Comprehensive Overview of Cellulase Mechanisms: From Glycosidic Bond Cleavage to Synergistic Enzyme Interactions

Cellulases, a type of glycosyl hydrolase (GH), play a crucial role in cellulolysis by breaking down glycosidic bonds in cellulose. The cleavage of β-1,4-glycosidic bonds in cellulose through acid hydrolysis involves a proton donor, a nucleophile or base, and can result in inversion or retention of the anomeric configuration at the carbon-1 (C-1) of the substrate-reducing end. In macromolecular complexes like cellulosomes, cellulolytic enzymes work synergistically.

The synergistic effects of cellulolytic enzyme combinations, whether free in solution or in an artificial cellulosome, are challenging to predict. Studies on fungi suggest that efficient bioconversion of polysaccharides into fermentable sugars requires interactions among three enzyme types: endoglucanases, cellobiohydrolase, and β-glucosidases. These enzymes, often glycoproteins with isoenzymes, are functionally categorized into carbohydrases, oxidative cellulases, and phosphorylases.

Carbohydrases, including endoglucanases (EGs), exoglucanases (EXs), and β-glucosidases (βGs), hydrolyze β-1,4-glycosidic bonds in cellulose. EGs, belonging to various GH families, can hydrolyze cellulose at amorphous and crystalline regions, generating new chain ends for cellobiohydrolase. EXs, grouped into cellodextrinases and cellobiohydrolases, cleave glycosidic linkages in cello-oligosaccharides. βGs hydrolyze bonds at nonreducing ends of cellooligosaccharides.

Oxidoreductive cellulases, like cellobiose dehydrogenase (CDH), participate in cellulose degradation through oxidative mechanisms. CDH reduces cellobiose inhibition, generates hydroxyl radicals, and may transfer electrons to polysaccharide monooxygenases (PMOs). Other enzymes in oxidoreductive systems include lactonase, glucose oxidase, and PMOs.

PMOs are copper-dependent enzymes that cleave glycosidic bonds in cellulose structures. They require molecular oxygen and electron donors like CDH. PMOs are categorized into Type 1, Type 2, and Type 3 based on substrate specificity.

Phosphorylases reduce cellobiose and cellodextrins to glucose using phosphates instead of water. This phosphorolytic process, while not directly part of the cellulolytic pathway, can accelerate cellulose degradation by removing inhibitory products and conserving energy.

In summary, cellulases, categorized into carbohydrases, oxidative cellulases, and phosphorylases, work synergistically to break down cellulose into fermentable sugars through complex mechanisms involving glycosidic bond cleavage, oxidation-reduction reactions, and phosphorolysis.

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