NREL Researchers Use Imaging Technologies to Solve Puzzle of Plant Architecture
Breakthrough could help optimize capture of sugars for biofuels
Monday, November 26, 2012
Scientists at the U.S. Department of Energy's
National Renewable Energy Laboratory (NREL) and the BioEnergy Science Center
(BESC) combined different microscopic imaging methods to gain a greater
understanding of the relationships between biomass cell wall structure and
enzyme digestibility, a breakthrough that could lead to optimizing sugar yields
and lowering the costs of making biofuels.
A paper on the breakthrough, "How Does Plant Cell
Wall Nanoscale Architecture Correlate with Enzymatic Digestibility?" appears in
the current issue of Science Magazine.
Principal Investigator Dr. Shi-You Ding of NREL said
the imaging technologies allowed the interdisciplinary team of scientists to
view the plants' architecture at scales ranging from millimeter to nanometer, a
range of 1 million to one.
That allowed them to learn not just the plant cell wall
architecture, but also the localization of the enzymes responsible for
deconstruction of the cell wall polymers and the effects of enzyme action on the
cell wall.
They didn't have to resort to wet chemistry, which
ascertains the molecular makeup of a substance at the cost of destroying the
spatial relationships. "The typical way to understand the structure of biomass
is to break down all the individual components so they can be analyzed," Ding, a
biologist, said. "The problem with that method is that then you don't know where
all the components came from. You lose the structural integrity."
That's a crucial loss, because an understanding of how
enzymes digest plants requires an understanding of where everything is inside
the cell walls.
"Our imaging techniques gave us a deeper understanding
of the cell wall structure and the process of enzyme hydrolysis of cell-wall
carbohydrate polymers to release simple sugars," Ding said. "That allows us to
optimize the process and reduce costs."
Dr. Paul Gilna, the director of the BESC, in which the
project was conducted, added: "This work greatly improves our ability to closely
examine the mechanisms behind the scientific improvements we have developed, all
of which are targeted at enabling the emergence of a sustainable cellulosic
biofuels industry." BESC is a multi-institutional Bioenergy Research Center
supported by the Office of Biological and Environmental Research in the
Department of Energy Office of Science.
The correlative imaging in real time allowed the team
to assess the impact of lignin removal on biomass hydrolysis and to see the
nanometer-scale changes in cell wall structure. And, that allowed them to see
how those changes affected the rate at which enzymes from two different
organisms digested the plant cell walls.
The aim in the biofuel industry is to access the
plants' polymeric carbohydrate structures without damaging the basic molecules
of which the polymers are constructed. "It's more like dis-assembling a building
with wrenches, hammers and crowbars to recover re-useable bricks, wiring, pipes
and structural steel than it is like using a wrecking ball or explosives," Gilna
said. Enzymes, unlike typical harsh chemical catalysts, excel at this relatively
gentle disassembly.
The NREL team examined two enzyme systems – one from a
fungus, the other from a bacterium – both holding promise as biocatalysts for
producing sugar intermediates for the biofuels industry.
The particular bacterial enzymes studied are organized
through a large scaffolding protein into a multi-enzyme complex from which they
make a coordinated attack on the cell walls. The separate fungal enzymes act
more individualistically, although the ultimate result is cooperative in that
case, as well.
The NREL team found that the easier the access to the
cell walls, the better and faster the enzymes will digest the
material.
In biofuels production, enzymes are needed to greatly
speed up the chemical reactions that break down the biomass during fermentation.
The NREL scientists found that the gummy, poly-aromatic
non-sugar lignin in plants interferes with enzymes' ability to access the
polysaccharides in the cell wall – the stuff that both the enzymes and the
industry want.
So, they concluded, ideal pre-treatment should focus on
getting rid of the lignin while leaving the structural polysaccharides within
the cell walls intact, thus leaving a relatively loose, porous native-like
structure that allows easy access by the enzymes and rapid digestion, as opposed
to pretreatments that remove some of the spongier carbohydrate polymers and
allow the remainder to collapse into tighter and less-accessible structures. To
continue the building dis-assembly and salvage analogy, removal of the lignin is
like unlocking all of the doors in the building so that the workers can get in
to pull out re-useable materials, but without collapsing the overall structure
so that access is blocked.
By understanding the changing structure of the plant
material, scientists can learn more about how enzymes work.
"The enzyme has evolved to deal with the real
structure, not the pretreated, artificially decomposed one," Ding said. "So to
understand how the enzyme goes about its business, it is really important to
know where cell wall components are located, as well as the various modes of
enzyme action."
"Then we can optimize the whole process," Ding said.
"By observing where cellulase enzymes are localized and the nanostructural
changes in the plant cell wall architecture that their actions produce, we hope
to suggest rational strategies for more cost effective pretreatments and better
enzymes."
NREL is the U.S. Department of Energy's primary national
laboratory for renewable energy and energy efficiency research and development.
NREL is operated for DOE by the Alliance for Sustainable Energy,
LLC.
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