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首页> 外文期刊>Biochemistry >The Reaction of Yeast Cystathionine #beta#-Synthase Is Rate-Limited by the Conversion of Aminoacrylate to Cystathionine
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The Reaction of Yeast Cystathionine #beta#-Synthase Is Rate-Limited by the Conversion of Aminoacrylate to Cystathionine

机译：氨基丙烯酸酯向胱硫醚的转化速率限制了胱硫代蛋氨酸＃β＃合酶的反应。

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Our studies of the reaction mechanism of cystathionine #beta#-synthase from Saccharomyces cerevisiae (yeast) are facilitated by the spectroscopic properties of the pyridoxal phosphate coenzyme that forms a series of intermediates in the reaction of L-serine and L-homocysteine to form L-crytathionine. To characterize these reaction intermediates, we have carried out rapid-scanning stopped-flow and single-wavelength stopped-flow kinetic measurements under pre-steady-state conditions, as well as circular dichroism and fluorescence spectroscopy under steady-sate condition. We find that the gem-diamine and external aldimine of aminoacrylate are the primary intermediates in the forward half-reaction with L-serine and that the external aldimine of aminoacrylate or its complex with L-homocysteine is the primary intermediate in the reverse half-reaction with L-cystathionne. The second forward half-reaction of aminoacrylate with L-homocysteine is rapid. No primary kinetic isotope effect was obtained in the forward half-reaction with L-serine. The results provide evidence (1) that the formation of the external aldimine of L-serine is faster than the formation of the aminoacrylate intermediate, (2) that aminoacrylate is formed by the concerted removal of the #alpha#-proton and the hydroxyl group of L-serine, and (3) that the rate of the overall reaction is rate-limited by the conversion of aminoacrylate to L-cystathionine. We compare our results with cystathionine #beta#-synthase with those of related investigations of tryptophan synthase and O-acetylserine sulfhydrylase.
机译：吡of醛磷酸辅酶的光谱性质促进了我们对酿酒酵母（酵母）中的胱硫醚β-合酶反应机理的研究，该酶在L-丝氨酸和L-高半胱氨酸形成L的反应中形成了一系列中间体-甲硫氨酸。为了表征这些反应中间体，我们在稳态之前进行了快速扫描的停止流和单波长停止流动力学测量，并在稳态条件下进行了圆二色性和荧光光谱。我们发现，氨基丙烯酸的宝石二胺和外部亚胺是与L-丝氨酸的正向半反应的主要中间体，而氨基丙烯酸的外部亚胺或其与L-高半胱氨酸的配合物是反向半反应的主要中间体。与L-胱硫醚。氨基丙烯酸酯与L-高半胱氨酸的第二正向半反应是快速的。在与L-丝氨酸的正向半反应中未获得主要的动力学同位素效应。结果提供了证据（1）L-丝氨酸的外部醛亚胺的形成要快于氨基丙烯酸酯中间体的形成，（2）氨基丙烯酸酯是通过协同去除＃α＃质子和羟基而形成的（3）通过氨基丙烯酸酯向L-胱硫醚的转化来限制整个反应的速率。我们将胱硫醚＃beta＃-合酶的结果与色氨酸合酶和O-乙酰丝氨酸巯基化酶的相关研究进行了比较。

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