Cellulase Engineering
Current
Personnel:
We stand on the
brink of a cellulosic ethanol revolution. This will
bring growth and change to the energy industry and to the diverse
fields comprising the energy research community. Liquid fuels are
prized for their transportability and high energy density. Petroleum is
not a globally sustainable source of transportable fuel as a
consequence of its contribution to global warming, limited supply and
the political and economic hurdles that these limitations impose,
especially upon developing nations. U. S. oil production peaked over 20
years ago, and world oil production is expected to peak in our
lifetimes. A liquid fuel with zero net carbon dioxide emissions is
necessary to sustainably meet the world’s steadily rising
energy demand and mitigate the progression of global warming.
Cellulosic ethanol is the most feasible liquid fuel capable of being
produced on a sufficiently large scale while meeting these requirements.
Lignocellulosic biomass
is converted to ethanol through a three-step
process: pretreatment, hydrolysis of cellulose into glucose, and
fermentation of glucose to ethanol. The recalcitrance of lignocellulose
to hydrolysis arises from the presence of lignin and hemicellulose and
their entanglement with cellulose. Various pretreatment strategies are
available and in development by biofuels researchers to abate these
challenges through mechanisms such as the degradation of hemicellulose
and the removal of lignin. The hydrolysis of cellulose following
pretreatment can be accomplished through the use of a team of
cellulolytic enzymes. The glucose produced by these cellulases may be
fermented to ethanol using an appropriate microorganism, such as S.
cerevisiae. The Frances Arnold lab is working to engineer superior
cellulases for use in the hydrolysis of cellulose to glucose.
There are two classes of
cellulase systems: noncomplexed and complexed.
Noncomplexed cellulase systems consist of a synergistic set of three
soluble enzymes: 1) cellobiohydrolases, which cleave cellobiose units
from the ends of a cellulose fiber; 2) endoglucanases, which randomly
hydrolyze β-1,4-glycosidic linkages at the interior of a
cellulose fiber; 3) β-glucosidase, which hydrolyzes cellobiose
into its constituent glucose monomers. Noncomplexed cellulase systems
are typical of cellulose-degrading aerobic fungi. Complexed cellulase
systems are composed of the same three enzyme types found in
noncomplexed cellulase systems, but they are bound together forming a
multi-protein complex. Complexed cellulases are typically found in
anaerobic cellulose-degrading bacteria. Work in the lab is exploring
the potential of enzymes from these two classes of systems.
The Arnold Lab is using
the tools of protein recombination, directed
evolution and cell surface display on Saccharomyces cerivisiae to engineer superior cellulases and cellulolytic systems.