Page 57 - Advances in bioenergy (2016)

are stabilized by inter- and intramolecular hydrogen bonds and van der Waals interactions
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between glucose residues contributing to its recalcitrance. The bond network results in a
mostly uniform arrangement of fibers forming crystalline cellulose that lacks enzyme-
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accessible surface morphologies, further enhancing resistance to hydrolysis. Crystallinity
varies widely among plant cell wall from approximately 40–50% in plant cellulose to 65–80%
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in bacterial and algal cellulose. Glucan chains occur in hexagonal arrays of 36 (3 nm × 5 nm
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width) with exceptionally high degrees of polymerization (DPs). Although cellulose is
mainly present as crystalline fibers that are highly resistant to hydrolysis, its content in biomass
is typically larger compared to hemicellulose and, as a result, cellulases are the key enzymes
for bioethanol production. 3

Cellulose Degrading Enzymes

Cellulases have been widely applied in industry in different sectors, such as in the textile
industry for cotton softening and denim finishing, in the detergent market for color care,
cleaning, and antideposition, in the food industry for mashing and in the pulp and paper
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industries for deinking, improvement, and fiber modification. The cellulase market is
expected to expand dramatically for the hydrolysis of pretreated cellulosic materials into
sugars, which can be fermented to produce commodities such as bioethanol and other
bioproducts on a large scale. However, the high cost of fuel ethanol produced from
lignocellulosic biomass, is heavily depended on the cost of cellulases that needs to be
significantly reduced for their commercial use in biorefineries. 17

Box 2.1 Functional and Structural Diversity of Cellulases

Enzymes that modify complex carbohydrates, such as cellulases, together with their
accessory noncatalytic CBMs, have been grouped into sequence-based families on the
continuously updated CAZy database (http://www.cazy.org/). At the moment, CAZy
database describes around 130 GH families, whereas around 16 of them are populated by
many different cellulolytic enzymes. From a structural point of view, cellulases belong to
at least eight unrelated protein folds that are diffentiated into even more GH families,

underpinning that these enzymes have evolved convergently from diverse ancestors.

Classification and Structure of Cellulases

Enzymes that modify complex carbohydrates, such as cellulases, together with their accessory
noncatalytic carbohydrate binding modules (CBMs), have been grouped into sequence-based
families on the continuously updated Carbohydrate-Active EnZymes (CAZy) database
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(http://www.cazy.org/). Cellulolytic enzymes are classified into different glycoside
hydrolase (GH) families (Box 2.1). Starting with β-1,4-endoglucanases (EC 3.2.1.4), these
enzymes catalyze the endohydrolysis of (1,4)-β-D-glucosidic bonds in cellulose, which are
members of GH families 5–9, 12, 44, 45, 48, 51, 74, and 124. Cellobiohydrolases (EC
3.2.1.91) that are known to catalyze the release of cellobiose from either the non-reducing end