Laser Reveals Photosynthetic Processes
ST. LOUIS, Dec. 2, 2013 — Scientists have used an ultrafast laser to study a photosynthetic complex — arguably the most important bit of organic chemistry on the planet — in its complete functioning state.
When sunlight strikes a photosynthesizing organism, energy flashes between proteins just beneath its surface until it is trapped as separate electric charges. These tiny hits of energy are literally the sparks of life, eventually powering the growth and movement of all plants and animals.
The photosynthetic megacomplex from a cyanobacterium, which scientists have managed to isolate in its complete, functioning form, weighs about 6 million daltons. It has three parts: On top is a light-harvesting antenna complex called a phycobilisome that captures and funnels the energy in sunlight to two reaction centers, Photosystem II (the complex protruding beneath the antenna) and Photosystem I (the complexes to either side of Photosystem II). The megacomplex is embedded in a membrane shown as a green carpet. Courtesy of Haijun Liu.
Although in illustrations they look like random scrawls, three clumps of protein — a light-harvesting antenna called a phycobilisome and photosystems I and II — are positioned with exquisite precision to do their jobs. If the distances between proteins are too great, or the transfers are too slow, energy is wasted and the organism ultimately dies.
How these three clumps work together has been a mystery. Previous attempts to isolate them failed because the weak links that held them together broke, and the megacomplex fell apart.
Now, scientists at Washington University in St. Louis report a new technique involving tandem mass spectrometers and ultrafast lasers that finally allows the megacomplex to be plucked out in its entirety and examined as a functioning whole.
The work was done at PARC (Photosynthetic Antenna Research Center), which is focused on the scientific groundwork needed to maximize photosynthetic efficiency in living organisms and to design biohybrid or synthetic ones to drive chemical processes or generate photocurrent.
Dr. Robert Blankenship, PARC's director and the Lucille P. Markey Distinguished Professor of Arts & Sciences, said that one outcome of the work in the long term might be the ability to double or triple the efficiency of crop plants — now stuck at 1 to 3 percent. "We will need such a boost to feed the 9 or 10 billion people predicted to be alive by 2050," he said.
His team worked with the model organism often used to study photosynthesis in the lab, a cyanobacterium, sometimes called a blue-green alga.
The ultrafast laser system used to probe the split-second energy flow within the cyanobacterial megacomplex. Courtesy of Joe Angeles/WUSTL Photos.
All photosynthesizing organisms have light-harvesting antennas made up of many molecules that absorb light and transfer the excitation energy to reaction centers, where it is stored as charge separation.
In free-living cyanobacteria, the antenna, or phycobilisome, consists of splayed rods made up of disks of proteins containing intensely colored bilin pigments. The antenna sits directly above one reaction center, Photosystem II, and kitty-corner to the other, Photosystem I.
PARC research scientist Dr. Haijun Liu proposed stitching together the megacomplex and then engineered a strain of cyanobacteria that has a tag on the bottom of Photosystem II.
The mutant cells were treated with reagents that stitched together the complexes, then broken open, and the tag used to pull out Photosystem II and anything attached to it.
To determine how the proteins were interconnected, the scientists repeatedly cut or shattered the proteins, analyzing them by mass spectrometry down to the level of the individual amino acid.
Their work yielded the surprising result that the antenna feeds energy to both photosystems. It had previously been thought that the antenna fed Photosystem II, which then transferred energy to Photosystem I.
"The work provides a new level of understanding of the organization of these photosynthetic membranes, and that is something that a lot of people have tried to understand for a long time," Blankenship said. "It also introduces the methodology of the cross-linking and then the mass spectrometry analysis that could potentially be applicable to a lot of other complexes, not just photosynthetic ones."
The work appears in the Nov. 29 issue of Science.
For more information, visit: www.wustl.edu