SEARCH FOR BETTER BIOFUELS MICROBES LEADS TO HUMAN GUT
Scientists have
scoured cow rumens and termite guts for microbes that can efficiently break
down plant cell walls for the production of next-generation biofuels, but some
of the best microbial candidates actually may reside in the human lower
intestine, researchers report.
Their
study, reported in the Proceedings of the National Academy of Sciences,
is the first to use biochemical approaches to confirm the hypothesis that
microbes in the human gut can digest fiber, breaking it down into simple sugars
in order to ferment them into nutrients that nourish human cells. These
findings have significance for human health but also for biofuels production,
since the same sugars can be fed to yeast to generate ethanol and other liquid
fuels. The human microbes appear to be endowed with enzymes that break down a
complex plant fiber component more efficiently than the most efficient microbes
found in the cow rumen, the researchers report.
Their
work in cows led the researchers to the human microbes, said University of
Illinois animal sciences and Institute for Genomic Biology professor Isaac
Cann, who led the new analysis with his colleagues, animal sciences professor
Roderick Mackie and M.D./Ph.D. student Dylan Dodd. Cann also is a microbiology
professor and a principal investigator at the Energy Biosciences Institute.
Dodd is now at Stanford University.
"In
looking for biofuels microbes in the cow rumen, we found that Prevotella
bryantii, a bacterium that is known to efficiently break down (the plant
fiber) hemicellulose, gears up production of one gene more than others when it
is digesting plant matter," Cann said.
When
searching a database for similar genes in other organisms, the researchers
found them in microbes from the human gut. The team focused on two of these
human microbes, Bacteroides intestinalis and Bacteroides
ovatus, which belong to the same bacterial phylum as Prevotella from the
cow.
"We
expressed the human gut bacterial enzymes and found that for some related
enzymes, the human ones actually were more active (in breaking down
hemicellulose) than the enzymes from the cow," Cann said.
When
the researchers looked more closely at the structure of the human enzymes, they
saw something unusual: many single polypeptide (protein) chains actually
contained two enzymes, one of which was embedded in the other. Further analysis
of the most important protein revealed that the embedded component was a
carbohydrate-binding module (CBM), which, as its name implies, latches onto
carbohydrates such as hemicellulose. This enzyme shreds the plant fiber
hemicellulose so that other enzymes can work on it to break it down into its
unit sugars.
Working
with U. of I. biochemistry professor Satish Nair, the researchers also noticed
that the CBM "put a kink" in the fiber when it bound to it. This bending
action may bring the fiber close to the other enzyme in the protein so it can
get to work breaking the bonds between the sugars. Further research is needed
to confirm this hypothesis, Cann said.
The
study points to human microbes as a potentially potent source of microbes that
can aid in biofuels production, Cann said.
"In
addition to finding microbes in the cow rumen and termite gut, it looks like we
can actually make some contributions ourselves," he said. "And our
bugs seem to have some enzymes that are even better than those in the cow
rumen."
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