Project Summary Despite substantial advancements in small-molecule catalysis, general methods to control the stereoselectivity in radical-mediated transformations remain a formidable challenge facing synthetic chemists. On the other hand, enzymes are known for their ability to exert exquisite control over the stereochemical outcome of the reactions they catalyze. However, the catalytic repertoire of enzymes has been largely limited to their native biochemistry. Using an interdisciplinary approach combining ideas and technologies from organic chemistry, organometallic chemistry, biocatalysis, enzyme engineering and computational modeling, we will reprogram naturally occurring metalloenzymes to catalyze unnatural radical reactions with excellent stereoselectivity. Capitalizing on the innate redox property of metallocofactor present in a plethora of natural metalloproteins, we will develop metalloredox radical biocatalytic reactions with significant synthetic utility. First, we will develop new-to-nature metalloredox atom transfer radical addition reactions that proceed with excellent diastereo- and enantiocontrol. Second, we will advance stereoselective radical additions to aromatic compounds in a metalloenzyme-controlled fashion. Furthermore, enantioselective C-H functionalization reactions involving enzymatically formed nitrogen-centered radicals will be developed. All the three aims are supported by strong preliminary data gathered in our laboratory. Together, these novel metalloredox biocatalytic processes constitute a powerful toolbox for the rapid assembly of diverse bioactive small molecules that are highly valuable in biomedical sciences. Furthermore, the development of new-to-nature catalytic functions will dramatically broaden the biochemical landscape of metalloenzymes.