Li Xianghui, Jones Limei H, Pearson Arwen R, Wilmot Carrie M, Davidson Victor L
Department of Biochemistry, The University of Mississippi Medical Center, Jackson, Mississippi 39216, USA.
Biochemistry. 2006 Nov 7;45(44):13276-83. doi: 10.1021/bi061497d.
Tryptophan tryptophylquinone (TTQ), the prosthetic group of methylamine dehydrogenase, is formed by post-translational modifications of two tryptophan residues that result in the incorporation of two oxygens into one tryptophan side chain and the covalent cross-linking of that side chain to a second tryptophan residue. MauG is a novel 42 kDa di-heme protein, which is required for the biosynthesis of TTQ. An experimental system has been developed that allows the direct continuous monitoring of MauG-dependent TTQ biosynthesis in vitro. Four diverse electron donors, ascorbate, dithiothreitol, reduced glutathione, and NADH, were each able to provide reducing equivalents for MauG-dependent TTQ biosynthesis under aerobic conditions. The reaction with NADH was mediated by an NADH-dependent oxidoreductase. Under anaerobic conditions in the absence of an electron donor, H(2)O(2) could serve as a substrate for MauG-dependent TTQ biosynthesis. During the reaction with H(2)O(2), a discrete reaction intermediate was observed, which is likely the reduced quinol form of TTQ that then is oxidized to the quinone. These results suggest that not only the incorporation of oxygen into the monohydroxylated biosynthetic intermediate but also the subsequent oxidation of quinol MADH during TTQ biosynthesis is a MauG-dependent process. The implications of these results in elucidating the mechanism of MauG-dependent TTQ biosynthesis and identifying potential physiologic electron and oxygen donors for TTQ biosynthesis in vivo are discussed.
色氨酸色氨酰醌（TTQ）是甲胺脱氢酶的辅基，由两个色氨酸残基的翻译后修饰形成，该修饰导致一个氧原子掺入一个色氨酸侧链，并使该侧链与第二个色氨酸残基共价交联。MauG是一种新型的42 kDa双血红素蛋白，是TTQ生物合成所必需的。已开发出一种实验系统，可在体外直接连续监测依赖MauG的TTQ生物合成。在有氧条件下，四种不同的电子供体，即抗坏血酸、二硫苏糖醇、还原型谷胱甘肽和NADH，均能够为依赖MauG的TTQ生物合成提供还原当量。与NADH的反应由一种依赖NADH的氧化还原酶介导。在无氧条件下且没有电子供体时，H₂O₂可作为依赖MauG的TTQ生物合成的底物。在与H₂O₂反应过程中，观察到一种离散的反应中间体，它可能是TTQ的还原型醌醇形式，随后被氧化为醌。这些结果表明，不仅将氧掺入单羟基化生物合成中间体，而且在TTQ生物合成过程中醌醇MADH的后续氧化都是依赖MauG的过程。讨论了这些结果在阐明依赖MauG的TTQ生物合成机制以及确定体内TTQ生物合成潜在的生理电子和氧供体方面的意义。
