ADVANCES IN CELLULOSIC ENZYME TECHNOLOGIES FOR ENHANCED STABILITY AND CATALYSIS

Abstract

Cellulosic ethanol has been the most promising second-generation biofuel in terms of raw material availability. The production process mandates efficient removal of lignin followed by a three-step sequential enzymatic conversion of cellulose to glucose. Cellobiase (E.C. 3.2.1.21), a β-glucosidase (BGL), obtained preferably from filamentous fungi catalyzes the final rate limiting step of this reaction, namely, the hydrolysis of cellobiose to glucose. It is therefore the most sought after model for cellulosic enzyme research. Efficient conversion of cellulosic biomass to glucose requires enhanced stability and superior catalysis. This in turn mandates strong producer organism able to secrete a high titer of the enzyme into the extracellular medium, optimized media formulation and improvised technologies for catalysis. Particularly, stabilization of the big cellobiose aggregates remains a significant technological bottleneck in this regard. Large aggregates of cellulosic enzymes are indispensible for industrial scale catalysis; however, these enzymes are prone to spontaneous dissociation by sheer dilution. Conventional immobilization and cross-linking approaches involving glutaraldehyde or entrapment in alginate beads have either proven cost-ineffective or have resulted in retention of poor specific activity for efficient catalysis. Over the last decade, new generation enzyme technologies such as synthetic multienzyme cellulosome complex, use of protein stabilizing osmolytes and reducing agents to maximize substrate exposure has opened up new avenues for cross-linker free stabilization and enhanced catalysis. The review is a fresh update of the producer strains, media optimizations and enzyme technologies to boost the production of cellulosic ethanol.