Department of Physics and Center for Interdisciplinary Research on Complex Systems, Northeastern University , Boston, Massachusetts 02115, United States.
Departments of Molecular Biology and Biochemistry, Chemistry, and Pharmaceutical Sciences, University of California , Irvine, California 92697, United States.
J Am Chem Soc. 2017 Nov 8;139(44):15738-15747. doi: 10.1021/jacs.7b07507. Epub 2017 Oct 24.
The ultrafast kinetics of CO rebinding to carbon monoxide oxidation activator protein (ChCooA) are measured over a wide temperature range and compared with the kinetics of CO binding in other heme systems such as myoglobin (Mb) and hemoglobin (Hb). The Arrhenius prefactor for CO binding to ChCooA and protoheme (∼10 s) is similar to what is found for spin-allowed NO binding to heme proteins and is several orders of magnitude larger than the prefactor of Mb and Hb (∼10 s). This indicates that the CO binding reaction is adiabatic, in contrast to the commonly held view that it is nonadiabatic due to spin-forbidden (ΔS = 2) selection rules. Under the adiabatic condition, entropic factors, rather than spin-selection rules, are the source of the reduced Arrhenius prefactors associated with CO binding in Mb and Hb. The kinetic response of ChCooA-CO is nonexponential at all temperatures, including 298 K, and is described quantitatively using a distribution of enthalpic rebinding barriers associated with heterogeneity in the heme doming conformation. Above the solvent glass transition (T ∼ 180 K), the rebinding progress slows as temperature increases, and this is ascribed to an evolution of the distribution toward increased heme doming and larger enthalpic barriers. Between T and ∼60 K, the nonexponential rebinding slows down as the temperature is lowered and the survival fraction follows the predictions expected for a quenched barrier distribution. Below ∼60 K the rebinding kinetics do not follow these predictions unless quantum mechanical tunneling along the heme doming coordinate is also included as an active channel for CO binding.
一氧化碳重新结合到一氧化碳氧化激活蛋白（ChCooA）的超快动力学在很宽的温度范围内进行测量，并与其他血红素系统（如肌红蛋白（Mb）和血红蛋白（Hb））中 CO 结合的动力学进行比较。ChCooA 和原血红素（∼10 s）与 spin-allowed 允许的 NO 结合到血红素蛋白的 CO 结合的 Arrhenius 前因子相似，比 Mb 和 Hb 的前因子（∼10 s）大几个数量级。这表明 CO 结合反应是绝热的，与通常认为的由于 spin-forbidden（ΔS = 2）选择规则而不是绝热的观点相反。在绝热条件下，熵因素而不是 spin 选择规则是导致 Mb 和 Hb 中 CO 结合的 Arrhenius 前因子减小的原因。ChCooA-CO 的动力学响应在所有温度下都不是指数的，包括 298 K，并使用与血红素穹顶构象异质性相关的焓复绑定势垒分布进行定量描述。在溶剂玻璃化转变（T ∼ 180 K）以上，随着温度的升高，复绑定速度会减慢，这归因于分布向增加的血红素穹顶和更大的焓势垒的演变。在 T 和 ∼60 K 之间，随着温度降低，非指数复绑定速度会减慢，而存活分数遵循预期的淬灭势垒分布的预测。在 ∼60 K 以下，除非沿着血红素穹顶坐标的量子力学隧道也被包括作为 CO 结合的活性通道，否则复绑定动力学不会遵循这些预测。
