalyst in organometallic reactions, such as the cyclopropanation of diaza carbonyl compounds with olefins in aqueous solution. The catalytic properties of this first dirhodium enzyme will be fine-tuned by the highly modular secondary ligand environment of the protein. The dirhodium binding site will also be introduced at the inner mouth of the fourfold channel, resulting in an active site inside the capsid. High local substrate concentrations and the presence of a second, complementary reaction center inside the capsid might allow multi-enzyme cascade catalysis, thus paving the way toward artificial nanoreactors with tailored properties."