A complete molecular-scale picture of how plants convert sunlight to chemical energy has been obtained at Purdue University, offering potential new insights into animal metabolism as well.

Using advanced imaging techniques, a team of Purdue biologists has determined the structure of the cytochrome, a protein complex that governs photosynthesis in a blue-green bacterium. While their work does not immediately suggest any industrial applications, it does reveal a wealth of information not only about a chemical process crucial to all life on the planet, but also about how cells handle and distribute energy. According to team member William Cramer, the study is a great leap forward in our understanding of photosynthesis. "Where we once could see merely the tip of the iceberg, we can now perceive the entire mechanism of photosynthesis," said Cramer, the Henry Koffler Distinguished Professor of Biological Sciences in Purdue's School of Science. "Before we found a way to crystallize the cytochrome, we had a general picture of the photosynthetic process, but possessed only a fraction of a percent of the information we now have. Now that we can examine these proteins closely with X-ray crystallography, it could lead to knowledge about how all cells exchange energy with their environment." Cramer also said that the study is an important contribution to the young field of proteomics research because there is little data on the important family of membrane-embedded proteins in the total protein database.

The Purdue University biologists who determined the structure of the cytochrome protein complex, which is critical for photosynthesis, are, from left, professor Janet Smith, associate research scientist Huamin Zhang, visiting scholar Genji Kurisu and distinguished professor William Cramer. (Purdue Department of Biological Sciences photo/T. Geders)

"Membrane proteins are involved in a cell's interactions with its environment, making them an essential component of metabolism," he said. "However, they are difficult to crystallize for study. This research could clarify our understanding of energy flow in human cells as well, giving us better insight into respiration and the absorption of antioxidants in animal cells." The report appears today (Thursday, 10/2) in the journal Science's online edition, Science Express. The first two authors on the manuscript are Genji Kurisu, visiting scholar from Osaka University, Japan, and Huamin Zhang, associate research scientist in the Department of Biological Sciences at Purdue, who made major contributions to the crystallographic and biochemical part of the analysis. The report paints a picture of the complex motion of electrons and protons across the bacterium's cell membrane, the boundary between the cell and its surroundings. "Plant cell membranes are like the two ends of a battery," said Janet Smith, professor of biological sciences and the team member responsible for much of the structure analysis. "They are positive on the inside and negative on the outside, and they are charged up when solar energy excites electrons from hydrogen within the cell. The electrons travel up into the cell membrane via proteins that conduct them just like wires. Of course, because of their high energy level, the electrons want to 'fall back' like water over a dam, releasing the energy a plant harnesses to stay alive." While this general picture has been common knowledge to scientists for decades, the complex motion of electrons and protons in the membrane have not.

Shown is an illustration of the cytochrome b6f protein complex, which is critical for photosynthesis. The eight colors represent the eight protein components of the cytochrome complex; the cylinders are the 26 segments of the complex that cross the photosynthetic membrane; the colored rings made of little balls that are embedded in protein are the groups that actually carry the electrons stimulated by light absorbed in photosynthesis. Purdue University biologists determined the structure of the complex using X-ray crystallography. (Purdue Department of Biological Sciences illustration/H. Zhang)

"It's a bit like watching electrons move through a computer chip," Smith said. "A microprocessor has far more complex and numerous routes for its electricity to follow than, say, a flashlight, which only has one. But while a chip uses electrons to flip tiny digital switches back and forth for calculations, the membrane uses them to drive the cell's metabolism."

The cell that provided the proteins for the team's work was a cyanobacterium, a single-celled thermophile plant commonly found in hot springs such as those in Yellowstone. The particular cyanobacterium used in these studies was isolated by Swiss researchers at a hot spring in Iceland.

While animals do not employ photosynthesis, their cells do make use of similar proteins for respiration. The similarities could lead to a better understanding of our own metabolic processes.

"What we see when we examine these proteins is the nature of their partial similarity," said Cramer. "The differences can now be explored more easily."

Examining the membrane proteins has itself been the challenge for the research team, which is reaping the benefits of its breakthrough work with protein crystallization. While proteomics specialists have been crystallizing protein molecules for years to obtain their structure, membrane proteins have proven difficult because they do not dissolve in water, a crucial step in the crystallization process.

"This difficulty has left a gap in our knowledge of membrane proteins, which total about 30 percent of the proteins in living things," Cramer said. "After finding a way to crystallize a membrane protein earlier this year, it only took a few months before we were able to look at photosynthesis in such detail."

The team is hopeful that their method can be applied to other membrane proteins, which they consider a variety of vast untapped potential.

"If cells were countries, membrane proteins would control all the international commerce," Cramer said. "They are the border guards that regulate all the energy transfer and material exchange across the boundary between the cell and its environment. If you want to get a drug into a cell where it can be of use, you have to deal with the membrane proteins – that's why they're so tempting a subject to study."

Funding for the research was provided in part by the National Institute of General Medical Sciences (NIGMS), a branch of the National Institutes of Health. NIGMS's Dr. Peter Preusch agreed with Cramer's assessment of the value of membrane protein research, saying the team's work could lead to significant discoveries.

"New insights provided by Dr. Cramer's elegant studies underscore the value of searching for biological secrets in model systems," he said. "The findings will advance the study of energy metabolism in humans."

Members of the team are affiliated with several research centers at Purdue, including the Markey Center for Structural Biology, the Bindley Bioscience Center at Discovery Park, the Interdepartmental Program in Biochemistry and Molecular Biology, and the Purdue Cancer Center.

Writer: Chad Boutin, (765) 494-2081, cboutin@purdue.edu
Sources:
William Cramer, (765) 494-4956, wac@bilbo.bio.purdue.edu
Janet Smith, (765) 494-9246, smithj@purdue.edu

Related news release:
Purdue biologists crystallize technique to expand protein research

ABSTRACT

Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity
by G. Kurisu, H. Zhang, J.L. Smith and W.A. Cramer

The cytochrome b6f complex provides the electronic connection between the photosystem I and photosystem II reaction centers of oxygenic photosynthesis and generates a trans-membrane electrochemical proton gradient for ATP synthesis. A 3.0-Å crystal structure of the dimeric b6f complex from the thermophilic cyanobacterium, Mastigocladus laminosus, reveals a large quinone exchange cavity, stabilized by lipid, in which plastoquinone, a quinone analogue inhibitor, and a novel heme are bound. The core of the b6f complex is similar to the analogous respiratory cytochrome bc1 complex, but the domain arrangement outside the core and the complement of prosthetic groups are strikingly different. The motion of the Rieske iron-sulfur protein extrinsic domain, essential for electron transfer, must also be different in the b6f complex.