Researchers have trained an enzyme to perform a reaction not seen in nature but could be used for making pharmaceuticals, insecticides, and a portfolio of hormones and pheromones. California Institute of Technology’s Frances H. Arnold and her colleagues engineered a bacterial enzyme called cytochrome P450BM3 to turn olefins into cyclopropanes. That reaction previously could be carried out only with inorganic catalysts (Science, DOI: 10.1126/science.1231434). Cytochrome P450BM3 is a member of a protein family famous for catalyzing hydroxylations and other oxidative transformations. The team reasoned that it could be adapted for olefin cyclopropanation. Using a library of mutant P450BM3 enzymes, they found a handful that could perform the desired reaction, but with low yield and poor enantiospecificity. When the team tweaked amino acids in several P450BM3 enzymes’ active sites , they observed reactions with high turnover, enantioselectivity, and diastereoselectivity. “The established reaction promiscuity of natural enzymes and the surprising ease with which cyclopropanation activity could be installed into P450BM3 suggest that this approach will be useful for other synthetically important transformations for which biological counterparts do not yet exist,” the authors note.