Effective, efficient, and economical conversion technologies are needed to meet the potential of using biobased feedstocks to produce liquid fuels and other high-value chemicals. Enzymatic hydrolysis of plant cell wall structural carbohydrates into soluble and fermentable sugars has been technically achievable for decades. Despite significant advances in the past five years, the economical production and use of cellulase enzymes for biomass hydrolysis remain key hurdles. This is true whether one is seeking hydrocarbons or economical cellulosic ethanol.
The key to conquering these obstacles according to the collaborating PIs Minko of Clarkson University in Potsdam, NY and Voronov and Pryor of North Dakota State University at Fargo rests in placing the enzymes in nanostructured capsules. These carefully designed and fabricated, hybrid organic-inorganic microcapsules are loaded with a cocktail of cellulase enzymes for the conversion of cellulose into fermentable glucose. These unique capsules protect the enzymes and preserve their activity, allow for a simple reuse/recovery process for the enzymes, and provide an opportunity to regulate enzymatic reactions using external signals, such as pH. This enzyme recovery and reuse, facilitated through the encapsulation process and magnetic separation, are expected to have significant impacts on processing costs to produce biomass-derived sugars.
The PIs plan to create a website as a means of sharing data and plans between the research groups at the two universities, and to allow public access to follow aspects of the project. They intend to utilize REU opportunities and high school researchers as a part of their outreach efforts. This is a challenging project for all of the researchers. There are many synthetic chemistry components that all must come together in the fabrication of the encapsulated enzymes. The successful development of reversible encapsulation technology for applications of enzymes would have far-reaching implications in a number of industrial processes and would constitute a significant advance in the field.
The environmental, economic, and political consequences of US dependence on foreign sources of petroleum have fueled the expanding interest in developing renewable, biobased transportation fuels. The 2007 Energy Independence and Security Act calls for 36 billion gallons of biobased fuels by 20221. Of that, 16 billion gallons must come from cellulosic biomass and another 5 billion gallons should be "Advanced Biofuels," which may come from cellulosic biomass or other nontraditional feedstocks. Biomass availability and political will are both necessary to begin the transition to a more biobased economy. Effective, efficient, and economical conversion technologies must also be available to meet the potential of using all available biobased feedstocks for liquid fuels or other high-value chemicals. Several significant challenges remain in the development of economical cellulosic ethanol. Enzymatic hydrolysis of plant cell wall structural carbohydrates into soluble and fermentable sugars has been technically achievable for decades. Despite significant advances in the past five years, the economical production and use of cellulase enzymes for biomass hydrolysis remain a key hurdle and focus area for industry and government agencies. Knowledge gaps for the effective use of cellulase enzymes include the following: 1) understanding the structure-function relationship of cellulase enzyme complexes, 2) understanding the synergistic relationships among the suite of cellulolytic enzymes (e.g., endoglucanases, exoglucanases, and cellobiohydrolases) required for complete biomass hydrolysis, and 3) improving enzyme stability through a spectrum of temperatures, pressures, and pHs and in the presence of solvents or chemical inhibitors. Encapsulation of some or the entire suite of cellulase enzymes is expected to have the immediately tangible benefits of increasing stability and enabling separation of enzymes from biomass hydrolyzate and/or fermentation broth. This enzyme recovery and reuse, facilitated through the encapsulation process, are expected to have significant impacts on processing costs. In this project, engineered novel, "smart," pH-responsive capsules were developed for the enhanced storage, delivery, and recovery of cellulase enzymes for the conversion of cellulosic biomass to glucose and fermentation to ethanol or any other fermentation product. The results show this method of enzyme immobilization is technologically feasible and the enzymes function in a fundamentally different way than other techniques for enzyme immobilization. Enzymes are not rigidly attached to the polymers but are rather just attracted to the outside layer, called a brush, where the enzymes remain mobile. The mobility of the enzymes within the brush layer allows them to function more similarly to the way enzymes would when they are freely dispersed in the solution. The enzymes have a high affinity for the polymer brush but remain reasonably effective at breaking down the cellulose into glucose. The fact that the enzymes are concentrated on the capsule surface appears to be beneficial in keeping the enzymes located at the surface of the solid biomass. The experiments demonstrated that this method allows the enzymes to produce sugar, or biofuel, yields approximately four-fold larger than with the conventional single use of cellulase enzymes. The project resulted in training of two graduate students, a postdoc and an international visitor. The results of the project were disseminated for scientific and public communities. The work included international collaboration with German partners.
