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February 13, 2006
Mutant enzyme could help plants reduce global warming

BY Dana GOLDMAN

Global warming may have just met its match. New research completed in the School of Medicine has found a mutant enzyme that could enable plants to use and convert carbon dioxide faster. The process could allow a greater amount of greenhouse gasses to be stripped from the atmosphere.

The new research was published online on Jan. 19 and was scheduled to be printed in the February issue of Protein Engineering Design and Selection.

Ichiro Matsumura, assistant professor of biochemistry, is senior author and principal investigator; first author is research specialist Monal Parikh.

Greenhouse gases generally are produced from energy use. The emissions trap heat in the atmosphere and have been cited in the persistent rise of the earth’s temperature over time.

During photosynthesis, plants and some bacteria convert sunlight and carbon dioxide into usable chemical energy. Scientists have long known that this process relies on the enzyme rubulose 1,5-bisphosphate carboxylase/oxygenase, also called RuBisCO. While RuBisCO is the most abundant enzyme in the world, it is also one of the least efficient.

As Matsumura said, “All life pretty much depends on the function on this enzyme. It actually has had billions of years to improve, but remains about a thousand times slower than most other enzymes. Plants have to make tons of it just to stay alive.”

RuBisCO’s inefficiency limits plant growth and inhibits organisms from using and assimilating all the carbon dioxide in the atmosphere. Since photosynthesis has not kept pace with the level of gas in the atmosphere, the gas builds up. The resulting buildup is one cause of global warming.

A 2004 report by the National Science Foundation estimates that atmospheric carbon dioxide concentrations remained steady, between 200 and 280 parts per million (ppm), for thousands of years, but those levels have risen dramatically since the Industrial Revolution of the 19th century, leading to today’s concentration of 380 ppm of carbon dioxide in the atmosphere.

Scientists have struggled for decades to engineer a variant of the enzyme that would more quickly convert carbon dioxide. Their efforts primarily focused on mutating specific amino acids within RuBisCO, and then seeing if the change affected carbon dioxide conversion. Because of RuBisCO’s structural complexity, the mutations did not have the desired outcome.

For their experiment, Matsumura and his colleagues decided to use a process called “directed evolution.” That approach calls for isolating and randomly mutating genes, and then inserting the mutated genes into bacteria (in this case Escherichia coli, or E. coli). The researchers then screened the resulting mutant proteins for the fastest and most efficient enzymes. “We decided to do what nature does, but at a much faster pace,” Matsumura said. “Essentially, we’re using evolution as a tool to engineer the protein.”

Because E. coli does not normally participate in photosynthesis or carbon dioxide conversion, it does not usually carry the RuBisCO enzyme. In this study, Matsumura’s team added the genes encoding RuBisCO and a helper enzyme to E. coli, enabling it to change carbon dioxide into consumable energy. The scientists withheld other nutrients from this genetically modified organism so that it would need RuBisCO and carbon dioxide to survive under these stringent conditions.

The research team randomly mutated the RuBisCO gene, and added these mutant genes to the modified E. coli. The fastest growing strains carried mutated RuBisCO genes that produced a larger quantity of the enzyme, leading to faster assimilation of carbon dioxide gas.

These mutations caused a 500 percent increase in RuBisCO expression,” Matsumura said. “We are excited because such large changes could potentially lead to faster plant growth. This result also suggests that the enzyme is evolving in our laboratory in the same way it did in nature.”

Matsumura’s team has just published its results, but the group is continuing its research on the RuBisCO enzyme, planning to experiment by increasing the mutation rates on genes during directed evolution and looking for undiscovered connections between the enzyme’s structure and function. Perhaps, with a little more evolution, RuBisCO might be able to shed its reputation as the slowest of plant enzymes.

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