Research Stories

--by Joe Caspermeyer

Photosynthesis is a remarkable biological process that supports life on Earth. Plants and certain microbes can harvest light and use it as an energy source to produce their food. In the process, they also provide vital oxygen for animals and people.

Scientists have come up with a surprising twist to photosynthesis. The researchers devised a method for swapping a key metal necessary for turning sunlight into chemical energy. The international collaboration includes scientists from Arizona State University, the University of California San Diego, and the University of British Columbia. The team described their findings in the May 11 early online edition of the Proceedings of the National Academies of Science.

In the heart of every green leaf are pigments called chlorophyll. These pigments give most plants their color. In addition to green, there are also yellow and orange carotenoid pigments. The pigments are key molecules that work to harvest light across the spectrum.

Magnesium is an important metal found in all plant chlorophylls. It is held tightly within the molecule’s center.

During photosynthesis, plants have two mechanisms that work in tandem. Scientists call these mechanisms photosystem I and photosystem II. To peer at the inner workings of photosynthesis, the researchers used a hardy, well-studied, photosynthetic bacterium called Rhodobacter sphaeroides. The purple bacteria possess a simplified system similar to photosystem II. A similar organism was likely one of the earliest photosynthetic bacteria to evolve.

The center stage of photosynthesis is the reaction center. Light energy is funneled into specialized chlorophyll binding proteins inside the reaction center. Scientists know that the movement of the reaction center proteins during photosynthesis assists the light-driven movement of electrons between molecules in the reaction center. This helps the plant or bacteria to harness light energy efficiently even if conditions aren’t optimal.

The researchers introduced disruptions into this electron pathway. Despite the disruptions, the proteins were able to compensate by moving and energetically guiding the electrons through their biological circuit.

“One of our research strategies is to introduce mutations into the bacteria. We study how these affect the energy conversion efficiency of the reaction center,” says Su Lin, senior researcher at ASU’s Department of Chemistry and Biochemistry and Biodesign Institute. Lin is lead author of the study.

“Carefully designed aberrations provide extensive information about the normal mechanism of energy conversion in reaction centers,” Lin adds. “It is similar to how studying a disease clarifies the parameters of health for the involved biochemical pathways and tissues. We are learning a lot about the most basic mechanisms of photosynthesis.”

The reactions that convert light to chemical energy happen in a millionth of a millionth of a second. This makes experimental observation extremely challenging. Sponsored by the National Science Foundation, Lin designed and built a premier ultrafast laser spectroscopic detection system. The device acts like a high-speed motion picture camera.

The premier ultrafast laser spectroscopic detection system allows scientists to take a "movie" of the energy transfer events of photosynthesis.

Lin’s device splits the light spectrum into infinitesimally discrete slivers. Scientists use it to capture vast amounts of ultrafast frames from the components of these exceedingly rapid reactions. The frames are then mathematically assembled. The researchers can make a figurative "movie" of the energy transfer events taking place during photosynthesis.

Graduate student Paul Jaschke got the current research rolling while working with J. Thomas Beatty in the Department of Microbiology and Immunology at the University of British Columbia. He discovered a mutant that replaced the magnesium metal found in the reaction center with zinc.

“We initially thought this reaction center was non-functional,” says Beatty. “We were forced to think in new ways to explain the surprising results. This led to some nice insight.”

Lin carefully measured the light absorption spectra for the naturally occurring magnesium reaction center. She then compared it to the mutant reaction center that was replaced with zinc bacteriochlorophylls.

The ASU scientist found that the zinc-coordinated reaction center is comprised of six bacteriochlorophylls. However, after changing their structure to a configuration similar to that used in photosystem I reaction centers, surprisingly, the data from the reaction kinetics and the energy conversion efficiency were almost identical to the magnesium-containing reaction center.

“Amazingly, the reaction center still works with essentially the same physical chemical properties as the normal system,” says Neal Woodbury, deputy director of the Biodesign Institute. “This was a real puzzle when Su first did these measurements, but she was able to figure out why.”

“The electron transfer driving force can be determined by either the properties of the metal cofactors themselves or through their interaction with the protein,” says Lin. “In the case of the zinc reaction center, the driving force is regulated through the coordination of the metal.”

“Once again, biology shows its resilience,” Woodbury explains. “The changes in one area are compensated by changes in others and to the protein’s ability to dynamically adjust.”

So why are scientists excited by this work? The results may help them better explore a deeper understanding of the structure, function, and evolution of photosynthesis reaction centers in photosystems I and II.

Of particular interest are studies that focus on the interaction between chlorophylls and protein, which differs in naturally occurring reaction center variants. The researchers may also conduct future experiments to understand the metal substitution limitations of the reaction center. They will track the protein movements that may be occurring in the reaction center that helps to optimize photosynthesis.

Their results may have long-term practical applications for the development of next-generation solar cells. Through biomimicry of photosynthesis, scientists could greatly boost the energy efficiency compared with current technology. The robustness of the natural system may offer some useful lessons for engineers working to improve on current technologies. Better efficiency could lower the cost of solar panels for the average consumer.

Team members on the project include ASU scientists Su Lin, Neal Woodbury, Aaron Tufts, and James P. Allen. Their UBC colleagues are J. Thomas Beatty, Paul R. Jaschke, Federico I. Rosell, and A. Grant Mauk. Mark Paddock works at UCSD. Haiyu Wang is a researcher at Jilin University in China.