In plants, photosynthesis is carried out by intracellular, membrane-enclosed organelles called chloroplasts. Many of the reactions that take place in the chloroplasts involve the compound NADPH, which acts as an electron donor for the processes responsible to convert CO2 into carbohydrates. During photosynthesis in green plants, NADPH is synthesized by the enzyme FNR (Ferredoxin-NADP(H)-Oxidoreductase) in the so-called stroma, the central soluble compartment of the chloroplast. "Because all metabolic processes are dependent on an adequate supply of NADPH, the enzyme FNR plays a crucial role," says Privatdozentin Dr. Bettina Bolter of the Department of Biology I at LMU Munich.
In collaboration with Professor Michael Groll of the TUM, Bolter and her colleagues have now shown that, by interacting with a second protein called Tic62, FNR regulates biomass accumulation depending on levels of ambient light. Tic62 itself forms part of the molecular complex that enables specific proteins to be imported into the chloroplast from the surrounding cytoplasm, and was already known to anchor FNR to the so-called thylakoids, the photosynthetic membranes within the chloroplast. "This is the first time that a direct functional link has been demonstrated between protein uptake into the chloroplast and the photosynthetic process itself," says Bolter.
Structural studies using X-ray crystallography subsequently showed that each Tic62 molecule binds two FNR enzymes in a back-to-back configuration, leaving their chemically active surfaces free. Most importantly, it turns out that the binding affinity of Tic62 for FNR varies with light intensity. Bolter and her coworkers found that the FNR sandwich is particularly stable when the stroma surrounding the thylakoids is acidic. This is the case in the dark, when photosynthesis is inactive. The transformation of light energy into chemical energy, on the other hand, causes the stroma to become alkaline, and this weakens the affinity of FNR for Tic62.
"In the dark, or when light levels are low, there is less demand for NADPH, and the chloroplasts park the excess FNR molecules on the thylakoid membranes," explains Michael Groll. This makes it possible for the plant to regulate its photosynthetic performance in accordance with light conditions. "Binding of FNR to the thylakoid membranes may just be an elegant way of storing the enzyme when it is not needed," says Bolter. "The new findings represent one further step in the quest to understand photosynthesis in detail. Although they have no immediate application, they may well contribute to enhancing crop yields in the future."
"Ferredoxin: NADP(H) Oxidoreductase is recruited to thylakoids by binding to a polyproline type II helix in a pH-dependent manner"
Ferdinand Alte, Anna Stengel, J. Philipp Benz, Eike Petersen, Jurgen Soll, Michael Groll, Bettina Bolter
Proceedings of the National Academy (PNAS) online before print, 25 October 2010
Source: LMU Munich