光是調(diào)節(jié)生物體重要生理過程（如生物鐘等）的一個基本信號。植物,，真菌和細菌中含有很多光敏色素,，形成了一系列紅色光敏感的感光器。他們在紅光吸收態(tài)（PR）和遠紅光吸收態(tài)（PFR）之間的經(jīng)過可逆的光轉(zhuǎn)化,，從而將光信號最終轉(zhuǎn)換成一個調(diào)解后續(xù)細胞反應(yīng)的獨特生物信號,。
已有幾種微生物光敏色素，在紅光吸收態(tài)（PR）或遠紅光吸收態(tài)（PFR）的暗適應(yīng)過程中的結(jié)構(gòu)已確定,。然而,，最初的光化學(xué)事件的結(jié)構(gòu)性質(zhì)并沒有被衍射分析表征。
在這里,，我們報告了綠膿桿菌光敏色素的光化學(xué)反應(yīng)中三個中間體的晶體結(jié)構(gòu),。我們使用冷凝阱晶體捕捉中間體，然后掃描光化學(xué)反應(yīng)進行時發(fā)生結(jié)構(gòu)性的變化時的溫度,。綠膿桿菌光敏色素光誘導(dǎo)的構(gòu)象變化起源于環(huán)膽綠素（BV）發(fā)光基團的D環(huán),，而C15=C16的環(huán)C和D之間雙鍵連接的E-to-Z異構(gòu)化觸發(fā)最初的光化學(xué)事件。當發(fā)光基團松弛,，C15次甲基橋的兩個二面角的扭曲是相反的,。結(jié)構(gòu)變化會進一步擴展到A環(huán)和B環(huán)，及周圍的蛋白質(zhì)區(qū)域,。這些數(shù)據(jù)表明,，綠膿桿菌光敏色素在遠紅光吸收態(tài)（PFR）吸收一個光子，通過扭曲和解纜在密閉的蛋白質(zhì)腔中的線性卟啉上的次甲基橋結(jié)構(gòu)將光信號轉(zhuǎn)換成光信號,。(生物谷bioon.com)
doi:10.1038/nature10506
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Temperature-scan cryocrystallography reveals reaction intermediates in bacteriophytochrome
Xiaojing Yang, Zhong Ren,Jane Kuk & Keith Moffat
Light is a fundamental signal that regulates important physiological processes such as development and circadian rhythm in living organisms. Phytochromes form a major family of photoreceptors responsible for red light perception in plants, fungi and bacteria. They undergo reversible photoconversion between red-absorbing (Pr) and far-red-absorbing (Pfr) states, thereby ultimately converting a light signal into a distinct biological signal that mediates subsequent cellular responses. Several structures of microbial phytochromes have been determined in their dark-adapted Pr or Pfr states. However, the structural nature of initial photochemical events has not been characterized by crystallography. Here we report the crystal structures of three intermediates in the photoreaction of Pseudomonas aeruginosa bacteriophytochrome (PaBphP). We used cryotrapping crystallography to capture intermediates, and followed structural changes by scanning the temperature at which the photoreaction proceeded. Light-induced conformational changes in PaBphP originate in ring D of the biliverdin (BV) chromophore, and E-to-Z isomerization about the C15 = C16 double bond between rings C and D is the initial photochemical event. As the chromophore relaxes, the twist of the C15 methine bridge about its two dihedral angles is reversed. Structural changes extend further to rings B and A, and to the surrounding protein regions. These data indicate that absorption of a photon by the Pfr state of PaBphP converts a light signal into a structural signal via twisting and untwisting of the methine bridges in the linear tetrapyrrole within the confined protein cavity.
