Published: July 30, 2015

Orange is the new red

Contact(s): Lynn Yarris Berkeley Center for Structural Biology office: (510) 486-5375, Layne Cameron Media Communications office: (517) 353-8819 cell: (765) 748-4827

Overexposure to sunlight, which is damaging to natural photosynthetic systems of green plants and cyanobacteria, is also expected to be damaging to artificial photosynthetic systems.

animated gif of translocation of the carotenoid pigment Translocation of the carotenoid pigment within a critical light-sensitive protein called the Orange Carotenoid Protein triggers a shifting of the protein from the light-absorbing orange state to the energy- quenching red state, providing cyanobacteria with protection from too much sunlight. Image credit: Berkeley Center for Structural Biology

Nature has solved the problem through a photoprotection mechanism called “nonphotochemical-quenching.” This allows solar energy to be safely dissipated as heat from one molecular system to another.

With an eye on learning from nature’s success, a team led by Cheryl Kerfeld, the Hannah Distinguished Professor of Structural Bioengineering in the Michigan State University-DOE Plant Research Lab who is also an affiliate of Berkeley Lab’s Physical Biosciences Division, has discovered a surprising key event in this energy-quenching process.

In a recent paper published in Science, the team discovered that in cyanobacteria the energy-quenching mechanism is triggered by an unprecedented, large-scale (relatively speaking) shift of a single carotenoid pigment within a protein.

As a result of this shift, the carotenoid changes its shape slightly and interacts with a different set of amino acid neighbors causing the protein to change from an orange light-sensing state to a red photoprotective state.

“Prior to our work, the assumption was that carotenoids are static, held in place by the protein scaffold,” Kerfeld said. “Having shown that the translocation of carotenoid within the protein is a functional trigger for photoprotection, scientists will need to revisit other carotenoid-binding protein complexes to see if translocation could play a role in those systems as well. Understanding the dynamic function of carotenoids should be useful for the design of future artificial photosynthetic systems.”

Through photosynthesis, plants are able to harvest solar energy and convert it to chemical energy. Creating an efficient artificial version of photosynthesis would realize the dream of solar power as the ultimate green and renewable source of electrical energy. However, if a sunlight-harvesting system becomes overloaded with absorbed solar energy, it most likely will suffer some form of damage.

Ryan Leverenz, the paper’s lead author who is a research scientist in Kerfeld’s MSU lab, first identified the movement of the carotenoid. Now that the mechanism has been identified, future efforts will focus on finding ways to improve the process.

“I think it’s quite elegant that one molecule – specifically a carotenoid – senses the intensity of the sunlight, and that the same molecule acts to dissipate the extra energy being absorbed by the cell,” Leverenz said. “Now that we’ve identified how this molecular switch works, we can potentially fine-tune the process in order to improve cyanobacteria’s viability as a biofuel.”

This research was supported by the DOE Office of Science.

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