'Beating heart' of photosynthesis exposed


Rebecca Pool

Wednesday, November 20, 2019 - 15:15
Protein complex cytochrome b6f [University of Sheffield]
UK-based researchers have solved the structure of one of the key components of photosynthesis, a discovery that could lead to photosynthesis being 'redesigned' to achieve higher yields and meet urgent food security needs.
The study, led by the University of Sheffield and published in Nature, reveals the structure of cytochrome b6f, the protein complex that influences plant growth via photosynthesis.
Using a high-resolution structural model, Lorna Malone from the Department of Molecular Biology and Biotechnology, and colleagues, found that the protein complex provides the electrical connection between the two light-powered chlorophyll-proteins, Photosystems I and II, found in the plant cell chloroplast.
"Our study provides important new insights into how cytochrome b6f utilises the electrical current passing through it to power up a 'proton battery',” she says. “This stored energy can then be used to make ATP, the energy currency of living cells.”
“Ultimately this reaction provides the energy that plants need to turn carbon dioxide into the carbohydrates and biomass that sustain the global food chain," she adds.
The cytochrome b₆f complex is an enzyme found in the membrane in chloroplasts of plants, cyanobacteria, and green algae.
To determine the structure of the complex, Malone and colleagues used single particle cryo-electron microscopy.
After purifying the complex from spinach leaves, they placed a solution of the purified complexes on a carbon grid and plunged the entire set-up into liquid ethane cooled to about -190 degrees Celsius.
Data acquisition was carried out on a Titan Krios microscope operated at 300 kV, equipped with an energy filtered Gatan K2 summit direct electron detector.
An incredible 6,035 micrographs were generated with researchers manually selecting more than 400,000 particles.
Particle data was processed to generate a 3D model of the cytochrome b6f, revealing component parts down to the level of individual molecules, at 3.6 Å resolution.
The high-resolution structural model showed new details of the additional role of cytochrome b6f as a sensor to tune photosynthetic efficiency in response to ever-changing environmental conditions. 
As the researchers point out, this response mechanism protects the plant from damage during exposure to harsh conditions such as drought or excess light.
As Dr Matt Johnson, from Biochemistry, highlights: "Cytochrome b6f is the beating heart of photosynthesis which plays a crucial role in regulating photosynthetic efficiency.”
"Previous studies have shown that by manipulating the levels of this complex we can grow bigger and better plants,” he says. “With the new insights we have obtained from our structure we can hope to rationally redesign photosynthesis in crop plants to achieve the higher yields we urgently need to sustain a projected global population of 9-10 billion by 2050.”
Researchers now aim to establish how cytochrome b6f is controlled by myriad regulatory proteins and how these regulators affect the function of this complex.
Research is published in Nature.
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