Unlabeled D- and L-alanine were racemized in deuterium oxide with an alanine racemase of Bacillus stearothermophilus at saturated concentration of substrate, and various p2H and temperature. Samples of the solution were taken at intervals, and all alanine isomers in the samples were transformed into a mixture of diastereomeric derivatives of methyl N-(–)-camphanylalaninate. Their ratio was measured on a GC-Mass, and the relative rate was calculated at the initial stage of the reaction. There was little difference in the decrease rate of the optical rotation between the enantiomers. Internal proton-transfer to the antipode was almost zero for either substrate. The α-hydrogen was abstracted 1.2–2.3 times faster from D-alanine than from L-alanine. D-Alanine gave an almost even mixture of deuterium labeled D- and L-alanine, while L-alanine gave a mixture of labeled D- and L-alanine at a ratio of 3:1. These results suggest the racemase builds two different bases in the active site. The base for D-alanine may be closer to the enzyme surface, and that for L-alanine inside.
在底物浓度饱和、p2H 和温度不同的条件下,用硬嗜热脂肪芽孢杆菌的丙
氨酸消旋酶在氧化
氘中对未标记的 D-和
L-丙氨酸进行消旋化。每隔一段时间对溶液进行取样,将样品中的所有丙
氨酸异构体转化为 N-(-)-
樟脑丙
氨酸甲酯非对映异构体衍
生物的混合物。在气相色谱-质谱仪上测量它们的比率,并计算反应初始阶段的相对速率。对映体之间的光学旋转下降率差别不大。两种底物向对映体的内部质子转移几乎为零。从
D-丙氨酸中抽取 α-氢的速度是
L-丙氨酸的 1.2-2.3 倍。D- 丙
氨酸产生的
氘标记 D- 和 L- 丙
氨酸混合物几乎是均匀的,而 L- 丙
氨酸产生的标记 D- 和 L- 丙
氨酸混合物的比例为 3:1。这些结果表明,消旋酶在活性位点中构建了两种不同的碱基。
D-丙氨酸的碱基可能更靠近酶的表面,而
L-丙氨酸的碱基则在酶的内部。