Composition and Structure of the Inorganic Core of Relaxed Intermediate X(Y122F) of Escherichia coli Ribonucleotide Reductase

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• 作者：Peter E. Doan ; Muralidharan Shanmugam ; JoAnne Stubbe ; Brian M. Hoffman
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：December 16, 2015
• 年：2015
• 卷：137
• 期：49
• 页码：15558-15566
• 全文大小：502K
• ISSN：1520-5126

文摘

Activation of the diferrous center of the 尾2 (R2) subunit of the class 1a Escherichia coli ribonucleotide reductases by reaction with O₂ followed by one-electron reduction yields a spin-coupled, paramagnetic Fe(III)/Fe(IV) intermediate, denoted X, whose identity has been sought by multiple investigators for over a quarter of a century. To determine the composition and structure of X, the present study has applied p>57p>Fe, p>14,15p>N, p>17p>O, and p>1p>H electron nuclear double resonance (ENDOR) measurements combined with quantitative measurements of p>17p>O and p>1p>H electron paramagnetic resonance line-broadening studies to wild-type X, which is very short-lived, and to X prepared with the Y122F mutant, which has a lifetime of many seconds. Previous studies have established that over several seconds the as-formed X(Y122F) relaxes to an equilibrium structure. The present study focuses on the relaxed structure. It establishes that the inorganic core of relaxed X has the composition [(OHp>鈥?/sup>)Fep>IIIp>鈥揙鈥揊ep>IVp>]: there is no second inorganic oxygenic bridge, neither oxo nor hydroxo. Geometric analysis of the p>14p>N ENDOR data, together with recent extended X-ray absorption fine structure measurements of the Fe鈥揊e distance (Dassama, L. M.; et al. J. Am. Chem. Soc. 2013, 135, 16758), supports the view that X contains a 鈥渄iamond-core鈥?Fe(III)/Fe(IV) center, with the irons bridged by two ligands. One bridging ligand is the oxo bridge (O_Br) derived from O₂ gas. Given the absence of a second inorganic oxygenic bridge, the second bridging ligand must be protein derived, and is most plausibly assigned as a carboxyl oxygen from E238.