Adiabatic Ligand Binding in Heme Proteins: Ultrafast Kinetics of Methionine Rebinding in Ferrous Cytochrome c
Adiabatic Ligand Binding in Heme Proteins: Ultrafast Kinetics of Methionine Rebinding in Ferrous Cytochrome c
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DOI：
10.1021/acs.jpcb.8b07355
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发表时间：
2018-12-13
影响因子：
3.3
通讯作者：
Champion, Paul M.
中科院分区：
文献类型：
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作者：
Benabbas, Abdelkrim;Champion, Paul M.
在较宽的温度范围内研究了亚铁细胞色素c光解后蛋氨酸偕复合的动力学。的动力学响应，以上的溶剂玻璃化转变（T-g），几乎是单指数，并显示出弱的温度依赖性。低于T-g，再结合动力学是非指数的，可以解释使用淬灭分布的hemic再结合障碍，从一个相对较窄的分布血红素面外位移。该反应（Δ S = 2）的Arrhenius前因子类似于10（11）s（-1），这与血红素蛋白中（Δ S = 1）NO结合反应的结果相似。这一观察结果，沿着其他超快CO结合的例子，提供了强有力的证据，配体结合血红素是一个绝热反应与自旋无关的前因子。为了同时考虑反应的绝热性质以及超快CO和甲硫氨酸双生再结合的温度依赖性，提出了自旋三重态在过渡态区域中沿沿着反应坐标与反应物（S = 2）和产物（S = 0）状态表面相交并强烈耦合。它还建议，相交的三重态和反应路径的性质可能取决于邻近的光解配体相对于铁原子。在温度低于类似60 K，动力学数据表明，有一个意想不到的延迟血红素光产物松弛或重原子量子力学隧道变得显着。
The dynamics of methionine geminate recombination following photodissociation in ferrous cytochrome c is investigated over a broad temperature range. The kinetic response, above the solvent glass transition (T-g), is nearly monoexponential and displays a weak temperature dependence. Below T-g, the rebinding kinetics are nonexponential and can be explained using a quenched distribution of enthalpic rebinding barriers, arising from a relatively narrow distribution of heme out-of-plane displacements. The Arrhenius prefactor of this (Delta S = 2) reaction is similar to 10(11) s(-1), which is similar to what has been found for the (Delta S = 1) NO binding reaction in heme proteins. This observation, along with other examples of ultrafast CO binding, provides strong evidence that ligand binding to heme is an adiabatic reaction with a spin independent prefactor. In order to simultaneously account for the adiabatic nature of the reaction as well as the temperature dependence of both ultrafast CO and methionine geminate rebinding, it is proposed that a spin triplet state intersects and strongly couples to the reactant (S = 2) and product (S = 0) state surfaces in the transition state region along the reaction coordinate. It is also suggested that the nature of the intersecting triplet state and the reaction path may depend upon the proximity of the photolyzed ligand relative to the iron atom. At temperatures below similar to 60 K, the kinetic data suggest that there is either an unexpected retardation of the heme photoproduct relaxation or that heavy atom quantum mechanical tunneling becomes significant.