Investigation of homochiral origin on the basis of comparison between enzymatic activity and active site for both enantiomers

通过实验确定了D-色氨酸和L-色氨酸的活性部位的相对位置，得到了以下结果。结论1.在高浓度磷酸氢二铵溶液中，D-色氨酸完全不消旋。分析了丙酮酸钾和吲哚丙酮酸钾对L-色氨酸降解反应的抑制类型。前者是竞争性的，而后者随着磷酸氢二铵浓度的增加而从竞争性转变为非竞争性。γ-色氨酸酶在γ-射线照射下对D-色氨酸失活，即使在磷酸氢二铵存在下也具有竞争性抑制作用。根据上述结果，D-色氨酸和L-色氨酸的活性中心之间的位置关系可以说明如下。也就是说，D-色氨酸的杂环部分与远离L-色氨酸结合部位的另一个结合部位结合。D-色氨酸的位置与色氨酸酶的反应有一个合适的角度。因此，D-色氨酸可以变成色氨酸酶。尽管酶的立体选择性曾经被认为是永久严格的，但这项研究表明，酶周围的环境变化使其变得模糊。我们将研究结构变化如何对手性立体选择性变化有效。

Experiments were performed to identify the relative position of an active site for D-tryptophan with that for L-tryptophan, and the following results were obtained.1. D-tryptophan was not racemized at all in highly concentrated diammoniumhydrogen phosphate solution.2. The inhibition types of potassium pyruvate and indolepyruvate were analyzed for L-tryptohan degradation reaction. The former was competitive, but the latter switched from competitive to noncompetitive type with increasing diammoniumhydrogen phosphate concentrations.3. γ-tryptophanase, which was inactivated to D-tryptophan due to γ-ray irradiation, inhibited competitively even in the presence of diammoniumhydrogen phosphate.4. On the basis of the above results, positional relationship between an active site for D-tryptophan and one for L-tryptophan could be illustrated as follows. That is, heterocyclic moiety of D-tryptophan binds with another binding site remote from a binding site for L-tryptophan. The location of D-tryptophan has an appropriate angle to react with tryptophanase. Thus, D-tryptophan can become to tryptophanase.5. Although the stereoselectivity of enzyme has been ever considered to be permanently strict, it was shown by this study that environmental change around the enzyme made it ambiguous.6. We will investigate how a structural change effective to chirally stereoselective change is.