Enzymatic Hydrolysis of Cellulose
In the enzymatic hydrolysis, cellulose is degraded by highly specific cellulase enzymes. This process occurs in three steps: adsorption of enzymes onto the cellulose surface and release of separate chains, hydrolysis of 1,4-P-D -glycosidic linkages in cellulose to reducing fermentable sugars, and desorption of the enzyme. Cellulase refers to a suite of enzymes produced mainly by fungi, bacteria and protozoans. However, there are also cellulases produced by other types of organisms such as plants and animals. For instance, there are cellulases produced by some termites and the microbial intestinal symbionts of other termites [6]. In the most familiar case of cellulase activity, the enzyme complex breaks down cellulose to D-glucose. This type of cellulase is produced mainly by symbiotic bacteria in the ruminating chambers of herbivores. Enzymes which can hydrolyze hemicellulose are usually referred to as hemicellu — lases and these enzymes are also generally classified under cellu — lases. There are three major types of cellulases based on the type of reaction that are known in the common enzymatic depolymerization of cellulose to D-glucose:
1. Endocellulase (EC 3.2.1.4) randomly cleaves internal bonds at amorphous sites that create new chain ends.
2. Exocellulase (EC 3.2.1.91) cleaves two to four units from the ends of the exposed chains produced by endocel — lulase, resulting in tetrasaccharides or disaccharides such as cellobiose. Exocellulases that are producing cellobiose are known as cellobiohydrolases (CBH). There are two main types of cellobiohydrolases (CBH). These two types are known as CBHI and CBHII. Of the two types, CBHI works processively from the reducing end of cellulose and CBHII works processively from the nonreducing end of cellulose.
3. Cellobiase (EC 3.2.1.21), also known as P-glucosidase, hydrolyses the exocellulase product into individual D-glucose molecules.
Within the above types of cellulases there are also progressive (also known as processive) and nonprogressive types. Progressive cellu — lase will continue to interact with a single polysaccharide strand; nonprogressive cellulase will interact once then disengage and engage another polysaccharide strand. Most fungal cellulases have a two-domain structure, with one catalytic domain and one cellulose binding domain that are connected by a flexible linker. This structure is adapted for working on an insoluble substrate, and it allows the enzyme to diffuse two-dimensionally on a surface in a caterpillar-like fashion. However, there are also cellulases (mostly endoglu — canases) that lack cellulose binding domains. These enzymes might have a swelling function.