Enzymatic Hydrolysis of Lignocellulose
Enzymatic Hydrolysis of Cellulose
As defined by IUBMB (International Union of Biochemistry and Molecular Biology), glycoside hydrolases (GHs) are enzymes which hydrolyze glycosidic interlinkages between carbohydrate molecule and carbohydrate and noncarbohydrate moieties. Cellulase is the subset of GH that cleaves glycosidic-p-1-4 bonds present in the cellulose (Fig. 2.2) (Garvey et al. 2013). Cellulase comprises of three major classes involved in the hydrolysis: (1) endoglucanase (EG, endo-1,4-D- glucanohydrolase, or EC 18.104.22.168), which attacks regions of low crystallinity in the cellulose, creating free chain ends; (2) exoglucanase including 1,4-p-D-glucan glu- canohydrolases (also known as cellodextrinases) (EC 22.214.171.124) and 1,4-p-D-glucan cellobiohydrolases (cellobiohydrolases) (EC 126.96.36.199), which removes cellobiose units from the free chain ends; and (3) p-glucosidase or p-glucoside glucohydrolases (EC 188.8.131.52), which hydrolyses cellobiose to glucose (Henrissat et al. 1998; Chandel et al. 2012). There are 132 GH families available and are organized into 14 clans (http://www.cazy.org). Cellulolytic enzymes are distributed among different GH families, for example, endoglucanases (EC: 184.108.40.206) belongs to families 5, 9, and 12, exoglucanases (EC: 220.127.116.11/176) in families 6 and 7, and p-glucosidases (EC: 18.104.22.168) belongs to families 1 and 3 (Garvey et al. 2013).
Cellulolytic system has a modular structure and can be present in two forms: (1) as individual enzymes that consist of a catalytic domain intended for hydrolysis and carbohydrate-binding modules (CBMs) for substrate recognition and binding of the enzymes to the substrate and (2) as multienzymatic complexes called cellulosome, which is a combination of many tethered cellulolytic enzymes that are interrelated and augmented with each other (Gefen et al. 2012). The structure of cellulosomes consists of scaffolding backbone with several cohesion domains that have capability for interaction with dockering domain-carrying enzyme. Several cellulosomes are joined together to form giant cellulase and other GH enzymes and get attached to the cell surface (Mazzoli et al. 2012). Close proximity of enzymes increases the synergy between enzymes and product of one enzyme becomes substrate for another and thus cellulose degradation becomes fast and efficient with a minimum of feedback products (Hyeon et al. 2011).
On interaction with substrate, cellulosome gets tagged with one or more CBMs on the cellulose and sometimes these cellulosomes modify their composition and activity to resemble the substrate (Elkins et al. 2010). Cellulosomes play important role in cellulose degradation and mediate the recycling of fixed carbon via photosynthesis in nature and are found in anaerobic cellulolytic bacteria such as Clostridium spp. (Ding et al. 2012).