Graduate School of Pharmaceutical Sciences, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8675, Japan.
Elements Chemistry Laboratory, RIKEN, and RIKEN Center for Sustainable Resource Science (Wako campus) 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan.
PLoS One. 2018 Jun 13;13(6):e0198944. doi: 10.1371/journal.pone.0198944. eCollection 2018.
In the late stage of anthocyanin biosynthesis, dihydroflavonol reductase (DFR) and anthocyanidin synthase (ANS) mediate a formal tautomerization. However, such oxidation/reduction process requires high energy and appears to be unnecessary, as the oxidation state does not change during the transformation. Thus, a non-enzymatic pathway of tautomerization has also been proposed. To resolve the long-standing issue of whether this non-enzymatic pathway is the main contributor for the biosynthesis, we carried out density functional theory (DFT) calculations to examine this non-enzymatic pathway from dihydroflavonol to anthocyanidin. We show here that the activation barriers for the proposed non-enzymatic tautomerization are too high to enable the reaction to proceed under normal aqueous conditions in plants. The calculations also explain the experimentally observed requirement for acidic conditions during the final step of conversion of 2-flaven-3,4-diol to anthocyanidin; a thermodynamically and kinetically favorable concerted pathway can operate under these conditions.
在花色苷生物合成的晚期阶段，二氢黄酮醇还原酶（DFR）和花青素合酶（ANS）介导了一个正式的互变异构。然而，这种氧化/还原过程需要很高的能量，而且似乎是不必要的，因为在转化过程中氧化态不会发生变化。因此，也提出了一种非酶促的互变异构途径。为了解决这个长期存在的问题，即非酶促途径是否是生物合成的主要贡献者，我们进行了密度泛函理论（DFT）计算，以研究从二氢黄酮醇到花青素的非酶促互变异构途径。我们在这里表明，所提出的非酶促互变异构的活化势垒太高，以至于在植物的正常水相条件下无法进行反应。这些计算还解释了实验观察到的在 2- flavan-3,4-二醇转化为花青素的最后一步需要酸性条件的原因；在这些条件下，可以进行热力学和动力学有利的协同途径。
