Scientists reveal water pathways in photosynthesis

Structure of the Photosystem II protein complex form Arabidopsis thaliana created using cryo-electron microscopy. Global resolution: 2.44Å; local resolution illustrated by color: 2.0Å violet, 2.5Å blue, 3.0Å green, 3.5Å yellow.
Illustration Credit: Jack Forsman, J. Messinger & W. Schröder group

Scientific Frontline: Extended "At a Glance" Summary: Water Pathways in Photosystem II

The Core Concept: Researchers have mapped the precise structural pathways of Photosystem II in plants, revealing exactly how water molecules navigate to the active site for the critical water-splitting reaction that initiates photosynthesis.

Key Distinction/Mechanism: By comparing the molecular structure in Arabidopsis thaliana (thale cress) with that of cyanobacteria, scientists discovered a specialized "water valve." This structural bottleneck is positioned directly before the manganese-containing catalytic center. While the channels leading to the valve vary across species, the valve itself enforces strict control, ensuring water molecules are in exact, necessary positions to correctly interact with the catalyst.

Major Frameworks/Components: 

• Photosystem II (PSII): The essential protein complex and molecular machinery driving the light-dependent reactions of photosynthesis, specifically water oxidation.
• Cryo-Electron Microscopy (Cryo-EM): The advanced structural biology technique used to achieve a 2.44 Å global resolution, allowing scientists to identify individual water molecules and hydrogen atoms.
• Manganese-Catalytic Center: The highly conserved, metallic active site where water is split to release oxygen, alongside the electrons and energy required for carbon fixation.
• The "Water Valve": The newly identified structural bottleneck within the water channel that dictates the flow and precise spatial alignment of water molecules prior to catalysis.

Branch of Science: Structural Biology, Plant Physiology, and Biochemistry.

Future Application: The structural blueprints from this biological water network can be applied to the development of artificial photosynthesis. This paves the way for designing synthetic, sustainable energy technologies capable of producing fuels from water, carbon dioxide, and sunlight utilizing abundant elements like manganese, rather than relying on rare and expensive metals.

Why It Matters: Unlocking the mechanics of biological oxygen production fundamentally alters the approach to designing biomimetic catalysts. The data establishes that engineering efficient artificial photosynthesis requires designing not just the metal catalyst, but also the highly specific surrounding water delivery networks. Furthermore, it provides profound evolutionary insights into how photosynthetic machinery has independently adapted over a billion years.

Johannes Messinger wants to understand how water is split on the molecular level during photosynthesis.
Photo Credit: Mattias Pettersson

Plants use sunlight to turn water and carbon dioxide into energy-rich biomolecules such as sugars, a process that also produces the oxygen in the air. But exactly how water reaches the part of the system where the initial steps of this reaction happen – the active site for water splitting - has remained unclear. Now, researchers have revealed a detailed structure of this system in plants, uncovering what they describe as a “water valve” that helps control the flow of water during photosynthesis. 

Comparing the structures of Photosystem II from Arabidopsis and cyanobacteria showed us which areas are the same, and therefore functionally important 

Two years ago, researchers from Umeå University revealed the detailed structure of Photosystem II, the molecular machinery that drives photosynthesis, in cyanobacteria. For the first time, they were able to see this structure at very high resolution using cryo-electron microscopy, even identifying individual water molecules and hydrogen atoms inside the system. 

Now, the research team led by Johannes Messinger has taken the next step by resolving the same structure in plants, specifically in Arabidopsis or thale cress. The study, published in New Phytologist, compares Photosystem II in plants and photosynthetic bacteria, revealing how it has evolved independently over almost one billion years. 

Key parts of photosystem II are conserved across species 

“Comparing the structures of Photosystem II from Arabidopsis and cyanobacteria showed us which areas are the same, and therefore functionally important,” explains Johannes Messinger, professor at Umeå University and group leader at Umeå Plant Science Centre. “We assume that those areas that are different are less critical, as they can change without affecting photosynthesis.” 

An early step in photosynthesis is the splitting of water, a reaction that releases oxygen and provides the electrons and part of the energy needed to convert carbon dioxide into sugars. This is the process the researchers are focused on. 

The team was particularly interested in how water molecules move through Photosystem II and how they interact with the manganese-containing catalytic center, the part of the system where water is split. 

“We were looking at water molecules and water channels in both structures. Around the catalytic center, they were almost identical, suggesting that the arrangement of the water molecules is very important for the function of photosystem II,” says Jack Forsman, one of the two shared first authors of the study. “However, further away, the picture was very different and the channels deviated significantly.” 

The researchers identified a narrow bottleneck in one of these channels, which they call ‘the water valve’. This point sits just before the catalytic center and likely plays a key role in controlling how water is delivered to it. 

Jack Forsman (right), André T. Graça (left) and Wolfgang Schröder (middle). Forsman and Graça share first authorship of the study, led by Johannes Messinger and Wolfgang Schröder.
Photo Credit: André T. Graça

A “water valve” controls how water reaches the reaction center 

“Before ‘the water valve’, the only requirement is that water can easily reach this point, which is why the channels can vary between plants and cyanobacteria without affecting function,” explains Wolfgang Schröder, one of the authors and leader of the previous study. “After the bottleneck, however, water molecules need to be in very specific positions to interact correctly with the catalytic center.” 

Our data clearly show that it is important not only to design the metal catalyst itself, but also the surrounding water network 

Understanding how water is transported and positioned in this system could help scientists design materials for artificial photosynthesis, technologies that aim to produce fuels from water, carbon dioxide, and sunlight. Today, such reactions often rely on rare and expensive metals, but insights from plants could help enable the use of more abundant elements such as manganese, opening new possibilities for developing more sustainable energy technologies in the future.   

“Our data clearly show that it is important not only to design the metal catalyst itself, but also the surrounding water network,” says Johannes Messinger. “In future, we will focus on how these bottlenecks affect water flow and water-splitting, as well as study Photosystem II in additional species.” 

Published in journal: New Phytologist

Title: The structure of intact and active Photosystem II from Arabidopsis thaliana at 2.44 Å resolution

Authors: Jack Forsman, André T. Graça, Abuzer Orkun Aydin, Michael Hall, Rana Hussein, Wolfgang P. Schröder, and Johannes Messinger

Source/Credit: Umeå University | Anne Honsel

Reference Number: phgy041826_01

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