School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia;
Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6;
Proc Natl Acad Sci U S A. 2014 Jul 1;111(26):E2666-75. doi: 10.1073/pnas.1402538111. Epub 2014 Jun 16.
Observation of coherent oscillations in the 2D electronic spectra (2D ES) of photosynthetic proteins has led researchers to ask whether nontrivial quantum phenomena are biologically significant. Coherent oscillations have been reported for the soluble light-harvesting phycobiliprotein (PBP) antenna isolated from cryptophyte algae. To probe the link between spectral properties and protein structure, we determined crystal structures of three PBP light-harvesting complexes isolated from different species. Each PBP is a dimer of αβ subunits in which the structure of the αβ monomer is conserved. However, we discovered two dramatically distinct quaternary conformations, one of which is specific to the genus Hemiselmis. Because of steric effects emerging from the insertion of a single amino acid, the two αβ monomers are rotated by ∼73° to an "open" configuration in contrast to the "closed" configuration of other cryptophyte PBPs. This structural change is significant for the light-harvesting function because it disrupts the strong excitonic coupling between two central chromophores in the closed form. The 2D ES show marked cross-peak oscillations assigned to electronic and vibrational coherences in the closed-form PC645. However, such features appear to be reduced, or perhaps absent, in the open structures. Thus cryptophytes have evolved a structural switch controlled by an amino acid insertion to modulate excitonic interactions and therefore the mechanisms used for light harvesting.
对光合蛋白的二维电子光谱（2D ES）中的相干振荡的观察，促使研究人员开始探讨非平凡的量子现象是否在生物学上具有重要意义。已经报道了从隐藻中分离的可溶性捕光藻胆蛋白（PBP）天线的相干振荡。为了探究光谱性质与蛋白质结构之间的联系，我们确定了三种从不同物种中分离的 PBP 光收集复合物的晶体结构。每个 PBP 都是 αβ 亚基的二聚体，其中 αβ 单体的结构是保守的。然而，我们发现了两种截然不同的四级构象，其中一种仅存在于半枝藻属中。由于单个氨基酸的插入引起的空间位阻效应，两个 αβ 单体旋转约 73°至“打开”构型，与其他隐藻 PBPs 的“关闭”构型形成对比。这种结构变化对于光收集功能非常重要，因为它破坏了封闭形式中两个中央发色团之间的强激子耦合。2D ES 显示出明显的交叉峰振荡，归因于封闭形式 PC645 中的电子和振动相干性。然而，在开放结构中，这些特征似乎减少或缺失。因此，隐藻进化出了一种由氨基酸插入控制的结构开关，以调节激子相互作用，从而调节用于光收集的机制。
