Study Sheds New Light on Water Splitting Reaction of Photosynthesis

A special combination of chemical analysis and nanoscale imaging has allowed an international research team to identify a crucial step in the molecular mechanism that contributes to the water-splitting reaction of photosynthesis. This latest discovery may lead to the development of renewable energy technology.

Life depends on the oxygen that plants and algae split from water; how they do it is still a mystery, but scientists, including our team, are slowly peeling away the layers to get to the answer.

Vittal K. Yachandra, Study Co-Lead Author and Chemist Senior Scientist, Lawrence Berkeley Laboratory, Department of Energy

Yachandra continued, “If we can understand this step of natural photosynthesis, it would enable us to use those design principles for building artificial photosynthetic systems that produce clean and renewable energy from sunlight and water.”

The study was published in the PNAS journal.

The researchers had earlier designed and fabricated an instrument that they employed to examine photosynthetic proteins using both X-ray emission spectroscopy and X-ray crystallography.

This dual technique, which the researchers had initially developed and have been improving for the last decade, produces chemical and protein structure data from the same sample simultaneously. The imaging was carried out with the X-ray free-electron laser (XFEL) at SACLA based in Japan and at the LCLS based at SLAC National Laboratory.

With this technique, we get the overall picture of how the entire protein structure dynamically changes and we see the chemical intricacies occurring at the reaction site. The X-ray free electron laser produces extremely bright, short bursts of X-rays that allow us to not only analyze a protein at room temperature, which is how these reactions occur in nature, but also capture various moments over the reaction time scale.

Junko Yano, Study Co-Lead Author and Chemist Senior Scientist, Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley Laboratory

In conventional crystallography techniques, the sample proteins had to be usually frozen and, as a result, they can only produce images of static proteins. Due to this limitation, researchers find it difficult to get a grip of the accurate behavior of proteins in living organisms. This is because the molecules deform between varying physical states at the time of chemical reactions.

“The water-splitting reaction in photosynthesis is a cyclical process that needs four photons and cycles between four stable ‘states’,” Yano added. “Previously, we could only take pictures of these four states. But by taking multiple snapshots in time, we now can visualize how one state goes to the other.”

We saw, really nicely, how the structure changes step-by-step as it transforms from one state to the next state. It is pretty exciting, because we can see the ‘cause and effect’ and the role that each moving atom plays in this transition.

Jan F. Kern, Study Co-Author and Chemist, Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley Laboratory

Nicholas K. Sauter, the study’s co-author and computational senior scientist at Biophysics and Integrated Bioimaging Division, added, “Essentially, we’re trying to take a ‘movie’ of a chemical reaction. We made a lot of progress to get to this point, in terms of our technology and our computational analyses.”

Sauter continued, “The work of our co-author Paul Adams and others in MBIB was critical to interpreting the XFEL and X-ray data But we still have to get the other frames to see how the reaction is completed and the enzyme is ready for the next cycle.”

The scientists at Lawrence Berkeley Laboratory Lab are hoping to pursue the new project as soon as the various research sites—situated in the United States, South Korea, Switzerland, and Japan—begin to operate normally after the COVID-19 pandemic. Incidentally, the entire international research community depends on these research sites.

Kern further observed that the technological milestone demonstrated in this study has considerably gained from the different know-how of the authors from Humboldt University based in Germany, SLAC, as well as Uppsala and Umeå Universities based in Sweden.

The study also benefited from the capabilities of five DOE Office of Science user facilities—the Stanford Synchrotron Radiation Lightsource and LCLS at SLAC National Accelerator Laboratory, and the Advanced Light Source, Energy Sciences Network, and National Energy Research Scientific Computing Center at Lawrence Berkeley Laboratory.

Other researchers from Lawrence Berkeley Laboratory who contributed to this study include Ruchira Chatterjee, Louise Lassalle, Kyle D. Sutherlin, Iris D. Young, Sheraz Gul, In-Sik Kim, Philipp S. Simon, Isabel Bogacz, Cindy C. Pham, Nicholas Saichek, Trent Northen, Asmit Bhowmick, Robert Bolotovsky, Derek Mendez, Nigel W. Moriarty, James M. Holton, Aaron S. Brewster, and David Skinner.

The study was mainly funded by the DOE Office of Science and by grants from the National Institutes of Health.

Journal Reference:

Ibrahim, M., et al. (2020) Untangling the sequence of events during the S₂ → S₃ transition in photosystem II and implications for the water oxidation mechanism. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2000529117.