Evidence That the 尾 Subunit of Chlamydia trachomatis Ribonucleotide Reductase Is Active with the Manganese Ion of Its Manganese(IV)/Iron(III) Cofactor in Site 1

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• 作者：Laura M. K. Dassama ; Amie K. Boal ; Carsten Krebs ; Amy C. Rosenzweig ; J. Martin Bollinger ; Jr.
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：February 8, 2012
• 年：2012
• 卷：134
• 期：5
• 页码：2520-2523
• 全文大小：243K
• 年卷期：v.134,no.5(February 8, 2012)
• ISSN：1520-5126

文摘

The reaction of a class I ribonucleotide reductase (RNR) begins when a cofactor in the 尾 subunit oxidizes a cysteine residue 35 脜 away in the 伪 subunit, generating a thiyl radical. In the class Ic enzyme from Chlamydia trachomatis (Ct), the cysteine oxidant is the Mn^IV ion of a Mn^IV/Fe^III cluster, which assembles in a reaction between O₂ and the Mn^II/Fe^II complex of 尾. The heterodinuclear nature of the cofactor raises the question of which site, 1 or 2, contains the Mn^IV ion. Because site 1 is closer to the conserved location of the cysteine-oxidizing tyrosyl radical of class Ia and Ib RNRs, we suggested that the Mn^IV ion most likely resides in this site (i.e., ¹Mn^IV/²Fe^III), but a subsequent computational study favored its occupation of site 2 (¹Fe^III/²Mn^IV). In this work, we have sought to resolve the location of the Mn^IV ion in Ct RNR-尾 by correlating X-ray crystallographic anomalous scattering intensities with catalytic activity for samples of the protein reconstituted in vitro by two different procedures. In samples containing primarily Mn^IV/Fe^III clusters, Mn preferentially occupies site 1, but some anomalous scattering from site 2 is observed, implying that both ¹Mn^II/²Fe^II and ¹Fe^II/²Mn^II complexes are competent to react with O₂ to produce the corresponding oxidized states. However, with diminished Mn^II loading in the reconstitution, there is no evidence for Mn occupancy of site 2, and the greater activity of these 鈥渓ow-Mn鈥?samples on a per-Mn basis implies that the ¹Mn^IV/²Fe^III-尾 is at least the more active of the two oxidized forms and may be the only active form.