Tamogami Jun, Kikukawa Takashi, Nara Toshifumi, Demura Makoto, Kimura-Someya Tomomi, Shirouzu Mikako, Yokoyama Shigeyuki, Miyauchi Seiji, Shimono Kazumi, Kamo Naoki
College of Pharmaceutical Sciences, Matsuyama University, Matsuyama, Ehime 790-8578, Japan.
Faculty of Advanced Life Science, Hokkaido University, Sapporo 060-0810, Japan.
Biophys Physicobiol. 2017 Mar 1;14:49-55. doi: 10.2142/biophysico.14.0_49. eCollection 2017.
A spectrally silent change is often observed in the photocycle of microbial rhodopsins. Here, we suggest the presence of two O intermediates in the photocycle of rhodopsin II (ARII or also called Ace2), a light-driven algal proton pump from . ARII exhibits a photocycle including a quasi-equilibrium state of M, N, and O (M⇄N⇄O→) at near neutral and above pH values. However, acidification of the medium below pH ~5.5 causes no accumulation of N, resulting in that the photocycle of ARII can be described as an irreversible scheme (M→O→). This may facilitate the investigation of the latter part of the photocycle, especially the rise and decay of O, during which molecular events have not been sufficiently understood. Thus we analyzed the photocycle under acidic conditions (pH ≤ 5.5). Analysis of the absorbance change at 610 nm, which mainly monitors the fractional concentration changes of K and O, was performed and revealed a photocycle scheme containing two sequential O-states with the different molar extinction coefficients. These photoproducts, termed O and O, may be even produced at physiological pH, although they are not clearly observed under this condition due to the existence of a long M-N-O equilibrium.
在微生物视紫红质的光循环中经常观察到一种光谱上无信号的变化。在此，我们提出在视紫红质II（ARII，也称为Ace2，一种来自[藻类名称未给出]的光驱动藻类质子泵）的光循环中存在两种O中间体。ARII在接近中性及更高pH值时表现出包括M、N和O的准平衡态（M⇄N⇄O→）的光循环。然而，介质酸化至pH约5.5以下时不会积累N，导致ARII的光循环可描述为不可逆过程（M→O→）。这可能有助于研究光循环的后半部分，特别是O的上升和衰减，在此期间分子事件尚未得到充分理解。因此，我们分析了酸性条件（pH≤5.5）下的光循环。对610 nm处吸光度变化进行了分析，该变化主要监测K和O的分数浓度变化，结果揭示了一个包含两个具有不同摩尔消光系数的连续O态的光循环过程。这些光产物，称为O和O，甚至可能在生理pH下产生，尽管由于存在长的M - N - O平衡，在这种条件下它们未被清晰观察到。
