Plays Key Role in Photosynthesis : Structure of Plant Protein Unraveled

After 18 years of work, a UCLA molecular biologist has determined the three-dimensional structure of the world’s most common protein--an enzyme that plays a key role in the process by which plants convert carbon dioxide and sunlight into sugars that virtually all living organisms use for energy.

The structural analysis, reported in today’s Science magazine, will provide new impetus to efforts to genetically engineer plants so that they can operate at a higher efficiency and grow more quickly, experts said.

“It’s a very important advance,” said biochemist Bruce McFadden of Washington State University.

The enzyme, now known as RuBisCo, was discovered in 1947 by UCLA biologist Samuel Wildman. It initiates the process of photosynthesis, in which energy in sunlight is trapped in a chemical form usable by plants and accounts for more than 50% of the protein in the leaves of plants. Scientists estimate its worldwide production at about 1 metric ton per second.

Received Advice

“When I came to UCLA in 1970, Wildman told me it was a good thing to work on,” said David S. Eisenberg. “So I have been working on it ever since, along with a procession of graduate students and postdocs (post-doctoral students).”

To analyze structures of proteins, researchers must first isolate the protein in a very pure form, then induce it to precipitate from solution as crystals. A beam of X-rays is then shone through the crystal, and the scattered X-rays recorded on film. By analyzing where the X-rays strike the film, it is possible to determine the exact location of each atom within the proteins.

For small proteins, such analysis is relatively easy and can be performed in a few days. As the number of atoms increases, the task becomes progressively more difficult and takes proportionately longer. RuBisCo contains 37,792 atoms and is the largest protein whose structure has ever been determined, Eisenberg said.

Eisenberg and his students have gone through this process repeatedly, producing first a very crude picture of the overall shape of the molecule, then a series of pictures with increasing resolution until now they have located the position of each atom to within less than a billionth of an inch.

Intrigues Biologists

RuBisCo intrigues biologists, McFadden said, “because it catalyzes the slowest step in photosynthesis.”

“That means that if you can accelerate that step by genetic engineering, you might be able to get faster-growing plants,” he said.