Are carboxylations involving biotin concerted or nonconcerted?
Are carboxylations involving biotin concerted or nonconcerted?
复制标题
涉及生物素的羧化是协同的还是非协同的？
DOI：
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发表时间：
1980
影响因子：
4.8
通讯作者：
R. Abeles
中科院分区：
文献类型：
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作者：
J. Stubbe;S. Fish;R. Abeles
转羧化酶和丙酰辅酶a羧化酶催化-氟丙酰辅酶a中HF的消除生成丙烯酰辅酶a。未检出氟丙酰辅酶a羧化产物。消除的速度与正常的羧基化反应大致相同。丙炔辅酶a羧化酶需要ATP （HCO3-也存在），并水解成ADP和Pi。ADP的生成速率与丙烯酰辅酶a的生成速率相等。以前的报道（Stubbe， J. A, and Abeles， R. H.(1977)）。化学。252,8338—8340)丙烯酰辅酶a的形成比ADP的形成更快是错误的。在转羧化酶作用下，丙烯酰辅酶a的生成需要草酸酯，丙酮酸酯的生成需要草酸酯。丙酮酸的生成速率与丙烯酰辅酶a的生成速率相等。我们得出的结论是，该酶催化从-氟丙酰辅酶a中去除HF的能力表明，该酶可以催化底物α质子的提取，而不会伴随底物的羧化。我们还得出结论，因此，正常的催化反应可能涉及碳离子中间体，而不是像经常提出的那样通过协调过程进行。
Transcarboxylase and propionyl-CoA carboxylase catalyze the elimination of HF from beta-fluoropropionyl-CoA to form acrylyl-CoA. No carboxylation products of fluoropropionyl-CoA could be detected. The elimination proceeds at approximately the same rate as the normal carboxylation reactions. With propionyl-CoA carboxylase, ATP was required (HCO3- was also present) and was hydrolyzed to ADP and Pi. The rate of ADP formation was equal to that of acrylyl-CoA formation. A previous report (Stubbe, J. A., and Abeles, R. H. (1977) J. Biol. Chem. 252, 8338--8340) that acrylyl-CoA formation is faster than ADP formation is in error. With transcarboxylase, oxalacetate was required for acrylyl-CoA formation, and pyruvate was produced. The rate of pyruvate formation was equal to that of acrylyl-CoA formation. We conclude that the ability of the enzyme to catalyze the elimination of HF from beta-fluoropropionyl-CoA indicates that the enzyme can catalyze the abstraction of the substrate alpha-proton without concomitant carboxylation of the substrate. We also conclude that the normal catalytic reaction, therefore, probably involves a carbanion intermediate and does not proceed through a concerted process as has frequently been proposed.