14 DAY TRIAL // Researchers from Emory University School of Medicine have discovered a mutant enzyme that enables plants to use and convert carbon dioxide more quickly, effectively removing more greenhouse gasses from the atmosphere.
During photosynthesis, plants, and some bacteria, convert sunlight and carbon dioxide into usable chemical energy. This process relies in its first stage (involving the "capture" of carbon dioxide inside a larger molecule) on an enzyme called RuBisCO. RuBisCo is the catalyst in the process by which the atoms of atmospheric carbon dioxide are made available to organisms in the form of energy-rich molecules such as sucrose.
While RuBisCO is the most abundant enzyme in the world, it is also one of the least efficient. RuBisCO is so slow that it can "capture" only a few carbon dioxide molecules each second and it is the main limitation of the rate by which plants produce energy. As Dr. Matsumura 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 hampers their ability of using and assimilating all the carbon dioxide in the atmosphere. Basically, the carbon dioxide gas, which is a greenhouse gas, has been building up in the atmosphere because photosynthesis could not keep pace with the amount of gas released in the atmosphere. One consequence is global warming. One solution is to reduce the amount of gas released in the atmosphere. The other one is to increase the efficiency of photosynthesis.
A 2004 report by the National Science Foundation has estimated that atmospheric carbon dioxide concentrations remained steady for thousands of years, but have risen dramatically since the Industrial Revolution of the 1800s.
Designing a better RuBisCo
For decades, scientists have struggled to artificially engineer a variant of the enzyme that would convert carbon dioxide more quickly. Their attempts involved mutating specific amino acids within RuBisCO, and then checking how the change affected carbon dioxide conversion. However, due to RuBisCO's structural complexity (see picture above), none of these mutations had the desired outcome.
Dr. Matsumura and his colleagues tried a different approach. They have used a process called "directed evolution", which is basically a lab recreation of the natural Darwinian evolution but in a highly accelerated fashion. They have mutated the RuBisCo enzyme and then inserted it into the E. coli bacteria. The point is that E. coli does not normally participate in photosynthesis or carbon dioxide conversion, thus, it does not usually carry the RuBisCO enzyme. Matsumura's team added the genes encoding RuBisCO and a helper enzyme to E. coli, enabling it to change carbon dioxide into consumable energy. In order to put the "directed evolution" (more specifically, the natural selection) at work for screening the efficient RuBisCo enzymes from the inefficient ones, the scientists withheld other nutrients from this genetically modified E. coli so that it would need RuBisCO and carbon dioxide to survive. In these harsh conditions some E. coli survived and multiplied while others did not. The ones that thrived had the most efficient enzymes.
"We decided to do what nature does, but at a much faster pace." Dr. Matsumura says. "Essentially we're using evolution as a tool to engineer the protein."
Thus, the fastest growing strains of E. coli carried those mutated RuBisCO genes that produced a larger quantity of the enzyme, leading to faster assimilation of carbon dioxide gas. This way the team managed to increase the efficiency of RuBisCo five times. "We are excited because such large changes could potentially lead to faster plant growth. This results also suggests that the enzyme is evolving in our laboratory in the same way that it did in nature," says Dr. Matsumura.