Soil organic matter consisting of large proportion of plant biomass comprise of hemicellulose, cellulose, lignin and pectin. Primary and secondary cell wall contain hemicellulose that strengthen the cell wall along with cellulose and lignin. Hemicellulose with pectin and lignin, form cell wall matrix and cross-link with cellulose microfibril through hydrogen bond (Brett & Waldron 1996). Hemicellulose a polysaccharide have β (1-4) linked backbone (Scheller & Ulvskov 2010) containing xylan, xyloglucan, and mannan. Cellulose, hemicellulose and lignin are bonded by non-covalent forces and covalent cross-links. Hemicellulose include:
The plant’s primary and secondary cell wall contain xylan in abundance. The linear xylan is involved in cross-linking of cellulose microfibril (Awano et al., 2001) and lignin (Hatfield et al., 1999). Xylan hemicellulose is the glucan with a backbone of 1, 4-β-D-xylopyranose and branch chain of 4-oxymethyl-glucuronic acid. Xylan composed of β-1, 4-xylosyl chain are substituted to varying degree.
Mannan are present in certain secondary cell wall. Mannan containing polysaccharide include mannan, galactomannan, glucomannan, glucuronic acid mannan etc.. Mannose residue are connected by a β (1-4) bond to form mannan, they form galactomannan if linked to galactose residue by α-(1-6) bond. The glucomannan backbone is made up of mannose and glucose linked by β (1-4) bond and when glucomannan contains one galactose residue as a branch chain then it is called galactoglucomannan (Chen 2014). Glucomannan are abundant in secondary cell wall.
Xyloglucan is present in primary cell wall and contains cellulosic backbone having β-(1-4) linked D-glucopyranose (Chen 2014).
Soil microorganisms (bacteria and fungi) can degrade these organic molecule by hydrolytic or oxidative enzymes. In contrast to cellulose which is crystalline and resistant to hydrolysis hemicellulose undergo hydrolysis easily, they are random amorphous structure with little strength.
Enzyme hemicellulase degrade hemicellulose such as xylan, xyloglucans, and glucomannan from plant biomass. Hemicellulases are classified according to the substrate they act upon, the bond they cleave and pattern of product formation. Xylan being the major component of hemicellulose consists of β-1, 4-linked D xylopyranosyl residue. The hydrolysis of xylan is achieved by a mixture of hydrolytic enzymes including endo β-1, 4-xylanase and β-D-xylosidase (Polizeli et al., 2005).
Endo-1, 4-xylanase produces oligosaccharide from arbitrary cleavage of xylan and xylan 1, 4-xylosidase acts on xylan oligosaccharide producing xylose (Jeffries 1994). Nine different enzymes are required to completely degrade hemicellulose.
S.No. EC Number Enzyme
- EC 126.96.36.199 Endo-1, 4-β-xylanase
- EC 188.8.131.52 Xylan 1, 4-β-xylosidase
- EC 184.108.40.206 Xylan endo-1, 3-β xylosidase
- EC 220.127.116.11 Xylan 1, 3-β xylosidase
- EC 18.104.22.168 ∝-L-Arabinofuranosidase
- EC 22.214.171.124 Arabinan endo-1, 5-∝-L-arabinosidase
- EC 126.96.36.199 Mannan endo-1, 4-β-mannosidase
- EC 188.8.131.52 Mannan 1, 4-β-mannobiosidase
- EC 184.108.40.206 Mannan endo-1, 6-β-mannosidase
Soil microorganisms releasing enzymes disintegrate the hemicellulose use it as an energy source and besides this makes recycling of the nutrient in the environment essential.
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