Enzymes that contain metal ions-that is, metalloenzymespossess the reactivity of a transition metal centre and the potential of molecular evolution to modulate the reactivity and substrateselectivity of the system 1 . By exploiting substrate promiscuity and protein engineering, the scope of reactions catalysed by native metalloenzymes has been expanded recently to include abiological transformations 2,3 . However, this strategy is limited by the inherent reactivity of metal centres in native metalloenzymes. To overcome this limitation, artificial metalloproteins have been created by incorporating complete, noble-metal complexes within proteins lacking native metal sites 1,4,5 . The interactions of the substrate with the protein in these systems are, however, distinct from those with the native protein because the metal complex occupies the substrate binding site. At the intersection of these approaches lies a third strategy, in which the native metal of a metalloenzyme is replaced with an abiological metal with reactivity different from that of the metal in a native protein 6-8 . This strategy could create artificial enzymes for abiological catalysis within the natural substrate binding site of an enzyme that can be subjected to directed evolution. Here we report the formal replacement of iron in Fe-porphyrin IX (Fe-PIX) proteins with abiological, noble metals to create enzymes that catalyse reactions not catalysed by native Fe-enzymes or other metalloenzymes 9,10 . In particular, we prepared modified myoglobins containing an Ir(Me) site that catalyse the functionalization of C-H bonds to form C-C bonds by carbene insertion and add carbenes to both β-substituted vinylarenes and unactivated aliphatic α-olefins. We conducted directed evolution of the Ir(Me)-myoglobin and generated mutants that form either enantiomer of the products of C-H insertion and catalyse the enantio-and diastereoselective cyclopropanation of unactivated olefins. The presented method of preparing artificial haem proteins containing abiological metal porphyrins sets the stage for the generation of artificial enzymes from innumerable combinations of PIX-protein scaffolds and unnatural metal cofactors to catalyse a wide range of abiological transformations.To create artificial metalloenzymes formed by combining abiological metals and natural metalloprotein scaffolds, we focused on haem proteins, which contain Fe-porphyrin IX (Fe-PIX) as a metal cofactor. Native haem enzymes catalyse reactions that include C-H oxidation and halogenation 11 , and they have been successfully evolved to oxidize abiological substrates 12,13 . Fe-PIX proteins have also been shown to catalyse abiological reactions involving the addition and insertion of carbenes and nitrenes to olefins and X-H bonds 2,3,9,14 . However, the reactivity of the Fe-centre in haem proteins limits the scope of these transformations. For example, Fe-PIX proteins catalyse the cyclopropanation of activated terminal vinylarenes 9,10 , but they do not catalyse reactions with internal vinylarenes ...

Natural enzymes contain highly evolved active sites that lead to fast rates and high selectivities. Although artificial metalloenzymes have been developed that catalyze abiological transformations with high stereoselectivity, the activities of these artificial enzymes are much lower than those of natural enzymes. Here, we report a reconstituted artificial metalloenzyme containing an iridium porphyrin that exhibits kinetic parameters similar to those of natural enzymes. In particular, variants of the P450 enzyme CYP119 containing iridium in place of iron catalyze insertions of carbenes into C-H bonds with up to 98% enantiomeric excess, 35,000 turnovers, and 2550 hours −1 turnover frequency. This activity leads to intramolecular carbene insertions into unactivated C-H bonds and intermolecular carbene insertions into C-H bonds. These results lift the restrictions on merging chemical catalysis and biocatalysis to create highly active, productive, and selective metalloenzymes for abiological reactions.

Multi-catalysis is an emerging field targeting the development of efficient catalytic transformations to quickly convert relatively simple starting materials into more complex valueadded products. Within multi-catalytic processes either multiple catalysts execute single reactions or precise sequences of multiple catalytic reactions occur in a 'one-pot' fashion. Attractively, multi-catalytic protocols not only enable transformations that are inaccessible through classic approaches, but also are able to significantly reduce the time, waste, and cost of the synthetic processes, making organic synthesis more resources efficient. In this Perspective article, we review different strategies in multi-catalysis that bring distinct challenges and opportunities. We divide this overarching field into three main categories: cooperative, domino, and relay catalysis. Each category is described along with representative examples to highlight its features. Special emphasis is dedicated to relay catalysis, which is further discussed in its sub-categories. Lastly, we provide an analysis of systems that incorporate higher levels of complexity and further underscore the potential of multi-catalytic systems.