Bajpai Xylanolytic Enzymes

1. Auflage 2014
ISBN: 978-0-12-801066-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 120 Seiten

Xylanolytic Enzymes describes the enzyme structure and its interaction with plant cell walls, the properties and production of different enzymes and their application, and the knowledge gathered on the hydrolysis mechanism of hemicellulose. The knowledge gathered about the hydrolysis mechanism of the hemicelluloses, especially xylans, has greatly promoted the rapid application of these enzymes in new areas. Recently there has been much industrial interest in xylan and its hydrolytic enzymatic complex, as a supplement and for the manufacturing of food, drinks, textiles, pulps and paper, and ethanol; and in xylitol production as a fermentation substrate for the production of enzymes. This book describes xylan as a major component of plant hemicelluloses. - Presents a thorough overview of all aspects of xylanolytic enzymes - Gives up-to-date authoritative information and cites pertinent research - Includes studies on xylanase regulation and synergistic action between multiple forms of xylanase

Dr. Pratima Bajpai is currently working as a Consultant in the field of Paper and Pulp. She has over 36 years of experience in research at the National Sugar Institute, University of Saskatchewan, the Universitiy of Western Ontario, in Canada, in addition to the Thapar Research and Industrial Development Centre, in India. She also worked as a visiting professor at the University of Waterloo, Canada and as a visiting researcher at Kyushu University, Fukuoka, Japan. She has been named among the World's Top 2% Scientists by Stanford University in the list published in October 2022. This is the third consecutive year that she has made it into the prestigious list. Dr. Bajpai's main areas of expertise are industrial biotechnology, pulp and paper, and environmental biotechnology. She has contributed immensely to the field of industrial biotechnology and is a recognized expert in the field. Dr. Bajpai has written several advanced level technical books on environmental and biotechnological aspects of pulp and paper which have been published by leading publishers in the USA and Europe. She has also contributed chapters to a number of books and encyclopedia, obtained 11 patents, written several technical reports, and has implemented several processes in Indian Paper mills. Dr. Bajpai is an active member of the American Society of Microbiologists and is a reviewer of many international research journals.

Autoren/Hrsg.

Bajpai, Pratima

Leseproben

Chapter 2 Xylan
Occurrence and Structure
The substrate of xylanases, xylan, is a major structural polysaccharide in plant cells. It is found in the cell walls of land plants, in which they may constitute more than 30% of the dry weight. Xylan structure is variable, ranging from linear 1,4-ß-linked polyxylose sugars other than D-xylose. The main chain of xylan is analogous to that of cellulose but composed of D-xylose instead of D-glucose. Branches consist of L-arabinofuranose linked to the 0–3 positions of D-xylose residues and of D-glucuronic acid or 4-O-methyl-D-glucuronic acid linked to the 0–2 position. Both side-chain sugars are linked a-glycosidically. The degree of branching varies depending on the source. Xylans of several wood species, particularly of hardwoods, are acetylated. The main heteropolymers of the hemicellulosic component are xylan, mannan, galactan, and arabinan. Xylan molecules are mainly constituted by D-xylose as the monomeric unit, and traces of L-arabinose are also present. Further, some substituents (i.e., acetyl, arabinosyl, and glucuronysyl) are found on the backbone of xylan. A relationship between the chemical structure of xylans and their botanical origins and cytological localizations is found which results in a certain degree of complexity of xylan-containing materials that may have several different xylan polymers of related structures but differ by more or less important features. Keywords
4-O-methyl-D-glucuronic acid; arabinan; cellulose; D-glucuronic acid; galactan; hardwood; hemicellulose; mannan; polysaccharide; softwood; xylan structure; xylose The substrate of xylanases, xylan, is a major structural polysaccharide in plant cells. It is found in the cell walls of land plants, in which it may constitute more than 30% of the dry weight. Aside from terrestrial plants, in which xylans are based on a ß-l,4-linked D-xylosyl backbone, marine algae also synthesize xylans of different chemical structure based on a ß-1,3-linked D-xylosyl backbone. In some species of Chlorophyceae and the Rhodophyceae in which cellulose is absent, xylans form a highly crystalline fibrillar material. Xylan structure is variable, ranging from linear 1,4-ß-linked polyxylose sugars to highly branched heteropolysaccharides. Some major structural features are summarized in Figure 2.1. The main chain of xylan is analogous to that of cellulose, but composed of D-xylose instead of D-glucose. Branches consist of L-arabinofuranose linked to the 0–3 positions of D-xylose residues and of D-glucuronic acid or 4-O-methyl-D-glucuronic acid linked to the 0–2 position. Both side-chain sugars are linked a-glycosidically. The degree of branching varies depending on the source. Xylans of several wood species—particularly of hardwoods—are acetylated. For example, birch xylan contain >1 mol of acetic acid per 2 mol of D-xylose. Acetylation occurs more frequently at the 0–3 than the 0–2 position, and double acetylation of a D-xylose unit has also been reported. The main structural elements commonly found in land plant cell wall xylans are shown in Table 2.1. Figure 2.1 A hypothetical plant xylan and the sites of its attack by microbial xylanolytic enzymes (Reproduced from Biely (1985), copyright (1985), with permission from Elsevier Science.) Table 2.1 Main Structural Elements Commonly found in Land Plant Cell Wall Xylans Structural Type Source Linear homoxylan Esparto grass
Tobacco stalk Arabinoxylan
 Low branching
 High branching
 Complex side chain Common barberry monocots
Primary walls
Flours
Gramineae pericarp Glucuronoxylan Soybean hull
Hardwood
Gramineae
Legumes Glucuronoarabinoxylan Softwoods
Gramineae
Dicot primary walls Based on Chanda et al., 1950; Timell, 1965; Eda et al., 1976; Wilkie, 1979; Bajpai, 1997 Hemicellulosic material constitutes around 30–35% of hardwood, 15–30% of graminaceous plants, and 7–12% of gymnosperms (Whistler and Richards, 1970; Wong et al., 1988; Georis et al., 2000; Bajpai, 1997, 2009). Schulze (1981) first used the term “hemicellulose” for the fractions that he collected from plant material isolated with diluted alkali. Plant biomass, in terms of dry weight, comprises three major polymeric constituents: (i) cellulose, an insoluble polymer composed of ß-D-glucopyranosyl residues linked by ß-1,4-glycosidic bonds; (ii) hemicellulose, a series of heteropolysaccharides that includes xylans, glucans, mannans, and arabinans; and (iii) lignin, a complex polyphenol, intimately interconnected with the hemicelluloses, forming a matrix that surrounds the orderly cellulose microfibrils. In wood, lignin in high concentration is the glue that binds contiguous cells. As a whole, biomass comprises on average 23% lignin, 40% cellulose, and 33% hemicellulose by dry weight. It does not accumulate in nature, but undergo microbial degradation as part of the carbon cycle (Biely, 1993). The main heteropolymers of the hemicellulosic component are xylan, mannan, galactans, and arabinans. D-xylose, D-mannose, D-galactose, and L-arabinose are examples of sugar moieties that are commonly attached with the heteropolymers and on the basis of which these heteropolymers are classified. Xylan molecules are mainly constituted by D-xylose as the monomeric unit, and traces of L-arabinose are also present (Bastawde, 1992). Further, some substituents, such as acetyl, arabinosyl, and glucuronysyl, are found on the backbone of xylan (Whistler and Richard, 1970). Xylan occupies the central position inbetween the sheath of lignin residues and also covalently linked and intertwined with this sheath at several points. Covalent linkage of xylan with lignin sheath and interwinedness through inter H-bonding gives an appearance of a “coat” around the cellulosic monomers (Biely, 1985; Joseleau et al., 1992). Intra-chain H-bonding occurs through the 0–3 position, giving unsubstituted xylan a helical twist. These cellulose monomers will act as a barrier against the hydrolyzing action of cellulose enzyme (Uffen, 1997). The xylan in hardwood, which may account for 10–35% of the total dry weight, is acetyl-4-O-methylglucuronoxylan with a degree of polymerization (DP) between 150 and 200. Approximately one in ten of the ß-D-xylopyranose backbone units is substituted at C-2 with a 1,2-linked 4-O-methyl-ß-Dglucuronic acid residue, while 70% is acetylated at C-2, C-3, or both. In fact, birch xylan contains more than 1 mol of acetic acid per 2 mols of xylose. The presence of these acetyl groups is responsible for the partial solubility of xylan in water. These groups are readily removed when xylan is subjected to alkali extraction. Softwoods contain 10–15% xylan as arabino-4-O-methylglucuronoxylan with a DP less than that of hardwood xylans, between 70 and 130. This material is not acetylated and contains ß-D-xylopyranose, 4-O-methyl-ß-Dglucuronic acid and L-arabinofuranose in a ratio of 100:20:13. The 4-O-methylglucuronose residues are attached to C-2 and the L-arabinofuranose residues linked by a-1,3 glycosidic bonds to the C-3 of the relevant xylopyranose backbone units. The arabinoyl constituents occur on almost 12% of the xylosyl residues. The ester-linked components (acetyl, feruloyl, and p-coumaroyl residues) may be lost from substrates prepared by solubilization in alkali. In native lignocellulolytic material, some or all of the feruloyl substituents may be involved in covalent cross-linking with other polysaccharides. However, additional natural functions of these phenolic side groups have been suggested, for example, regulation of cell-wall extension, stabilization, and defense against invading plant pathogens (Tenkanen and Poutanen, 1992). Homoxylans consisting exclusively of xylosyl residues are not widespread in nature; they have been isolated from a few sources like esparto grass, tobacco stalks, and guar seed husk (Sunna and Antranikian, 1997). However, on the basis of the nature of the substituents, a broad distinction may therefore be made among xylans in which complexity increases from linear to highly substituted xylans. There are four main families of xylan: Arabinoxylans, Glucuronoxylans, Glucuronoarabinoxylans, and Galactoglucuronoarabinoxylans (Voragen et al., 1992). Arabinoxylans have been identified in wheat, rye, barley, oat, rice, and sorghum, as well as in some other plants: pangola grass, bamboo shoots, and rye grass. Although these polysaccharides are minor components of entire cereal grains, they constitute an important part of plant cell walls (Izydorczyk and Biliaderis, 1995). Glucuronoxylans and glucuronoarabinoxylans are located mainly in the secondary wall and function as an adhesive by forming covalent and noncovalent bonds with lignin, cellulose, and other polymers essential to the integrity of the cell wall. Xylans are the principal class of hemicelluloses in angiosperms contributing 15–30% of the total dry weight, but are less abundant in gymnosperms, which contain 7–12% xylans (Haltrich et al., 1996). Glucuronoxylans are linear polymers of ß-D-xylopyranosyl units linked by...