Two types of raw materials are currently
used for biorefining and biofuel production: carbohydrates and lipids. Biofuels
are derived from carbohydrate raw materials such as sugars and lignocellulose,
while biodiesels are derived from another raw material, lipid-rich vegetable
oil.
In a study published online in Nature Biotechnology, researchers at the
UCLA Henry Samueli School of Engineering and Applied Science demonstrate for
the first time the feasibility of using proteins as a raw material for
biorefining and biofuel production.
“Proteins had been completely ignored
as a potential biomaterial because they’ve been thought of mainly as food. But
in fact, there are a lot of different proteins that cannot be used as
food,” said James C. Liao, the Chancellor’s Professor of Chemical and
Biomolecular Engineering at UCLA and senior author of the study. “These
proteins were overlooked as a resource for fuel or for chemicals because people
did not know how to utilize them or how to grow them. We’ve solved these
problems.”
“This research is the first attempt to
utilize protein as a carbon source for energy production and biorefining,”
said Kwang Myung Cho, a UCLA Engineering research scientist and an author of
the study. “To utilize protein as a carbon source, complex cellular
regulation in nitrogen metabolism had to be rewired. This study clearly showed
how to engineer microbial cells to control their cellular nitrogen metabolism.”
In nutrient-rich conditions, proteins are
the most abundant component in fast-growing microorganisms. The accumulation
rate of proteins is faster than that of any other raw materials, including
cellulose or lipids. In addition, protein does not have the recalcitrance
problems of lignocellulose or the de-watering problem of algal lipids. Protein
biomass can be easily digested to be used for microorganisms than cellulosic
biomass, which is very difficult to break down.
Further, cellulose and lipids don’t
contribute to the process of photosynthesis. But proteins are the major
component of fast-growing photosynthetic microorganisms.
The challenge in protein-based biorefining,
the researchers say, lies in the difficulties of effectively converting protein
hydrolysates to fuels and chemicals.
“Microorganisms tend to use proteins to
build their own proteins instead of converting them to other compounds,”
said Yi-xin Huo, a UCLA postdoctoral researcher and lead author of the study.
“So to achieve the protein-based biorefining, we have to completely
redirect the protein utilization system, which is one of the most highly
regulated systems in the cell.”
Liao’s team created an artificial metabolic
system to dump reduced nitrogen out of cells and tricked the cells to degrade
proteins without utilizing them for growth. Proteins contain both ammonia and
carbon; Liao’s team took away the ammonia and recycled it back for the growth
of the algae they worked with. Algae with rich ammonia fertilizers grow quickly
and were used only as a carrier to assimilate carbon dioxide and produce
protein, which results in more CO2 fixation and growth. With this
strategy, expensive photo-bioreactors can be eliminated.
“Today, nitrogen fertilizers used in
agriculture and biofuel production have become a major threat to many of the
world’s ecosystems, and the nitrogen-containing residuals in biofuel production
can eventually turn into nitrous oxide, which is about 300 times worse than CO2
as a greenhouse gas,” Liao said. “Our strategy effectively recycles
nitrogen back to the biofuel production process, thus approaching nitrogen
neutrality.
“Growing algae to produce protein is
like putting the interest back into the principal,” he said.
According to Liao’s team, the culture area
needed to produce 60 billion gallons of biofuels (30% of the United States’
current transportation fuel) based on the new technology could be as little as
24,600 square kilometers—equivalent to 1.9% of the agricultural land in the U.S.
“Developing large-scale systems is our
next step,” Huo said. “Harvesting of the protein biomass economically
is a bottleneck of advancing our technology.”
