Temperature-dependent heme kinetics with nonexponential binding and barrier relaxation in the absence of protein conformational substates.
Temperature-dependent heme kinetics with nonexponential binding and barrier relaxation in the absence of protein conformational substates.
复制标题
在不存在蛋白质构象亚状态的情况下，温度依赖性血红素动力学具有非指数结合和屏障松弛。
DOI：
10.1073/pnas.0702622104
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发表时间：
2007
影响因子：
11.1
通讯作者：
Champion,PaulM
中科院分区：
文献类型：
--
作者：
Ye,Xiong;Ionascu,Dan;Gruia,Florin;Yu,Anchi;Benabbas,Abdelkrim;Champion,PaulM
我们提出了超快双原子配体与“裸”原血红素（L1-FePPIX-L2，其中L1= H2O或2-甲基咪唑，L2= CO或NO）结合的温度依赖性动力学测量。我们发现，CO的结合是温度依赖性和非指数超过几十年的时间，而NO的结合是指数和温度无关。CO与血红素结合的非指数性质以及其在溶剂玻璃化转变以上的松弛，模仿了CO与肌红蛋白（Mb）结合的动力学，但在更快的时间尺度上。这表明，观察到的Mb的非指数动力学响应不一定是由于蛋白质构象substates的存在下，而是溶剂化血红素的固有属性。的非指数动力学数据进行了分析，通过使用线性耦合模型与分布的heme-CO重组时间比在较慢的时间尺度上波动的heme-CO的障碍。在低于溶剂玻璃化转变温度（Tg ≥ 180 K）时，H2O-PPIX-CO体系的平均再结合势垒为1.1kJ/mol。当Tg低于Tg时，势垒发生弛豫，在290 K时，势垒为1.6kJ/mol。从动力学数据中提取的血红素拱坐标分布的前两个时刻的值表明Tg以上的显着非谐性。与Mb相反，血红素没有显示出存在“远端”生物屏障的迹象。此外，在不同条件下，CO与血红素结合的Arabius前因子（109 ~ 1011 s −1）范围很广，这表明熵垒可能是这类生化反应的重要控制源。
We present temperature-dependent kinetic measurements of ultrafast diatomic ligand binding to the “bare” protoheme (L1-FePPIX-L2, where L1= H2O or 2-methyl imidazole and L2= CO or NO). We found that the binding of CO is temperature-dependent and nonexponential over many decades in time, whereas the binding of NO is exponential and temperature-independent. The nonexponential nature of CO binding to protoheme, as well as its relaxation above the solvent glass transition, mimics the kinetics of CO binding to myoglobin (Mb) but on faster time scales. This demonstrates that the nonexponential kinetic response observed for Mb is not necessarily due to the presence of protein conformational substates but rather is an inherent property of the solvated heme. The nonexponential kinetic data were analyzed by using a linear coupling model with a distribution of enthalpic barriers that fluctuate on slower time scales than the heme–CO recombination time. Below the solvent glass transition (Tg≈ 180 K), the average enthalpic rebinding barrier for H2O-PPIX-CO was found to be ≈1 kJ/mol. AboveTg, the barrier relaxes and is ≈6 kJ/mol at 290 K. Values for the first two moments of the heme doming coordinate distribution extracted from the kinetic data suggest significant anharmonicity aboveTg. In contrast to Mb, the protoheme shows no indication of the presence of “distal” enthalpic barriers. Moreover, the wide range of Arrhenius prefactors (109to 1011s−1) observed for CO binding to heme under differing conditions suggests that entropic barriers may be an important source of control in this class of biochemical reactions.