Enzymatically based cellulosic ethanol production technology was selected as a key area for biomass technology development in the 1980s, and the US Department of Energy (DOE) has actively supported the scale up of ethanol production since the Office of Alcohol Fuels was created in the DOE after the 'energy crisis' of the 1970s. Although biological conversion of cellulosic biomass to fuels and chemicals through enzymatic hydrolysis of cellulose offers the potential for higher yields, higher

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... tivity, lower energy costs and milder operating conditions than chemical processes, such technology was judged to be too high risk for industry to pursue at that time [1]. However, application of the emerging field of biotechnology offered the promise for significant advances that could dramatically reduce costs and make cellulosic ethanol competitive. Improvements in dilute acid pretreatment and cellulase produced by Trichoderma reesei discovered during World War II led to most of the historic cellulosic ethanol cost reductions in the 1980s [2][3][4]. Well-known T. reesei Rut C30 was derived at Rutgers University through classical mutagenesis and strain Biological conversion of cellulosic biomass to fuels and chemicals offers the high yields to products vital to economic success and the potential for very low costs. Enzymatic hydrolysis that converts lignocellulosic biomass to fermentable sugars may be the most complex step in this process due to substrate-related and enzyme-related effects and their interactions. Although enzymatic hydrolysis offers the potential for higher yields, higher selectivity, lower energy costs and milder operating conditions than chemical processes, the mechanism of enzymatic hydrolysis and the relationship between the substrate structure and function of various glycosyl hydrolase components is not well understood. Consequently, limited success has been realized in maximizing sugar yields at very low cost. This review highlights literature on the impact of key substrate and enzyme features that influence performance, to better understand fundamental strategies to advance enzymatic hydrolysis of cellulosic biomass for biological conversion to fuels and chemicals. Topics are summarized from a practical point of view including characteristics of cellulose (e.g., crystallinity, degree of polymerization and accessible surface area) and soluble and insoluble biomass components (e.g., oligomeric xylan and lignin) released in pretreatment, and their effects on the effectiveness of enzymatic hydrolysis. We further discuss the diversity, stability and activity of individual enzymes and their synergistic effects in deconstructing complex lignocellulosic biomass. Advanced technologies to discover and characterize novel enzymes and to improve enzyme characteristics by mutagenesis, post-translational modification and over-expression of selected enzymes and modifications in lignocellulosic biomass are also discussed.