Hemicellulose Biodegradation

Hemicellulases are mostly glycoside hydrolases (GHs), and a few belongs to carbohydrate esterases family that hydrolyze ester linkages present in acetate or ferulic acid side groups (Bourne and Henrissat 2001; Shallom and Shoham 2003). They are produced mainly from fungi and to a limited extent from bacteria and plants. Hemicellulases are part of 20 different GH families (1, 2, 3, 4, 5, 8, 10, 11, 26, 27, 36, 39, 43, 51, 52, 53, 54, 57, 62, and 67), and all of them except for four (families 4, 8, 52, and 57) have been found in fungi. All but one (family 7) of the seven different carbohydrate esterase families (1, 2, 3, 4, 5, 6, and 7) have been found in fungi (Cantarel et al. 2009). The two main groups of fungi responsible for hemicellulose degradation are basidiomycetous white-rot and brown-rot fungi. The main difference between these fungi is that unlike white-rot fungi, brown-rot fungi generate hydroxyl free radical (H2O2 + Fe2+ + H+ H2O + Fe3+ + »OH), which is one of the small oxidants that contributes to polysaccharide depolymerization (Illman et al. 1988). Aerobic fungi such as Trichoderma and Aspergillus secrete a wide variety of hemicellulases in high concentrations (8 and 12 hemicellulases, respectively) and they work synergistically (Shallom and Shoham 2003). Currently, commercial hemicellulases are produced from genetically manipulated Trichoderma and

Aspergillus fungal strains. The most favorable temperature for hemicellulases is below 70 °C and pH between 4 and 6 except some xylanases and mannanases which are produced from thermophilic bacteria and/or fungi (Ellis and Magnuson 2012; Lu et al. 2013).

Xylan is the most abundant hemicellulose found in nature and the main enzymes needed for depolymerization of xylan are called xylanases. Other accessory enzymes which help in xylan degradation by xylanase are p-xylosidases, arabinofuranosi- dases (Kormelink et al. 1993; Sprensen et al. 2007), acetylxylan esterases (Poutanen et al. 1990; Kormelink et al. 1993), ferulic acid esterases (Faulds and Williamson 1995; de Vries et al. 2002), and a-glucuronidases (de Vries et al. 1998; Biely et al. 2000; Golan et al. 2004). The structure of xylan is very complex and varies with the biomass (Allerdings et al. 2006). Some xylanases are produced from thermophilic fungus such as T. lanuginosus (Singh et al. 2003) while most commercial cellulolytic and xylanolytic enzymes are produced by fungi growing. T. lanuginosus has been shown to produce high levels of xylanase but does not produce cellulosedegrading enzymes (GroGwindhager et al. 1999). Xylanase production is dependent upon an inducer and demands a specific media composition (Shallom and Shoham 2003). Xylan is the best inducer of xylanase production.

The two main glycosyl hydrolases (GHs) involved in the depolymerization of the hemicellulose backbone are (1) endo-1,4-p-D-xylanase and (2) endo-1,4-p-D- mannanase (Suurnakki et al. 1997). Due to complexity of hemicelluloses, other accessory enzymes such as p-xylanase, p-xylosidase, and enzymes such as a-L- arabinofuranosidase, a-glucuronidase, acetylxylan esterase, and hydroxycinnamic acid esterases (cleave side chain residues from the xylan backbone) are required for complete hydrolysis. All of these enzymes work synergistically in xylan degradation (Sunna and Antranikian 1997). Xylanases randomly attack on the backbone of xylan, producing both substituted and non-substituted oligomers, xylobiose (Eriksson et al. 1990). Xylosidases further hydrolyze short chain xylooligosaccha- rides to xylose (Poutanen and Puls 1988). Other accessory enzymes such as arabi- nosidase, a-glucuronidase, and acetylxylan esterase act together with xylanases and xylosidases to release the constituents from the xylan backbone and achieve a complete hydrolysis of xylan to monosaccharides (Eriksson et al. 1990).

Three major enzymes involved in the hydrolysis of linear mannans and gluco- mannans are (1) p-mannanases (1,4-p-D-mannan mannohydrolases, EC 3.2.1.78), which are endo-acting hydrolases and attack the internal glycosidic bonds of the mannan backbone to release short p-1,4-manno-oligosaccharides; (2) p-manno- sidases (1,4-p-D-mannopyranoside hydrolases, EC 3.2.1.25), which are exo-acting hydrolases and act on terminal, nonreducing manno-oligosaccharides and mannobi- ose to release monomers; and (3) p-glucosidases (1,4-p-D-glucoside glucohydro- lases, EC 3.2.1.21), which attack on nonreducing end of glucomanno-oligomer derived from the degradation of glucomannan to release 1,4-glucopyranose units (Dhawan and Kaur 2007; Moreira and Filho 2008).

Accessory enzymes are required to remove side groups from galactomannan, glucomannan, and galactoglucomannan such as a-galactosidase (1,6-a-D-galactoside galactohydrolase) and acetyl mannan esterase. a-Galactosidases catalyzes the hydrolysis of terminal, nonreducing a-D-galactosides from galacto-oligosaccharides, galactomannan, and galactoglucomannan to remove a-1,6-linked galactosyl units from polymeric galactomannan (Viikari et al. 1993), whereas acetyl mannan esterases remove the acetyl groups from galactoglucomannan (Shallom and Shoham 2003). The pattern and extent of the substitution have inhibitory effect on exo- and endo-acting enzymes and greatly affect the rate of hydrolysis of mannan chain. The synergy between endo- and exo-acting hydrolases is required for complete hydrolysis of mannan. It has been reported that both hetero-synergy (between a main chain and a side chain-degrading enzyme) and homo-synergy (between two main chains or two side chain-degrading enzymes) are involved in mannan degradation (Shallom and Shoham 2003; Moreira and Filho 2008).

Mannanases are mainly produced from basidiomycetes especially Sclerotium rolfsii which have been shown to secrete high concentrations of p-mannanases (up to seven different p-mannanases) in amount exceeding that of xylanases and endo- glucanases (Haltrich et al. 1994; GroBwindhager et al. 1999). Cellulose, mannans, and manno-oligosaccharides act as the inducers for p-mannanases in S. rolfsii (Sachslehner et al. 1998). Other white-rot and brown-rot basidiomycetes, such as Pleurotus ostreatus (Baldrian et al. 2005), Trametes versicolor (Valaskova and Baldrian 2006; Baldrian 2008), Schizophyllum commune (Gubitz et al. 2000), Ceriporiopsis subvermispora (Heidorne et al. 2006), Piptoporus betulinus (Valaskova and Baldrian 2006), and Penicillium occitanis (Blibech et al. 2010), have also shown significant p-mannanase production. p-Mannanases are also produced from ascomycetes fungi commonly used for cellulases and other hemicellu- lase production such as Trichoderma and Aspergillus strains. Thermostable P-mannanases are produced from thermophilic fungi belonging to the genera, notably, Talaromyces, Thielavia, and Thermoascus thermomyces (Araujo and Ward 1990a, 1990b). A thermostable strain, Thermomyces lanuginosus (IMI 158749) was shown to produce p-mannanase activity of 30.0 U/mL with glucose as carbon source which is equivalent to the best known mannanase producer, S. rolfsii (Puchart et al.

1999). Another strain Thielavia terrestris was shown to produce four thermostable P-mannanases (Araujo and Ward 1990a, b). Mannanases are also produced from some Zygomycetes fungi such as Rhizopus niveus in small quantities as these fungi are better known for amylolytic and pectinolytic enzyme production (Henriksson et al. 1999; Kolarova and Augustin 2001).

Arabinan is another hemicelluloses sugar polymer and also one of the major constituent of plant cell walls. The arabinan consists of a linear backbone of a-1,5- linked L-arabinofuranosyl residues, with a-1,2- and/or a-1,3-linked L-arabinose side chains substitutions (McNeil et al. 1984). They occur either as homoglycans, arabi- nans, or as heteroglycans, namely, arabinoxylans and arabinogalactans. Two major enzymes are involved in arabinan hydrolysis: (1) endo-1,5-a-L-arabinanases (EC 3.2.1.99) belong to GH family 43 and are endo-acting hydrolases that cleave a-1,5- L-arabinofuranoside linkages via invert catalysis between arabinose units, and (2) a-L-arabinofuranosidases (EC 3.2.1.55) belong to glycoside hydrolase families GH3, GH43, GH51, GH54, and GH62 and display vast range of enzymatic activities. Thermostable arabinases are found only in bacteria. Perhaps the best character?ized endoarabinanases are those from Caldicellulosiruptor saccharolyticus and the previously described enzyme from B. thermodenitrificans (Takao et al. 2002; Hong et al. 2009).

Esterases are enzymes that are not classified within GH families and catalyze the hydrolysis of ester bonds. Currently there are 16 carbohydrate esterases which are grouped by structure of their catalytic domains (www.cazy.org). Mainly there are two types of carbohydrate esters: (1) acetylxylan esterases that catalyze the release of acetyl ester groups from C2 or C3 positions of D-xylopyranosyl units (Biely 2003), and (2) ferulic acid esterases that remove ferulic acid from the C2 or C5 positions of a-L-arabinofuranosyl side chains (Saulnier and Thibault 1999). Ferulic acid then esterifies to arabinofuranosyl side chains which forms a network with other ferulic acid esters via diferulolyl bridges forming interlinkages of polymers in cell wall of plants (Iiyama et al. 1994). Therefore, for complete hydrolysis of acetylated xylans, synergy between esterases and xylanolytic enzymes are required. Moreover, as a result of esterases action, phenolic acids are produced which can be potential precursor for a wide variety of high-value products (Graf 1992).

Previously it has been reported that certain fungal and bacterial metalloenzymes such as copper-containing lytic polysaccharide monooxygenases (Hemsworth et al. 2013) can degrade recalcitrant polysaccharides using an oxidative mechanism of action. These enzymes were originally classified as GH61 and CBM33 in the CAZy database, but reclassified as AA9 and AA10, respectively, as of March 2013 (www. cazy.org). These enzymes work together with polysaccharide hydrolases and other electron transfer components and significantly increase the conversion of cellulose and hemicellulose into respective oligosaccharides (Harris et al. 2010; Vaaje- Kolstad et al. 2010; Langston et al. 2011; Cannella et al. 2012; Horn et al. 2012).

 
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