Solvent Polarity Effects and Limited Acid Catalysis in Rearrangements of Model Radicals for the Methylmalonyl-CoA Mutase- and Isobutyryl-CoA Mutase-Catalyzed Isomerization Reactions
作者:Pierre Daublain、John H. Horner、Andriy Kuznetsov、Martin Newcomb
DOI:10.1021/ja049913+
日期:2004.5.1
The kinetics of reactions of models for the intermediate radicals formed in the methylmalonyl-CoA mutase- and isobutyryl-CoA mutase-catalyzed rearrangements were studied by laser flash photolysis methods. The aldehyde-containing model analogous to the propanal-3-yl radical reacted via 3-exo cyclization with rate constants that varied with solvent polarity (k in the range 2 x 105 to 1 x 107 s-1). The
通过激光闪光光解方法研究了在甲基丙二酰辅酶 A 变位酶和异丁酰辅酶 A 变位酶催化重排中形成的中间自由基模型的反应动力学。类似于丙醛-3-基自由基的含醛模型通过 3-外环化反应,其速率常数随溶剂极性(k 在 2 x 105 到 1 x 107 s-1 的范围内)而变化。类似的含甲基酮的自由基反应速度要慢 2 个数量级,而含乙硫羰基的自由基类似物反应太慢,无法进行动力学测量。在乙酸中没有观察到酸催化,但 CF3CO2H 络合的自由基比未络合的自由基反应快 1 个数量级。结果表明,酶中通过氢键与酸键合形成的自由基的 3-exo 自由基环化,即所谓的“部分质子化”,不足以将反应加速到动力学能力点。自然界中自由基重排的解离机制被认为是另一种选择。