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首页> 外文期刊>The FEBS journal >L-Galactono-gamma-lactone dehydrogenase from Arabidopsis thaliana, a flavoprotein involved in vitamin C biosynthesis
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L-Galactono-gamma-lactone dehydrogenase from Arabidopsis thaliana, a flavoprotein involved in vitamin C biosynthesis

机译：拟南芥中的L-半乳糖-γ-内酯脱氢酶，一种参与维生素C生物合成的黄素蛋白

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摘要

L-Galactono-1,4-lactone dehydrogenase (GALDH; ferricytochrome c oxidoreductase; EC 1.3.2.3) is a mitochondrial flavoenzyme that catalyzes the final step in the biosynthesis of vitamin C (L-ascorbic acid) in plants. In the present study, we report on the biochemical properties of recombinant Arabidopsis thaliana GALDH (AtGALDH). AtGALDH oxidizes, in addition to L-galactono-1,4-lactone (K-m = 0.17 mM, k(cat) = 134 s(-1)), L-gulono-1,4-lactone (K-m = 13.1 mM, k(cat) = 4.0 s(-1)) using cytochrome c as an electron acceptor. Aerobic reduction of AtGALDH with the lactone substrate generates the flavin hydroquinone. The two-electron reduced enzyme reacts poorly with molecular oxygen (k(ox) = 6 x 10(2)M(-1).s(-1)). Unlike most flavoprotein dehydrogenases, AtGALDH forms a flavin N5 sulfite adduct. Anaerobic photoreduction involves the transient stabilization of the anionic flavin semiquinone. Most aldonolactone oxidoreductases contain a histidyl-FAD as a covalently bound prosthetic group. AtGALDH lacks the histidine involved in covalent FAD binding, but contains a leucine instead (Leu56). Leu56 replacements did not result in covalent flavinylation but revealed the importance of Leu56 for both FAD-binding and catalysis. The Leu56 variants showed remarkable differences in Michaelis constants for both L-galactono-1,4-lactone and L-gulono-1,4-lactone and released their FAD cofactor more easily than wild-type AtGALDH. The present study provides the first biochemical characterization of AtGALDH and some active site variants. The role of GALDH and the possible involvement of other aldonolactone oxidoreductases in the biosynthesis of vitamin C in A. thaliana are also discussed.
机译：L-半乳糖-1,4-内酯脱氢酶（GALDH;亚铁色素c氧化还原酶; EC 1.3.2.3）是一种线粒体黄素酶，可催化植物体内维生素C（L-抗坏血酸）生物合成的最后一步。在本研究中，我们报告了重组拟南芥GALDH（AtGALDH）的生化特性。除L-半乳糖-1,4-内酯（Km = 0.17 mM，k（cat）= 134 s（-1））外，AtGALDH还会氧化L-gulono-1,4-内酯（Km = 13.1 mM，k （cat）= 4.0 s（-1）），使用细胞色素c作为电子受体。用内酯底物对AtGALDH进行有氧还原可生成黄素氢醌。两电子还原的酶与分子氧的反应很差（k（ox）= 6 x 10（2）M（-1）.s（-1））。与大多数黄素蛋白脱氢酶不同，AtGALDH形成黄素N5亚硫酸盐加合物。厌氧光还原涉及阴离子黄素半醌的瞬时稳定化。大多数醛糖内酯氧化还原酶都含有一个组氨酸-FAD作为共价结合的修复基团。 AtGALDH缺乏参与共价FAD结合的组氨酸，但包含亮氨酸（Leu56）。 Leu56替代品不会导致共价黄素化，但揭示了Leu56对FAD结合和催化的重要性。 Leu56变体显示L-半乳糖-1,4-内酯和L-gulono-1,4-内酯的Michaelis常数有显着差异，并且比野生型AtGALDH更容易释放FAD辅因子。本研究提供了AtGALDH和一些活性位点变异的第一个生化特征。还讨论了GALDH的作用以及其他醛糖内酯氧化还原酶可能参与拟南芥中维生素C的生物合成。

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