Conversion of Agricultural Residues to Bioethanol: The Roles of Cellulases and Cellulosomes

Some forms of agricultural residues represent an attractive resource for lignocellulosic biomass in our quest to reduce the dependence of the Western World on fossil fuels. After crops have been harvested, the residues usually represent relatively large amounts of cellulosic material that could be returned to the soil for its future enrichment in carbon and nutrients. However, today, many believe that a substantial portion of these residues could be made available for further conversion to biofuels. Likewise, animal wastes, particularly from herbivores and notably from ruminants, are high in cellulose content and can also be converted to liquid biofuels. Although such agricultural byproducts cannot compensate completely for the large volumes of liquid fuels required to sustain our transportation energy requirements, they can play a decisive local and regional role to fi ll these needs. However, in this case, nature and mankind have different agendas. The challenge regarding cellulosic biomass is that cellulose plays a critical structural role in the terrestrial plant cell wall. Glucose, the most desirable plant sugar for fermentation, is incorporated within the cellulose microfi brils that make up the complex plant cell wall. The most successful future bioconversion processes for the production of biofuels from lignocellulose may indeed ultimately mimic the concerted action of the cellulolytic microbes, the bacteria, and fungi that have evolved to produce cellulases and cellulosomes. It is now very clear that the major bottleneck in this process - both from a biochemical and economical point of view - is the deconstruction of the plant cell wall, liberating both C6 and C5 sugars. Nature has evolved microbes and their enzymes to deal primarily with damaged and decaying vegetation. Ultimately, much of this plant matter is again converted to a form that can be incorporated into living plant tissue. Nature thus has the time needed to manage the plant biosphere with low - energy consuming processes that can be less than ideal. We, on the other hand, must deploy rapid, effi cient, and most importantly, cost - effective conversion processes that will meet our future energy needs. The present chapter deals with the current status of our knowledge regarding the function of cellulases and cellulosomes, and how we might use them in processes for biomass conversion to biofuels. This includes a description of various types of cellulosic biomass in agricultural wastes and the pretreatment strategies required to enhance enzymatic attack and to avoid toxic byproducts that would interfere with enzyme action and fermentation. The effects of treatment with free cellulases versus treatment with cellulosomes are also detailed. The natural cellulases and cellulosomes, their various families, modular, and subunit architectures, are all documented. The search for novel enzymes, and strategies for mutation and modifi cation of cellulases and cellulosomes for future application to bioenergy initiatives are considered as well. We address some of the bottlenecks and pitfalls that await us in our current and future efforts to provide effi cient processes for conversion of cellulosic biomass to usable sugars for biofuel production.