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