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.