Biotechnological Applications of β-Glucosidases in Biomass Degradation

Abstract

The plant cell wall is chiefly composed of cellulose (35–50%), hemicellulose (20–35%) and lignin (10–15%). Structurally, cellulose is a simple biopolymer composed of D-glucose residues, linked by β-1,4-glucosidic bonds. It is, however, highly resistant to enzymatic hydrolysis due to its insoluble, crystalline microfibrillar structure and its complex association with lignin. Cellulose degradation requires the concerted action of three enzymes: exoglucanase (that attacks the terminal glucosidic bond), endoglucanase (that catalyses the hydrolysis of internal bonds) and β-glucosidase (that converts cellobiose to free glucose molecules). The process of cellulose degradation is often limited by the terminal step catalysed by β-glucosidases, and hence, an increased understanding of the regulatory aspects of this enzyme would help in increasing the efficiency of the reaction. β-glucosidases are naturally produced by some organisms like bacteria, fungi and termites, which are crucial for hydrolysing cellulose, the major carbohydrate produced by plants. In addition to their ecological roles, β-glucosidases find tremendous biotechnological applications in biofuel production, enhancement of flavours and nutritional quality, detoxification of cyanogenic glucosidases in food crops, waste paper recycling and many more. The fact that cellulose is the most abundant carbohydrate on earth makes it a promising source of biofuels through its conversion into sugars, followed by fermentation into ethanol. This process of lignocellulose bioconversion has an additional advantage of cellulose waste management. Hence, this chapter would mainly focus on the biotechnological applications of β-glucosidases with special emphasis on the regulations, limitations and approaches to enhance the enzyme activity in the process of lignocellulose conversion into biofuels.