Enzymatic Degradation Of Xylan
Due its heterogeneity and complex nature, the complete breakdown of xylan requires the action of a large variety of hydrolytic enzymes (Biely, 1985; Coughlan and Hazlewood, 1993). These enzymes can be classified into two main groups: those acting on the xylose backbone, and those cleaving the side chains. Degradation of the xylose backbone depends on xylanases, that cleave bonds within the polymer, and P-xylosidases that release xylose units from xylobiose and xylooligomers. Removal of xylan side chains is catalysed by a-L-arabinofuranosidases, a-D-glucuronidases, acetyl xylan esterases, ferulic acid esterases and p-coumaric acid esterases (Fig. 1). Xylan degradation is quite widespread among saprophytic
Figure 1. Structure of xylan and the sites of attack by xylanolytic enzymes. The backbone of xylan chains is composed of P-1,4-linked xylopyranose residues. This backbone can be variously substituted by side chains of arabinosyl, glucuronosyl, methylglucuronosyl, acetyl, feruloyl and p-coumaroyl residues. Hydrolysis of the xylan backbone is carried out by xylanases that hydrolyse internal linkages in xylan, and P-xylosidases that release xylose units from xylobiose and xylooligomers, while removal of xylan side chains is catalysed by a-L-arabinofuranosidases, a-D-glucuronidases, acetyl xylan esterases, ferulic acid esterases and p-coumaric acid esterases
Figure 1. Structure of xylan and the sites of attack by xylanolytic enzymes. The backbone of xylan chains is composed of P-1,4-linked xylopyranose residues. This backbone can be variously substituted by side chains of arabinosyl, glucuronosyl, methylglucuronosyl, acetyl, feruloyl and p-coumaroyl residues. Hydrolysis of the xylan backbone is carried out by xylanases that hydrolyse internal linkages in xylan, and P-xylosidases that release xylose units from xylobiose and xylooligomers, while removal of xylan side chains is catalysed by a-L-arabinofuranosidases, a-D-glucuronidases, acetyl xylan esterases, ferulic acid esterases and p-coumaric acid esterases micro-organisms, including bacteria and fungi, as well as in the rumen micro-biota, that possess complete xylanolytic enzyme systems (Biely, 1985; Sunna and Antranikian, 1997; Krause et al., 2003). Synergism between xylan degrading enzymes has been extensively studied and found to frequently occur between xylanases and side chain cleaving enzymes, between xylanases and ^-xylosidases, and also between different xylanases (Coughlan et al., 1993; de Vries et al., 2000). In this way, xylan degradation can proceed despite that the access of xylanases to their targets in the xylan backbone may be obstructed by side chain substituents and that these substituents may be more readily released from xylan fragments than from the polymeric substrate.
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