Stopped-flow absorption and freeze-quench electron paramagnetic resonance (EPR) and Mössbauer spectroscopies have been used to obtain evidence for the intermediacy of a (μ-1,2-peroxo)-diiron(III/III) complex on the pathway to the tyrosyl radical and (μ-oxo)diiron(III/III) cluster during assembly of the essential cofactor in the R2 subunit of ribonucleotide reductase from mouse. The complex accumulates to ∼0.4 equiv in the first few milliseconds of the reaction and decays concomitantly with accumulation of the previously detected diiron(III/IV) cluster, X, which generates the tyrosyl radical and product (μ-oxo)diiron(III/III) cluster. Kinetic complexities in the reaction suggest the existence of an anti-cooperative interaction of the monomers of the R2 homodimer in Fe(II) binding and perhaps O2 activation. The detection of the (μ-1,2-peroxo)diiron(III/III) complex, which has spectroscopic properties similar to those of complexes previously characterized in the reactions of soluble methane monooxygenase, stearoyl acyl carrier protein Δ9 desaturase, and variants of Escherichia coli R2 with the iron ligand substitution, D84E, provides support for the hypothesis that the reactions of the diiron-carboxylate oxidases and oxygenases commence with the formation of this common intermediate.
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