Photosynthetic organisms have achieved sophisticated photochemical reaction systems consisting of pigment-protein complexes. The light-harvesting antenna complex (LHC) absorbs sunlight and transfers the light energy to the reaction center (RC), where charge separation is induced. A series of light reactions occurs among pigment molecules embedded in the complexes. The molecular arrangement is highly optimized, leading to an extremely high efficiency of the light reaction. This explains why photosynthesis can continue under dim light conditions. Conversely, under intense light illumination, the system is likely to be photodamaged by an excess light energy. To respond to frequent and significant changes in actual light environments, phototrophs have a variety of photoprotection mechanisms.
The LHC contains multiple pigments, such as chlorophylls (Chls) and carotenoids (Cars). Chls are responsible for the excitation energy transfer via molecular interactions with each other. Conversely, Cars receive energy from Chls and dissipate it as heat, a process known as non-photochemical quenching, thus protecting against excess light energy. The quenching efficiency, which significantly depends on the relative arrangement of Chls and Cars, can be easily perturbed by even slight and local conformational changes around their binding sites in the protein. Considering that protein conformation is thermally fluctuating, the quenching efficiency by Cars should also fluctuate. Thus, to investigate the photoprotection mechanism by analyzing fluctuations in fluorescence properties, we applied single-molecule spectroscopy to the LHC protein.
The fluorescence of individual LHCs exhibited temporal variations in both intensity and lifetime. Statistical analyses of their time sequence data revealed frequent transitions between photoactive and inactive-quenched states, likely reflecting protein conformational fluctuations around the binding sites of Chl and Car. Additionally, under low pH conditions, in which the protein scaffold of LHCs has been reported to vary, the fluctuation behavior was restricted, and consequently, the quenching state was stabilized. The pH drop is induced by the photochemical reaction in the RC, especially under intense light conditions. From these results, it is suggested that LHCs sensitively sense light environmental changes through pH variations and then adjust quenching efficiency by flexibly altering their own structure. In this presentation, I will discuss photosynthetic light-harvesting regulation mechanism associated with protein dynamics.