Progress Toward Understanding Protein Control of Reaction Outcome in the Diverse Reactivity of Iron(II)‐ and 2‐Oxoglutarate‐dependent Oxygenases
Authors: Bollinger J. M. , Krebs Carsten , Boal Amie K. , Silakov Alexey , Price John C. , Matthews Megan L. , Chang Wei‐chen , Martinie Ryan J. , Pan Juan , Dunham Noah P. , Wenger Eliott S. , Copeland Rachelle A. , Lin Chi‐Yun 
Published: May 2022

In mammals, iron(II)‐ and 2‐oxoglutarate‐dependent (Fe/2OG) dioxygenases have roles in oxygen and body‐mass homeostasis, connective tissue synthesis, and control of transcription and epigenetic inheritance.1 Related enzymes in plants, fungi, and bacteria enable diverse biosynthetic pathways to valuable natural‐product drugs by catalyzing halogenation, epimerization, desaturation, cyclization, ring‐opening/expansion, and endoperoxidation reactions; some of these enzymes even catalyze multiple reaction types within the same pathway!2 Almost every enzyme in this class initiates its reaction by using a common oxoiron(IV) (ferryl) intermediate, first demonstrated by the Penn State group in 2003,3,4 to abstract a hydrogen atom from its target substrate (Figure 1). The iron(II)‐chelating co‐substrate, 2OG, is oxidatively decarboxylated to a succinate ligand in formation of the ferryl complex. To mediate an outcome other than hydroxylation, an enzyme must avoid what can be a facile “oxygen rebound,” Groves term for the coupling of the substrate radical with the iron‐coordinated oxygen just after it abstracts the hydrogen. By direct biophysical observation and “metallomimicry” of intermediates, we have rationalized several of these outcomes in terms of the alternative fates of the substrate radical and the structural features of the enzyme that avert oxygen rebound to enable these alternative fates.5‐11 Very recently, we explained how the most unusual Fe/2OG oxygenase discovered to date, the microbial ethylene‐forming enzyme (EFE), branches from the canonical pathway even before the ferryl intermediate is formed (Figure 2),12,13 leading to global fragmentation of 2OG to three carbon dioxide equivalents and ethylene, a reaction that requires but does not transform the amino acid L‐arginine. The elucidation of the unusual “radical‐polar‐crossover” mechanism of EFE13 significantly expands the paradigm of 2OG‐assisted dioxygen activation pathways and suggests unexpected uses of EFE in biotechnology.

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