The structure reveals key features that help block excess light absorption during photosynthesis

Photosynthetic organisms use light for fuel, but sometimes there are too many good things.

New research from the University of Washington in St. Louis reveals the basic structure of the light-collecting antenna of cyanobacteria or blue-green algae, including key features that collect energy and block absorption excessive light. The study, published on January 6 in Scientific advances, provides relevant information for future energy applications.

Scientists have built a model of the large protein complex called a phycobilisome which collects and transmits light energy. Phycobilisomes allow cyanobacteria to take advantage of different wavelengths of light than other photosynthetic organisms, such as green plants on dry land.

This ability dramatically increases the global productivity of photosynthesis across the solar energy spectrum, but it comes with many risks.

“For cyanobacteria, excessive light absorption is inevitable – and sometimes fatal,” said Haijun Liu, a chemistry researcher in the arts and sciences at the University of Washington. Liu is the principal investigator and corresponding author of the new study, funded by the Department of Energy (DOE), Basic Energy Sciences.

“We have found interesting structural features in the interface where energy is transferred and regulated,” he said. “One of the regulatory processes called non-photochemical quenching is carried out by a protein called the orange carotenoid protein. A high-resolution structure of the phycobilisome will allow us to understand these processes in detail.”

While the researchers already knew that the orange carotenoid protein helps protect cyanobacteria in bright light conditions, they did not have a clear picture of all the structural features at work.

They also didn’t know where or how the orange carotenoid protein is sequestered in a living cyanobacterial cell.

“We were blown away when we first hit the current model,” Liu said. “We immediately noticed that an inactive orange carotenoid protein can actually access the free space region between the phycobilisome and PSII (the protein complex that receives energy from the phycobilisome for photochemical reactions), or simply s’ fit perfectly into the free space region. recruited or activated by environmental signals. “

This structure was assembled by a team of analytical biochemists and structural biologists from the University of Washington, including Himadri Pakrasi, Professor George William and Irene Koechig Freiberg in Arts and Sciences.

The team used structural proteomics in combination with structural modeling to solve the structure. The method was first developed by Liu in Pakrasi’s laboratory a few years ago, in collaboration with members of a group led by Michael Gross, professor of chemistry in arts and sciences and immunology and internal medicine. at the Faculty of Medicine. The unique platform they created gave them significant advantages over other laboratories that had attempted to address similar biological questions with the electron microscope, cryoelectron microscope, and other techniques.

The fundamental scientific bases of this new research allow us to explain how living organisms maximize photosynthetic efficiency during the first events of photosynthesis. This theme was also supported by the Photosynthetic Antenna Research Center (PARC) at the University of Washington – one of 46 DOE Energy Frontier research centers, formerly headed by Robert E. Blankenship, Lucille P. Markey Emeritus Professor of the Arts and science.

The new work will contribute to future efforts to design biohybrid or synthetic systems that harness the energy of light.