Li Xianghui, Fu Rong, Lee Sheeyong, Krebs Carsten, Davidson Victor L, Liu Aimin
Department of Biochemistry, University of Mississippi Medical Center, 2500 North State Street, Jackson, MS 39216, USA.
Proc Natl Acad Sci U S A. 2008 Jun 24;105(25):8597-600. doi: 10.1073/pnas.0801643105. Epub 2008 Jun 18.
High-valent iron species are powerful oxidizing agents in chemical and biological catalysis. The best characterized form of an Fe(V) equivalent described in biological systems is the combination of a b-type heme with Fe(IV)=O and a porphyrin or amino acid cation radical (termed Compound I). This work describes an alternative natural mechanism to store two oxidizing equivalents above the ferric state for biological oxidation reactions. MauG is an enzyme that utilizes two covalently bound c-type hemes to catalyze the biosynthesis of the protein-derived cofactor tryptophan tryptophylquinone. Its natural substrate is a monohydroxylated tryptophan residue present in a 119-kDa precursor protein. An EPR-silent di-heme reaction intermediate of MauG was trapped. Mössbauer spectroscopy revealed the presence of two distinct Fe(IV) species. One is consistent with an Fe(IV)=O (ferryl) species (delta = 0.06 mm/s, DeltaE(Q) = 1.70 mm/s). The other is assigned to an Fe(IV) heme species with two axial ligands from protein (delta = 0.17 mm/s, DeltaE(Q) = 2.54 mm/s), which has never before been described in nature. This bis-Fe(IV) intermediate is remarkably stable but readily reacts with its native substrate. These findings broaden our views of how proteins can stabilize a highly reactive oxidizing species and the scope of enzyme-catalyzed posttranslational modifications.
高价铁物种在化学和生物催化中是强大的氧化剂。生物系统中描述的最具特征的Fe(V)等价物形式是b型血红素与Fe(IV)=O以及卟啉或氨基酸阳离子自由基的组合（称为化合物I）。这项工作描述了一种替代的天然机制，用于在铁离子状态之上存储两个氧化当量以进行生物氧化反应。MauG是一种利用两个共价结合的c型血红素来催化蛋白质衍生的辅因子色氨酸色氨酰醌生物合成的酶。其天然底物是存在于119 kDa前体蛋白中的单羟基化色氨酸残基。捕获了MauG的一种EPR沉默的双血红素反应中间体。穆斯堡尔光谱揭示了两种不同的Fe(IV)物种的存在。一种与Fe(IV)=O（高铁）物种一致（δ = 0.06 mm/s，ΔE(Q) = 1.70 mm/s）。另一种被指定为具有来自蛋白质的两个轴向配体的Fe(IV)血红素物种（δ = 0.17 mm/s，ΔE(Q) = 2.54 mm/s），这在自然界中从未被描述过。这种双Fe(IV)中间体非常稳定，但很容易与其天然底物反应。这些发现拓宽了我们对蛋白质如何稳定高活性氧化物种以及酶催化的翻译后修饰范围的认识。
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