Cellulolytic Activity in Microorganisms

Abstract

Cellulose is the most abundant polysaccharide occurring in plant materials. The cellulose content of higher plant is never fixed and the concentration changes with the age and type of the plant. It is specially predominant in woody substances as well as in straw, stubble and leaves. Cellulose molecules are linear polymers (unbranched long chains) of β-D-glucopyranose residues linked by β(1, 4)-glycosidic bonds. The residues in the cellulose chain are stabilized by hydrogen bonds between hydroxyl groups of adjacent glucose residues. Cellulose is soluble in acids but insoluble in alkaline solutions. Cellulose-decomposing microorganisms are found abundantly in nature. Due to cellulolytic potential these play an important role in the carbon cycle by recycling CO2 fixed through photosynthesis. Cellulosedecomposing microbes include a variety of aerobes and anaerobes; mesophiles as well as thermophiles. Fungi and bacteria, however, are mainly responsible for cellulose degradation in nature. The details of the mechanism involved in the breakdown of cellulose have been the subject of investigation for a long time. As per currently accepted three-enzyme group hypothesis, the complete degradation of native cellulose to glucose requires three enzymes - (a) endo-β-1, 4-glucanase (EG) or cellulase (CEL, EC 3.2.1.4); (b) Cellobiohydrolase (CBH, or exo-glucanase, EC 3.2.1.91) and (c) β-glucosidase (BG, EC 3.2.1.21). EG first hydrolyses amorphous regions of cellulose fibrils. The non-reducing ends thus generated are then attacked by CBH thereby releasing cellobiose. The action of CBH then proceeds into the crystalline region. BG hydrolyses cellobiose to glucose. These enzymes work synergistically to hydrolyse cellulose. The cellulolytic activity of microbes is greatly affected by different factor viz. availability of nutrients, optimum pH, temperature and moisture contents have been found to be a major controlling factor in the production of cellulolytic enzymes.