Cellulase
Cellulose, the most abundant and renewable material on earth, is a biopolymer of d-glucose units connected by β-1, 4 glycosidic linkages. The complete hydrolysis of cellulose into glucose involves the enzyme cellulase. Cellulase is a multi-enzyme system comprising of endo-β-1, 4-glucanases [EC 3. 2. 1. 4], exo-β-1,4-glucanases or cellobiohydrolases and β-1,4-glucosidases. Cellulases accounted for approximately 20% of world enzyme market between 2005 to 2010 [2] and its demand is thought to increase drastically due to its application in second generation bioethanol production. Cellulases are produced by bacteria, fungi, protozoans, plants and animals. Currently most of the commercial cellulases are obtained from fungi mainly Trichoderma, Humicola, Aspergillus, and Penicillium. However, bacterial cellulases are gaining attention because of their high natural diversity, higher growth rate, easier product recovery and ability to produce enzymes that withstand harsh environmental conditions.
Commercial Importance:
The cellulolytic potentials of bacteria belonging to different genera such as Acetivibrio, Bacillus, Bacteroides, Cellulomonas, Clostridium, Erwinia, Ruminococcus, and Thermomonospora have been well studied. Among them Bacillus spp. are known to produce and secrete large quantities of extracellular enzymes and hence dominate the bacterial workhorses. Moreover, the endospore forming ability and production of secondary metabolites give them an additional advantage over competitors under conditions of slow growth on cellulosic substrates. The strains of B. subtilis and B. sphaericus are excellent cellulase producers.
Cellulases have attracted much attention because of their application in various industrials processes, including food, textiles, laundry, pulp and paper as well as in agriculture.
The demand for cellulose is expected to increase by 100% within 2014. Presently, the majority of the commercial and laboratory cellulases are achieved by fungi due to their high enzyme activity, but several factors suggest that bacteria may have excellent potential.
Bacteria frequently have a higher growth rate than fungi allowing for higher rate of enzyme production. Most significantly, they show affinity to be more heat stable and are easier for genetic purpose. Various bacterial genera reported for cellulolytic activities include Bacillus, Clostridium, cellulomonas, Rummminococcus, Alteromonas, Acetivibrio etc. Among bacteria, Bacillus sp. including B. brevis are well recognized cellulase production under submerged condition. According to studies the cellulase activity in broad pH and temperature range of 4.0-10.0 and 30-105°C was observed, hence, clearly indicate the thermo-alkaline nature of this enzyme. Considering its stability under high temperature as well as mild alkaline condition, cellulase may be exploited for industrial usage.
They find use in saccharification of lignocellulosic agroresidues to fermentable sugars which can be used for production of bioethanol, lactic acid, and single-cell protein. Simultaneous saccharification and fermentation (SSF) process combines enzymatic hydrolysis of cellulose with subsequent fermentation of reducing sugar (glucose) to ethanol. SSF studies from lignocellulosic biomass such as wheat and rice straw, corn stalk, corn cobs, and forestry wastes using cellulase from natural sources have been reported. Owing to the inherent key enzymes for ethanol fermentation, alcohol dehydrogenase and pyruvate decarboxylase found in Zymomonas mobilis, research has been focused on it as a promising alternative ethanol producer for its high sugar uptake and improved ethanol tolerance.
Environmental Importance:
The increasing concern about the shortage of remnant energy, the release of green house gasses by incomplete burning of fossil fuel which create air pollution have resulted in rising center on the use of cellulases to carry out enzymatic hydrolysis of the lignocellulosic waste materials for the production of bioethanol