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"... Light-harvesting complex II (LHCII) is a complex of pigment molecules bound to proteins. It switches between two main functions—dissipating harmful excess light energy as heat under high light intensity through nonphotochemical quenching, and transferring absorbed light to the reaction center with almost a unit efficiency under low light. ...
Bioengineering studies have shown that accelerating the transition between these two functions can increase photosynthetic efficiency—for example, soybean yields have been reported to increase by up to 33%. However, the atomic-level dynamic structural changes in LHCII that activate such allosteric regulation had not been previously elucidated. ...
As part of their work, they reported a series of six cryo-EM structures, including the energy transfer state with LHCII in solution and the energy quenching state with laterally confined LHCII in membrane nanodiscs under both neutral and acidic conditions.
Comparison of these different structures shows that LHCII undergoes a conformational change upon acidification. This change allosterically alters the inter-pigment distance of the fluorescence quenching locus Lutein1 (Lut1)–Chlorophyll612 (Chl612) only when LHCII is confined in membrane nanodiscs, leading to the quenching of excited Chl612 by Lut1. ...
This distance regulates the energy transfer quantum channel in response to the lateral pressure on LHCII and the conformational change, that is, a slight change at its critical distance of 5.6 Å would allow reversible switching between light harvesting and excess energy dissipation. This mechanism enables a rapid response to changes in light intensity, ensuring both high efficiency in photosynthesis and balanced photoprotection with LHCII as a quantum switch. ..."
From the abstract:
"The major light-harvesting complex of photosystem II (LHCII) has a dual regulatory function in a process called non-photochemical quenching to avoid the formation of reactive oxygen. LHCII undergoes reversible conformation transitions to switch between a light-harvesting state for excited-state energy transfer and an energy-quenching state for dissipating excess energy under full sunshine. Here we report cryo-electron microscopy structures of LHCII in membrane nanodiscs, which mimic in vivo LHCII, and in detergent solution at pH 7.8 and 5.4, respectively. We found that, under low pH conditions, the salt bridges at the lumenal side of LHCII are broken, accompanied by the formation of two local α-helices on the lumen side. The formation of α-helices in turn triggers allosterically global protein conformational change, resulting in a smaller crossing angle between transmembrane helices. The fluorescence decay rates corresponding to different conformational states follow the Dexter energy transfer mechanism with a characteristic transition distance of 5.6 Å between Lut1 and Chl612. The experimental observations are consistent with the computed electronic coupling strengths using multistate density function theory."
Cryo-EM structures of LHCII in photo-active and photo-protecting states reveal allosteric regulation of light harvesting and excess energy dissipation (no public acess)
Fig. 1 Cryo-EM structures for LHCII in nanodisc and in detergent solution at pH 7.8 and 5.4.
Molecular mechanism of NPQ and acidity-induced changes in some key structural factors drive the LHCII trimer to switch between light-harvesting and energy-quenching states. Credit: Institute of Physics
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