Fritz Haber Center for Molecular Dynamics Research, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Department of Chemistry and Chemical Biology, Rutgers University, USA.
Phys Chem Chem Phys. 2021 Mar 28;23(12):7359-7367. doi: 10.1039/d0cp05314g. Epub 2021 Mar 23.
Cyanobacteriochromes are compact and spectrally diverse photoreceptor proteins that bind a linear tetrapyrrole as a chromophore. They show photochromicity by having two stable states that can be interconverted by the photoisomerization of the chromophore. These photochemical properties make them an attractive target for biotechnological applications. However, their application is impeded by structural heterogeneity that reduces the yield of the photoconversion. The heterogeneity can originate either from the chromophore structure or the protein environment. Here, we study the origin of the heterogeneity in AnPixJg2, a representative member of the red/green cyanobacteriochrome family, that has a red absorbing parental state and a green absorbing photoproduct state. Using molecular dynamics simulations and umbrella sampling we have identified the protonation state of a conserved histidine residue as a trigger for structural heterogeneity. When the histidine is in a neutral form, the chromophore structure is homogenous, while in a positively charged form, the chromophore is heterogeneous with two different conformations. We have identified a correlation between the protonation of the histidine and the structural heterogeneity of the chromophore by detailed characterization of the interactions in the protein binding site. Our findings reconcile seemingly contradicting spectroscopic studies that attribute the heterogeneity to different sources. Furthermore, we predict that circular dichroism can be used as a diagnostic tool to distinguish different substates.
蓝藻发色团是紧凑且光谱多样的光受体蛋白，它们结合线性四吡咯作为发色团。通过发色团的光异构化，它们具有两种稳定状态，可以相互转换，从而表现出光致变色性。这些光化学性质使它们成为生物技术应用的有吸引力的目标。然而，由于结构异质性降低了光转化的产率，它们的应用受到了阻碍。这种异质性可能源自发色团结构或蛋白质环境。在这里，我们研究了红/绿藻蓝细菌发色团代表性成员 AnPixJg2 中异质性的起源，该蛋白具有红色吸收母体状态和绿色吸收光产物状态。我们使用分子动力学模拟和伞状采样，确定了保守组氨酸残基的质子化状态是结构异质性的触发因素。当组氨酸呈中性形式时，发色团结构是均匀的，而当它呈正电荷形式时，发色团是异质的，具有两种不同的构象。我们通过对蛋白质结合位点相互作用的详细表征，确定了组氨酸的质子化与发色团结构异质性之间的相关性。我们的发现调和了看似矛盾的光谱研究，这些研究将异质性归因于不同的来源。此外，我们预测圆二色性可以用作区分不同亚状态的诊断工具。
