A theoretical study on the mechanism of camphor hydroxylation by compound I of cytochrome P450
A theoretical study on the mechanism of camphor hydroxylation by compound I of cytochrome P450
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DOI：
10.1021/ja0208862
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发表时间：
2003-04-16
影响因子：
15
通讯作者：
Yoshizawa, K
中科院分区：
文献类型：
--
作者：
Kamachi, T;Yoshizawa, K
通过 B3LYP DFT 计算讨论了细胞色素 P450 的化合物 I 铁氧物种将樟脑转化为 5-exo-羟基樟脑的机制和能量方面。该反应按照氧反弹机制所暗示的方式以两步方式发生。樟脑 C5 原子上 H 原子夺取的第一过渡态的活化能经计算超过 20 kcal/mol。 H 原子的提取是羟基化反应中的速率决定步骤，产生涉及碳自由基物质和铁羟基物质的反应中间体。连接反应中间体和产物醇络合物的回弹步骤的第二个过渡态比双峰和四峰势能表面上 H 原子提取的过渡态低几 kcal/mol。这种能量特征允许在两种自旋态下几乎无势垒的重组，与实验观察到的反应中间体的高立体选择性和短暂的寿命一致。还考虑和计算了樟脑羟基化催化机制的整体能量分布，特别是关于为什么在生理条件下可以获得氢原子提取的高活化能。根据涉及 Thr252、Asp251 和两个溶剂水分子的质子源模型 (Biochemisty 1998, 37, 9211)，研究了铁过氧物种转化为化合物 I 的能量学。在分子氧活化过程中释放出超过 50 kcal/mol 的大量能量。该化学过程中释放的能量是细胞色素 P450 烷烃羟基化的重要驱动力。该能量用于获得 H 原子抽象的高活化能。
Mechanistic and energetic aspects for the conversion of camphor to 5-exo-hydroxycamphor by the compound I iron-oxo species of cytochrome P450 are discussed from B3LYP DFT calculations. This reaction occurs in a two-step manner along the lines that the oxygen rebound mechanism suggests. The activation energy for the first transition state of the H atom abstraction at the C5 atom of camphor is computed to be more than 20 kcal/mol. This H atom abstraction is the rate-determining step in this hydroxylation reaction, leading to a reaction intermediate that involves a carbon radical species and the iron-hydroxo species. The second transition state of the rebound step that connects the reaction intermediate and the product alcohol complex lies a few kcal/mol below that for the H atom abstraction on the doublet and quartet potential energy surfaces. This energetic feature allows the virtually barrierless recombination in both spin states, being consistent with experimentally observed high stereoselectivity and brief lifetimes of the reaction intermediate. The overall energetic profile of the catalytic mechanism of camphor hydroxylation particularly with respect to why the high activation energy for the H atom abstraction is accessible under physiological conditions is also considered and calculated. According to a proton source model involving Thr252, Asp251, and two solvent water molecules (Biochemisty 1998, 37, 9211), the energetics for the conversion of the iron-peroxo species to compound I is studied. A significant energy over 50 kcal/mol is released in the course of this dioxygen activation process. The energy released in this chemical process is an important driving force in alkane hydroxylation by cytochrome P450. This energy is used for the access to the high activation energy for the H atom abstraction.