Cellulase

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Cellulase (β-1,4-glucan-4-glucanohydrolase) is a general term for a group of enzymes that degrade cellulose to produce glucose. It is not a single enzyme, but a multi-component that acts synergistically enzyme system is a composite enzyme, mainly composed of exo-β-glucanase, endo-β-glucanase and β-glucosidase, as well as high-activity xylanase. Acts on cellulose and products derived from cellulose. Microbial cellulase is of great significance in converting insoluble cellulose into glucose and destroying cell walls in fruit and vegetable juice to improve the yield of juice.

Classifications

• By composition and function

Cellulase enzymes can be divided into endoglucanases according to their catalytic reaction functions, EG from fungi, Cen from bacteria, exoglucanases, CBH from fungi, and Cex from bacteria. ) And β-glucosidase (β-1,4-glucosidase, EC.3.2.1.21) referred to as BG.

• According to degradation mechanism

The cellulase reaction is different from the general enzyme reaction. The main difference is that cellulase is a multi-component enzyme system and the substrate structure is extremely complex. Due to the water insolubility of the substrate, the adsorption of cellulase replaces the ES complex process formed by the enzyme and the substrate. Cellulase first specifically adsorbs on the substrate cellulose, and then decomposes the cellulose into glucose under the synergistic action of several components.

Structure

Most fungal cellulases have a two-domain structure, in which a catalytic domain and a cellulose binding domain are connected by a flexible linker. This structure is suitable for working on insoluble substrates and allows enzymes to spread two-dimensionally on the surface in a manner similar to caterpillars. However, there are also cellulases (mainly endoglucanases) that lack a cellulose binding domain. Both substrate binding and catalysis depend on the three-dimensional structure of the enzyme, which is the result of the level of protein folding. The amino acid sequence occurring in the active site and the arrangement of its residues (the position where the substrate binds) may affect various factors, such as the binding affinity of the ligand, the stabilization and catalysis of the substrate in the active site. The structure of the substrate is complementary to the precise active site structure of the enzyme. Changes in residue positions may cause distortions in one or more of these interactions.

Figure 1. Protein structure of cellulase.

Factors affecting the action of cellulase

The optimum pH of cellulase is generally 4.5 to 6.5. Gluconolactone can effectively inhibit cellulase. Heavy metal ions such as copper and mercury ions can also inhibit cellulase, but cysteine can eliminate their inhibitory effect and even further activate cellulase. Plant tissues contain natural cellulase inhibitors; it can protect plants from the rot of molds. These inhibitors are phenolic compounds. If there is high oxidase activity in plant tissues, then it can oxidize phenolic compounds into quinone compounds, which can inhibit cellulase.

Agricultural applications

Common livestock and poultry feeds such as grains, beans, wheat and processing by-products contain a lot of cellulose. In addition to ruminant animals that can use some of the rumen microbes, other animals such as pigs, chickens and other monogastric animals cannot use cellulose.

References

1. Guerriero G.; et al. Novel insights from comparative in silico analysis of green microalgae cellulases. Int. J. Mol. Sci. 2018, 19 (6), 1782.
2. Liu Y.; et al. Crystal structure of a glycoside hydrolase family 6 enzyme, CcCel6C, a cellulase constitutively produced by Coprinopsis cinerea. The FEBS Journal.2010, 277 (6): 1532–42.