Department of Chemistry, Georgia State University, P.O. Box 4098, Atlanta, Georgia 30302, USA.
Biochemistry. 2009 Dec 15;48(49):11603-5. doi: 10.1021/bi9017544.
MauG is a diheme enzyme that oxidizes two protein-bound tryptophan residues to generate a catalytic tryptophan tryptophylquinone cofactor within methylamine dehydrogenase. Upon the two-electron oxidation of bis-ferric MauG, the two c-type hemes exist as a spin-uncoupled bis-Fe(IV) species with only one binding oxygen, which is chemically equivalent to a single ferryl heme plus a pi porphyrin cation radical ( Li , X. et al. ( 2008 ) Proc. Natl. Acad. Sci. U.S.A. 105 , 8597 - 8600 ). The EPR spectrum of the nitrosyl complex of fully reduced MauG shows a single six-coordinate Fe(II)-NO species, which is characteristic of a histidine-ligated Fe(II)-NO moiety in the heme environment. Exposure of partially reduced MauG to NO reveals a redox equilibrium with facile electron transfer between hemes but with only one binding nitric oxide. Thus, the second heme is able to stabilize all three redox states of iron (Fe(II), Fe(III), and Fe(IV)) in a six-coordinate protein-bound heme without binding exogenous ligands. This is unprecedented behavior for a protein-bound heme for which each of these redox states is relevant to the overall catalytic mechanism. The results also illustrate the electronic communication between the two iron centers, which function as a diheme unit rather than independent heme cofactors.
MauG 是一种二血红素酶，它可氧化两个结合在蛋白质上的色氨酸残基，从而在亚甲胺脱氢酶内生成一个催化色氨酸三吡咯醌辅因子。在 MauG 的双 ferri 氧化作用下，两个 c 型血红素以自旋解耦的双 Fe(IV)物种存在，仅具有一个结合氧，这在化学上等同于一个 ferryl 血红素加一个π卟啉阳离子自由基（Li, X. 等人（2008 年）Proc. Natl. Acad. Sci. U.S.A. 105, 8597-8600）。完全还原的 MauG 的亚硝酰配合物的 EPR 光谱显示出单个六配位 Fe(II)-NO 物种，这是血红素环境中组氨酸配位的 Fe(II)-NO 部分的特征。部分还原的 MauG 暴露于 NO 下，揭示了一个氧化还原平衡，其中血红素之间易于发生电子转移，但仅结合一个一氧化氮。因此，第二个血红素能够在不结合外源配体的情况下稳定铁的所有三种氧化还原态（Fe(II)、Fe(III)和 Fe(IV))，这在蛋白质结合血红素中是前所未有的行为，其中每种氧化还原态都与整体催化机制相关。结果还说明了两个铁中心之间的电子通讯，它们作为一个二血红素单元而不是独立的血红素辅因子发挥作用。
