Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607.
Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607;
Proc Natl Acad Sci U S A. 2017 Jun 13;114(24):6286-6291. doi: 10.1073/pnas.1700956114. Epub 2017 May 30.
Photoprotection is essential for efficient photosynthesis. Cyanobacteria have evolved a unique photoprotective mechanism mediated by a water-soluble carotenoid-based photoreceptor known as orange carotenoid protein (OCP). OCP undergoes large conformational changes in response to intense blue light, and the photoactivated OCP facilitates dissipation of excess energy via direct interaction with allophycocyanins at the phycobilisome core. However, the structural events leading up to the OCP photoactivation remain elusive at the molecular level. Here we present direct observations of light-induced structural changes in OCP captured by dynamic crystallography. Difference electron densities between the dark and illuminated states reveal widespread and concerted atomic motions that lead to altered protein-pigment interactions, displacement of secondary structures, and domain separation. Based on these crystallographic observations together with site-directed mutagenesis, we propose a molecular mechanism for OCP light perception, in which the photochemical property of a conjugated carbonyl group is exploited. We hypothesize that the OCP photoactivation starts with keto-enol tautomerization of the essential 4-keto group in the carotenoid, which disrupts the strong hydrogen bonds between the bent chromophore and the protein moiety. Subsequent structural changes trapped in the crystal lattice offer a high-resolution glimpse of the initial molecular events as OCP begins to transition from the orange-absorbing state to the active red-absorbing state.
光保护对于高效光合作用至关重要。蓝藻进化出了一种独特的光保护机制，由一种水溶性类胡萝卜素基光受体介导，称为橙色类胡萝卜素蛋白（OCP）。OCP 对强光会发生大的构象变化，而光激活的 OCP 通过与藻胆体核心的别藻蓝蛋白直接相互作用，促进多余能量的耗散。然而，在分子水平上，导致 OCP 光激活的结构事件仍然难以捉摸。在这里，我们通过动态晶体学直接观察到 OCP 中光诱导结构变化。暗态和光照态之间的差分电子密度揭示了广泛而协调的原子运动，导致蛋白-色素相互作用改变、二级结构位移和结构域分离。基于这些晶体学观察结果和定点突变，我们提出了 OCP 光感知的分子机制，其中利用了共轭羰基的光化学性质。我们假设，OCP 的光激活始于类胡萝卜素中必需的 4-酮基的酮-烯醇互变异构，这破坏了弯曲生色团与蛋白质部分之间的强氢键。随后在晶格中捕获的结构变化提供了 OCP 从橙色吸收态向活性红色吸收态转变的初始分子事件的高分辨率观察。
