A manganese(IV)/iron(IV) intermediate in assembly of the manganese(IV)/iron(III) cofactor of Chlamydia trachomatis ribonucleotide reductase

We recently showed that the class Ic ribonucleotide reductase from the human pathogen Chlamydia trachomatis uses a Mn^IV/Fe^III cofactor to generate protein and substrate radicals in its catalytic mechanism [Jiang, W., Yun, D., Saleh, L., Barr, E. W., Xing, G., Hoffart, L. M., Maslak, M.-A., Krebs, C., and Bollinger, J. M., Jr. (2007) Science 316, 1188-1191]. Here, we have dissected the mechanism of formation of this novel heterobinuclear redox cofactor from the Mn^II/Fe^II cluster and O ₂. An intermediate with a g = 2 EPR signal that shows hyperfine coupling to both ⁵⁵Mn and ⁵⁷Fe accumulates almost quantitatively in a second-order reaction between O₂ and the reduced R2 complex. The otherwise slow decay of the intermediate to the active Mn ^IV/Fe^III-R2 complex is accelerated by the presence of the one-electron reductant, ascorbate, implying that the intermediate is more oxidized than Mn^IV/Fe^III. Mössbauer spectra show that the intermediate contains a high-spin Fe^IV center. Its chemical and spectroscopic properties establish that the intermediate is a Mn ^IV/Fe^IV-R2 complex with an S = 1/2 electronic ground state arising from antiferromagnetic coupling between the Mn^IV (S _Mn = 3/2) and high-spin Fe^IV (S_Fe = 2) sites.