Department of Chemistry, Yale University, New Haven, CT 06520.
Department of Chemistry, Yale University, New Haven, CT 06520
Proc Natl Acad Sci U S A. 2020 Oct 27;117(43):26626-26632. doi: 10.1073/pnas.2016719117. Epub 2020 Oct 9.
Blue light using flavin (BLUF) photoreceptor proteins are critical for many light-activated biological processes and are promising candidates for optogenetics because of their modular nature and long-range signaling capabilities. Although the photocycle of the Slr1694 BLUF domain has been characterized experimentally, the identity of the light-adapted state following photoexcitation of the bound flavin remains elusive. Herein hybrid quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations of this photocycle provide a nonequilibrium dynamical picture of a possible mechanism for the formation of the light-adapted state. Photoexcitation of the flavin induces a forward proton-coupled electron transfer (PCET) process that leads to the formation of an imidic acid tautomer of Gln50. The calculations herein show that the subsequent rotation of Gln50 allows a reverse PCET process that retains this tautomeric form. In the resulting purported light-adapted state, the glutamine tautomer forms a hydrogen bond with the flavin carbonyl group. Additional ensemble-averaged QM/MM calculations of the dark-adapted and purported light-adapted states demonstrate that the light-adapted state with the imidic acid glutamine tautomer reproduces the experimentally observed spectroscopic signatures. Specifically, the calculations reproduce the red shifts in the flavin electronic absorption and carbonyl stretch infrared spectra in the light-adapted state. Further hydrogen-bonding analyses suggest the formation of hydrogen-bonding interactions between the flavin and Arg65 in the light-adapted state, providing a plausible explanation for the experimental observation of faster photoinduced PCET in this state. These characteristics of the light-adapted state may also be essential for the long-range signaling capabilities of this photoreceptor protein.
蓝光利用黄素（BLUF）光感受器蛋白对于许多光激活的生物过程至关重要，并且由于其模块化性质和远程信号传递能力，是光遗传学的有前途的候选者。尽管已经通过实验表征了 Slr1694 BLUF 结构域的光循环，但在结合的黄素被光激发后，适应光的状态的身份仍然难以捉摸。在此，该光循环的混合量子力学/分子力学（QM/MM）分子动力学模拟提供了形成适应光状态的可能机制的非平衡动力学图像。黄素的光激发诱导向前质子耦合电子转移（PCET）过程，导致 Gln50 的亚氨基酸互变异构体的形成。本文中的计算表明，随后 Gln50 的旋转允许保留这种互变异构形式的反向 PCET 过程。在由此产生的假定的适应光状态中，谷氨酰胺互变异构体与黄素羰基形成氢键。对暗适应和假定的适应光状态的附加平均 QM/MM 计算表明，具有亚氨基酸谷氨酰胺互变异构体的适应光状态再现了实验观察到的光谱特征。具体来说，计算再现了适应光状态中黄素电子吸收和羰基伸展红外光谱的红移。进一步的氢键分析表明，在适应光状态下，黄素和 Arg65 之间形成氢键相互作用，为该状态下更快的光诱导 PCET 的实验观察提供了合理的解释。适应光状态的这些特征对于这种光感受器蛋白的远程信号传递能力也可能是必不可少的。
