exoglucanases

Exoglucanases are enzymes that act on cellulose by cleaving glucose units from the ends of cellulose chains, typically releasing cellobiose (and, after further hydrolysis by other enzymes, glucose). They are processive, exo-acting enzymes, in contrast to endoglucanases that cut at internal sites to create new chain ends. Exoglucanases play a crucial role in the synergistic breakdown of lignocellulose when combined with other cellulases.

Most exoglucanases that target crystalline cellulose belong to glycoside hydrolase families GH6, GH7, and, in some cases, GH48. They are commonly referred to as cellobiohydrolases (CBHs). These enzymes often consist of a catalytic domain and a carbohydrate-binding module (CBM) that anchors the enzyme to cellulose, boosting activity on crystalline substrates. There are fungi- and bacteria-derived exoglucanases, with prominent examples from Trichoderma, Aspergillus, and various thermophilic bacteria.

In natural and industrial settings, exoglucanases function alongside endoglucanases and oxidative auxiliary enzymes, such as lytic polysaccharide monooxygenases, to deconstruct plant cell walls. They are central to enzymatic cocktails used in the conversion of biomass to fermentable sugars for biofuel production and other biorefinery processes. Practical use requires consideration of product inhibition by cellobiose, often mitigated by coupling with β-glucosidases to produce glucose. Industrial enzymes are sourced from fungi and bacteria, with ongoing improvements yielding thermostable and pH-tolerant variants.

Structural and operational aspects include secretion into the extracellular milieu, dependency on pH and temperature, and enhancement through substrate accessibility and CBM-mediated binding to cellulose.