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Oral Presentation 2-01
The Microbial Cellulose Utilization Paradigm: Fundamentals and Implications for Consolidated Bioprocessing Lee R. Lynd1, Yiheng Zhang1, Zhiliang Fan1, Sunil Desai1, Michael Tyurin1 Danie La Grange2, Sharath Gundllapalli3, Ricardo Cordero Otero3 Willem H.
van Zyl2, Isak S. Pretorius3, and Paul J. Weimer4 1Thayer School of Engineering Dartmouth College, Hanover, NH
03755 2Department of Microbiology, University of Stellenbosch Stellenbosch 7600, South Africa 3Institute for Wine Biotechnology, University of Stellenbosch Stellenbosch 7600, South Africa 4U.S. Dairy Forage Research Center Madison, Wisconsin 53706 Telephone: (603) 646-2231, Fax: (603) 646-3856, E-mail: lee.lynd@dartmouth.edu The vast
majority of studies investigating cellulose hydrolysis over the last half
century have proceeded within the context of an enzymatically-oriented
intellectual paradigm. In terms of
fundamentals, this paradigm focuses on cellulose hydrolysis primarily as an
enzymatic phenomenon. In terms of
applications, the enzymatic paradigm anticipates processes featuring production
of cellulase in a step separate from cellulose hydrolysis. An alternative microbially-oriented paradigm
considers cellulose hydrolysis as a microbial phenomenon and anticipates
processes in a consolidated bioprocessing (CBP) configuration featuring
cellulase production and cellulose hydrolysis in a single step. The microbial paradigm naturally leads to an
emphasis on different fundamental issues, organisms, cellulase systems, and
applied milestones as compared to the enzymatic paradigm. Intriguing
hypotheses supported by the literature on cellulose utilization by anaerobic
microorganisms will be discussed. These
include: 1) the bioenergetic benefit of growth on cellulose is comparable to
and perhaps larger than the bioenergetic cost of cellulase synthesis; 2)
cellodextrins rather than glucose or cellobiose are the primary intermediates
in cellulose hydrolysis and cellular uptake; 3) cellulose hydrolysis does not proceed
primarily by the classic endoglucanase/cellobiohydrolase mechanism; 4) growth
rates on cellulose exhibit a strong correlation with temperature but little or
no correlation with the ATP available per unit substrate; 5) cell-specific
cellulose hydrolysis rates mediated by anaerobes are substantially higher than
for aerobes; 6) inhibition of naturally-occurring cellulolytic organisms during
growth at high substrate concentrations is due to salts added for pH control
and not inhibition by ethanol; 7) surface effects recently shown to play a key
role in determining the kinetics of acid-catalyzed cellulose hydrolysis may be
similarly important for cellularly-catalyzed hydrolysis. The possibility of cell-cellulase synergy in
which the presence of cells in a cellulose-enzyme-microbe complex enhances the
effectiveness of cellulase will also be examined. Selected
results will be presented from two lines of research aimed at the complementary
objectives of understanding cellulose utilization as a microbial phenomenon and
organism development for CBP: 1) studies
with thermophilic bacteria that utilize components of cellulosic biomass at a
high rate, including manipulation of such bacteria to improve properties
related to product-formation; 2) production
of heterologous saccharolytic enzymes in Saccharomyces cerevisiae. These results
will be considered in the context of fundamental issues such as those outlined
above as well as applied objectives.
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