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RuBISCO: the most lousy enzyme in the world?
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.
Questions:
1. What is the name of the molecule that reacts with carbon dioxide in the first stage of the
light-independent reactions of photosynthesis?
2. Why might changing the amino acids in RuBISCO change the activity of the enzyme?
3. Why do you think changing specific, targeted amino acids of RuBISCO in the laboratory is a
slow and often unsuccessful strategy for increasing the efficiency of the enzyme?
4. In the “directed evolution” process described, why are bacteria chosen as the organism in
which to carry out the experiment?
5. The researchers also added the genes for a second enzyme in addition to RuBISCO to E. coli
so that the bacteria would be able to use the molecules formed during the enzyme catalysed
by RuBISCO. What reaction might this second enzyme catalyse?
6. Why is it important to this study that the bacteria are able to actually use the products
catalysed by RuBISCO?
7. Why does withholding nutrients from the bacterial cultures aid the natural selection
process?
8. The researchers have increased the efficiency of RuBISCO by 5 times. How might they then
set about using this new, modified RuBISCO in a green plant such as rice?