Spectroscopic and Kinetic Studies of Catalytically Versatile Non-Heme Iron Enzymes

Iron(II) and 2-Oxoglutarate (2OG) dependent non-heme iron (Fe(II)/2OG) enzymes catalyze a large array of chemical transformations, including hydroxylation, desaturation, epoxidation, halogenation, epimerization, endoperoxidation, as well as ring expansion/contraction reactions. Many of these transformations are key chemical steps in the biosynthesis of natural products having antibiotic and antimicrobial activities. In recent years, studies on hydroxylation and halogenation reactions have revealed key details of Fe(II)/2OG enzyme catalysis, namely the discovery of the ferryl species as the key intermediate for C-H bond activation, and the utilization of the rebound mechanism (OH-rebound for hydroxylation and halide-rebound for halogenation) to complete functional group installation. However, mechanistic details are lacking on the reactions that the rebound mechanism is not operative, such as desaturation, epoxidation, and endoperoxidation. Furthermore, the governing factors that control reaction outcomes (rebound vs. non-rebound) are not fully elucidated, which prevent the emergence of a unified picture for Fe(II)/2OG enzyme catalysis. To this end, we are using spectroscopic and kinetic tools to elucidate reaction mechanisms of several newly discovered Fe(II)/2OG enzymes that utilize non-rebound pathways to catalyze reactions. In this talk, I will present, in detail, our recent results from the studies on a novel bi-functional Fe(II)/2OG enzymes through the detection and the characterization of reactive intermediates during enzyme catalysis under pre-steady state conditions. The enzyme is AsqJ from Aspergillus nidulans, which catalyzes a stepwise oxidation (desaturation and epoxidation) in the biosynthesis of a quinolone-type fungal alkaloid, 4’-methoxy-viridicatin. The discussion will further extended to another novel Fe(II)/2OG, FtmOx1 from Aspergillus fumigatus, which catalyzes a novel endoperoxide bond formation in the biosynthesis of a mycotoxin, verruculogen. The implications of these results to the overall understanding of Fe(II)/2OG enzyme catalysis will be discussed.