Cellulase

Cellulase refers to a class of enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis (i.e. the hydrolysis of cellulose). However, there are also cellulases produced by a few other types of organisms, such as some termites and the microbial intestinal symbionts of other termites.^[2] Several different kinds of cellulases are known, which differ structurally and mechanistically. The EC number for this group of enzymes is EC 3.2.1.4.

Reaction: Hydrolysis of 1,4-beta-D-glycosidic linkages in cellulose, lichenin and cereal beta-D-glucans.

Other names for 'endoglucanases' are: endo-1,4-beta-glucanase, carboxymethyl cellulase (CMCase), endo-1,4-beta-D-glucanase, beta-1,4-glucanase, beta-1,4-endoglucan hydrolase, and celludextrinase. The other types of cellulases belong to exocellulases. Beta-glucosidases can also be considered as yet another group of cellulases. The expression 'avicelase' refers to the total cellulase activity of a given sample of the enzyme(s). The cellulase may be the result of the action of more than one type of enzymes.

Types and action

Five general types of cellulases based on the type of reaction catalyzed:

• Endocellulase breaks internal bonds to disrupt the crystalline structure of cellulose and expose individual cellulose polysaccharide chains
• Exocellulase cleaves two to four units from the ends of the exposed chains produced by endocellulase, resulting in the tetrasaccharides or disaccharides, such as cellobiose. There are two main types of exocellulases [or cellobiohydrolases (CBH)] - CBHI works processively from the reducing end, and CBHII works processively from the nonreducing end of cellulose.
• Cellobiase or beta-glucosidase hydrolyses the exocellulase product into individual monosaccharides.
• Oxidative cellulases depolymerize cellulose by radical reactions, as for instance cellobiose dehydrogenase (acceptor).
• Cellulose phosphorylases depolymerize cellulose using phosphates instead of water.

In the most familiar case of cellulase activity, the enzyme complex breaks down cellulose to beta-glucose. This type of cellulase is produced mainly by symbiotic bacteria in the ruminating chambers of herbivores. Aside from ruminants, most animals (including humans) do not produce cellulase in their bodies and can only partially break down cellulose through fermentation, limiting their ability to use energy in fibrous plant material. Enzymes that hydrolyze hemicellulose are usually referred to as hemicellulase and are usually classified under cellulase in general. Enzymes that cleave lignin are occasionally classified as cellulase, but this is usually considered erroneous.

Within the above types there are also progressive (also known as processive) and nonprogressive types. Progressive cellulase 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 adaption for working on an insoluble substrate, and it allows the enzyme to diffuse two-dimensionally on a surface in a caterpillar way. However, there are also cellulases (mostly endoglucanases) that lack cellulose binding domains. These enzymes might have a swelling function.

In many bacteria, cellulases in-vivo are complex enzyme structures organized in supramolecular complexes, the cellulosomes. They contain roughly five different enzymatic subunits representing namely endocellulases, exocellulases, cellobiases, oxidative cellulases and cellulose phosphorylases wherein only endocellulases and cellobiases participate in the actual hydrolysis of the β(1→ 4) linkage. Recent work on the molecular biology of cellulosomes had led to the discovery of numerous cellulosome-related “signature” sequences known as dockerins and cohesins. Depending on their amino acid sequence and tertiary structures, cellulases are divided into clans and families.^[3]

Mechanism of cellulolysis

Uses

Cellulase is used for commercial food processing in coffee. It performs hydrolysis of cellulose during drying of beans. Furthermore, cellulases are widely used in textile industry and in laundry detergents. They have also been used in the pulp and paper industry for various purposes, and they are even used for pharmaceutical applications. Cellulase is used in the fermentation of biomass into biofuels, although this process is relatively experimental at present. Cellulase is used as a treatment for phytobezoars, a form of cellulose bezoar found in the human stomach.

References

• Chapin III, F.S., P.A. Matson, H.A. Mooney. Principles of Terrestrial Ecosystem Ecology. Springer-Verlag New York, NY. 2002
• The Merck Manual of Diagnosis and Therapy, Chapter 24

See also

• Hemicellulose