Mechanistic Consequences of Chiral Radical Clock Probes: Analysis of the Mononuclear Non-Heme Iron Enzyme HppE with 2-Hydroxy-3-methylenecyclopropyl Radical Clock Substrates

(S)-2-Hydroxypropylphosphonic acid [(S)-HPP] epoxidase (HppE) is a mononuclear iron enzyme that catalyzes the last step in the biosynthesis of the antibiotic fosfomycin. HppE also processes the (R)-enantiomer of HPP but converts it to 2-oxo-propylphosphonic acid. In this study, all four stereoisomers of 3-methylenecyclopropyl-containing substrate analogues, (2R, 3R)-8, (2R, 3S)-8, (2S, 3R)-8, and (2S, 3S)-8, were synthesized and used as radical probes to investigate the mechanism of the HppE-catalyzed reaction. Upon treatment with HppE, (2S, 3R)-8 and (2S, 3S)-8 were converted via a C1 radical intermediate to the corresponding epoxide products, as anticipated. In contrast, incubation of HppE with (2R, 3R)-8 led to enzyme inactivation, and incubation of HppE with (2R, 3S)-8 yielded the 2-keto product. The former finding is consistent with the formation of a C2 radical intermediate, where the inactivation is likely triggered by radical-induced ring cleavage of the methylenecyclopropyl group. Reaction with (2R, 3S)-8 is predicted to also proceed via a C2 radical intermediate, but no enzyme inactivation and no ring-opened product were detected. These results strongly suggest that an internal electron transfer to the iron center subsequent to C-H homolysis competes with ring-opening in the processing of the C2 radical intermediate. The different outcomes of the reactions with (2R, 3R)-8 and (2R, 3S)-8 demonstrate the need to carefully consider the chirality of substituted cyclopropyl groups as radical reporting groups in studies of enzymatic mechanisms.