Hybrid radical-polar pathway for excision of ethylene from 2-oxoglutarate by an iron oxygenase

Microbial ethylene-forming enzyme (EFE) converts the C3-C4 fragment of the ubiquitous primary metabolite, 2-oxoglutarate (2OG), to its namesake alkene product. This reaction is very different from the simple decarboxylation of 2OG to succinate promoted by related enzymes and has inspired disparate mechanistic hypotheses. We show that EFE produces stereochemically random (equal cis and trans) 1,2-[2H2]-ethylene from (3S,4R)-[2H2]-2OG, appends an oxygen from O2 upon the C1-derived (bi)carbonate, and can be diverted to ω-hydroxylated monoacid products by modifications to 2OG or the enzyme. These results implicate an unusual radical-polar hybrid mechanism involving iron(II)-coordinated acylperoxycarbonate and alkylcarbonate intermediates. The mechanism explains how EFE accesses a high-energy carboxyl radical to initiate its fragmentation cascade, and it hints at new capabilities of 2OG-dependent enzymes that may await discovery and exploitation.