Semialjac Marija, Schwarz Helmut
Institut für Chemie der Technischen Universität Berlin, D-10623 Berlin, Germany.
J Am Chem Soc. 2002 Jul 31;124(30):8974-83. doi: 10.1021/ja020101s.
DFT (B3LYP/6-31G) and ab initio molecular orbital theory (QCISD/cc-pVDZ) are used to investigate several possible mechanisms involving free radical intermediates as well as their protonated forms for processes related to the coenzyme B(12)-dependent rearrangement catalyzed by ethanolamine ammonia lyase. Two major types of rearrangements are discussed in detail, intramolecular migration and dissociation of the amine/ammonia groups, for both of which several scenarios are considered. According to the calculations, the complete dissociation of the migrating group and its subsequent association constitute an unlikely route for both the protonated and the unprotonated reactant because of the high-energy barriers (more than 23 kcal/mol) involved in these steps. Direct migration of the protonated amine group is far more favorable (10.4 kcal/mol) and therefore presents the most likely candidate for the actual enzymatic reaction. The calculations further imply that the direct loss of an ammonium cation (10.6 kcal/mol) represents a feasible pathway as well. Comparing the rearrangements for the aminoethanol radical and its protonated counterpart, in line with previous findings reported by Golding, Radom, and co-workers, we find that the migration of a protonated group is in general associated with lower energy barriers, suggesting that the actual enzyme substrate quite likely corresponds to (partially) protonated aminoethanol. As the extent of the substrate protonation/deprotonation by the active site of the enzyme may vary, the actual energy barriers are expected to range between the values calculated for the two extreme cases of a substrate, that is, the aminoethanol radical 2 and its fully protonated form 6.
采用密度泛函理论（DFT，B3LYP/6 - 31G）和从头算分子轨道理论（QCISD/cc - pVDZ），研究了几种可能涉及自由基中间体及其质子化形式的机制，这些机制与乙醇胺氨裂合酶催化的依赖辅酶B12的重排过程相关。详细讨论了两种主要类型的重排，即分子内迁移以及胺/氨基团的解离，并对这两种情况考虑了几种情形。根据计算，迁移基团的完全解离及其随后的缔合对于质子化和未质子化的反应物而言都不太可能是一条途径，因为这些步骤涉及高能垒（超过23千卡/摩尔）。质子化胺基团的直接迁移要有利得多（10.4千卡/摩尔），因此是实际酶促反应最有可能的候选途径。计算还表明，铵阳离子的直接损失（10.6千卡/摩尔）也是一条可行的途径。比较氨基乙醇自由基及其质子化对应物的重排情况，与戈尔丁、拉多姆及其同事先前报道的结果一致，我们发现质子化基团的迁移通常与较低的能垒相关，这表明实际的酶底物很可能对应于（部分）质子化的氨基乙醇。由于酶的活性位点对底物进行质子化/去质子化的程度可能不同，实际的能垒预计介于针对底物的两种极端情况（即氨基乙醇自由基2及其完全质子化形式6）计算出的值之间。
