Diverse engineered heme proteins enable stereodivergent cyclopropanation of unactivated alkenes
Stereodivergent syntheses leading to the different stereoisomers of a product are useful in the discovery and testing of drugs and agrochemicals. A longstanding challenge in catalysis, developing sets of stereodivergent catalysts is often solved for enzymes by screening Nature’s diversity for biocatalysts with complementary stereoselectivities. Here, Nature’s protein diversity has been leveraged to develop stereodivergent catalysts for a reaction not known in biology, cyclopropanation via carbene transfer. By screening diverse native and engineered heme proteins, we identified globins and serine-ligated cytochromes P450 with promiscuous activity for cyclopropanation of unactivated alkene substrates. Their activities and stereoselectivities were enhanced by directed evolution: 1-3 rounds of site-saturation mutagenesis and screening generated enzymes that catalyze the stereodivergent cyclopropanation to form each of the four stereoisomers of unactivated alkenes and electron-deficient alkenes with up to 5,400 total turnovers and 98% enantiomeric excess. These fully genetically encoded biocatalysts function in whole E. coli cells in mild, aqueous conditions and provide the first example of enantioselective, intermolecular iron-catalyzed cyclopropanation of unactivated alkenes via carbene transfer.