Plant enzyme efficiency may hold key to global warming

Global warming just may have met its match. In research recently completed at Emory University School of Medicine, scientists have discovered a mutant enzyme that could enable plants to use and convert carbon dioxide more quickly, effectively taking more of that gas out of the atmosphere.

The findings were published online on January 19 and will appear in the February issue of the journal Protein Engineering Design and Selection. Ichiro Matsumura, PhD, assistant professor of biochemistry at Emory University School of Medicine, is the senior author and principal investigator. The lead author is research specialist Monal R. Parikh.

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 Dr. Matsu-mura says, “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 stops organisms from using and assimilating all the carbon dioxide in the atmosphere, even as the amount of gas in the atmosphere con-tinues to grow. The resulting gas buildup is one cause of global warming. A 2004 report by the National Science Foundation estimates that atmospheric carbon dioxide concentrations remained steady at between 200 and 280 parts per million for thousands of years, but that carbon dioxide levels have risen dramatically since the Industrial Revolution of the 1800s, leading to 380 parts per million of carbon dioxide in the atmosphere today.

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

For their own study, Dr. Matsumura and his colleagues decided to use a process called “di-rected evolution” which involved isolating and randomly mutating genes, and then inserting the mutated genes into bacteria (in this case Escherichia coli, or E. coli). They 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.” Dr. Matsumura says. “Essentially we’re using evolu-tion 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 con-sumable 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.

They then 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” Dr. Matsumura says. “We are excited because such large changes could potentially lead to faster plant growth. This results also suggests that the en-zyme is evolving in our laboratory in the same way that it did in nature.”

Even as these results are published, Matsumura and his team are continuing their research on the RuBisCO enzyme. To start, they’ll experiment with increasing mutation rates on genes during di-rected evolution and look for undiscovered connections between the enzyme’s structure and func-tion. Perhaps, with a little more evolution, RuBisCO might be able to shed its ignominious reputa-tion as the slowest of plant enzymes.

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