Geometric and Electronic Structure of the Mn(IV)Fe(III) Cofactor in Class Ic Ribonucleotide Reductase: Correlation to the Class Ia Binuclear Non-Heme Iron Enzyme

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• 作者：Yeonju Kwak ; Wei Jiang ; Laura M. K. Dassama ; Kiyoung Park ; Caleb B. Bell ; III ; Lei V. Liu ; Shaun D. Wong ; Makina Saito ; Yasuhiro Kobayashi ; Shinji Kitao ; Makoto Seto ; Yoshitaka Yoda ; E. Ercan Alp ; Jiyong Zhao ; J. Martin Bollinger ; Jr. ; Carsten Krebs ; Edward I. Solomon
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：November 20, 2013
• 年：2013
• 卷：135
• 期：46
• 页码：17573-17584
• 全文大小：701K
• 年卷期：v.135,no.46(November 20, 2013)
• ISSN：1520-5126

文摘

The class Ic ribonucleotide reductase (RNR) from Chlamydia trachomatis (Ct) utilizes a Mn/Fe heterobinuclear cofactor, rather than the Fe/Fe cofactor found in the 尾 (R2) subunit of the class Ia enzymes, to react with O₂. This reaction produces a stable Mn^IVFe^III cofactor that initiates a radical, which transfers to the adjacent 伪 (R1) subunit and reacts with the substrate. We have studied the Mn^IVFe^III cofactor using nuclear resonance vibrational spectroscopy (NRVS) and absorption (Abs)/circular dichroism (CD)/magnetic CD (MCD)/variable temperature, variable field (VTVH) MCD spectroscopies to obtain detailed insight into its geometric/electronic structure and to correlate structure with reactivity; NRVS focuses on the Fe^III, whereas MCD reflects the spin-allowed transitions mostly on the Mn^IV. We have evaluated 18 systematically varied structures. Comparison of the simulated NRVS spectra to the experimental data shows that the cofactor has one carboxylate bridge, with Mn^IV at the site proximal to Phe₁₂₇. Abs/CD/MCD/VTVH MCD data exhibit 12 transitions that are assigned as d鈥揹 and oxo and OH^{鈥?/sup> to metal charge-transfer (CT) transitions. Assignments are based on MCD/Abs intensity ratios, transition energies, polarizations, and derivative-shaped pseudo-A term CT transitions. Correlating these results with TD-DFT calculations defines the MnIVFeIII cofactor as having a 渭-oxo, 渭-hydroxo core and a terminal hydroxo ligand on the MnIV. From DFT calculations, the MnIV at site 1 is necessary to tune the redox potential to a value similar to that of the tyrosine radical in class Ia RNR, and the OH鈥?/sup> terminal ligand on this MnIV provides a high proton affinity that could gate radical translocation to the 伪 (R1) subunit.}