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. Author manuscript; available in PMC: 2019 Mar 14.
Published in final edited form as: Chem Rev. 2018 Feb 5;118(5):2554–2592. doi: 10.1021/acs.chemrev.7b00457

Scheme 1. Possible Intermediates in the O2 Activation Chemistry of Nonheme Diiron Enzymes Primarily Based on the Canonical Mechanism for sMMOHa.

Scheme 1

aRed is the diferrous starting point and Ox represents the diferric end point of the cycle. P is a peroxodiferric species; Q is the bis(μ-oxo)diiron(IV) oxidant associated with sMMOH; X is the (μ-oxo)diiron(III,IV) species that generates the catalytically essential Y122• radical in RNR R2; S is the diiron(II,III)-superoxo species proposed for T4MOH.