| Abstract
| - We recently showed that the class Ic ribonucleotide reductase from the human pathogenChlamydia trachomatis uses a MnIV/FeIII cofactor to generate protein and substrate radicals in its catalyticmechanism [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 offormation of this novel heterobinuclear redox cofactor from the MnII/FeII cluster and O2. An intermediatewith a g = 2 EPR signal that shows hyperfine coupling to both 55Mn and 57Fe accumulates almostquantitatively in a second-order reaction between O2 and the reduced R2 complex. The otherwise slowdecay of the intermediate to the active MnIV/FeIII−R2 complex is accelerated by the presence of theone-electron reductant, ascorbate, implying that the intermediate is more oxidized than MnIV/FeIII. Mössbauerspectra show that the intermediate contains a high-spin FeIV center. Its chemical and spectroscopic propertiesestablish that the intermediate is a MnIV/FeIV−R2 complex with an S = 1/2 electronic ground state arisingfrom antiferromagnetic coupling between the MnIV (SMn = 3/2) and high-spin FeIV (SFe = 2) sites.
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