Cellulose makes up a significant portion of plant organic matter, accounting for 25-30% of herbaceous plants, 40-50% of mature wood, and 98% of cotton. Cellulose is a linear polymer composed of ß-D-glucose units linked by a ß-1,4-glucosidic bond. Adjacent glucose molecules are 180-oriented, the repeating unit being cellobiose. Hydrogen bonding helps maintain the brittle, rigid structure and insolubility of the cellulose molecule at all glycosidic bonds. If the glycosidic bond alone is cleaved, the hydrogen bridges maintain a spatial structure that is sufficient to restore the glycosidic bond. Adjacent molecules are linked by hydrogen bonds and van der Waal's forces, and as a result, their parallel rows form the crystalline structure of the cellulosic fibers. As a significant portion of cellulose, as a renewable energy source, appears as industrial, urban, and agricultural waste, it is important to understand the physical, chemical, and ecological factors that affect its degradation. It follows from the structure of cellulose and its role in nature that it is a highly resistant compound. In its natural state (plant cell wall), its hydrolysis is influenced by a number of factors, in particular the close relationship with other cell wall constituents.The enzymatic degradation of cellulose is a very difficult task due to the physical nature of the substrate. In the cell wall, the crystalline cellulose fibers are held together by hydrogen bonds and chemically linked by hemicellulose and lignin. Nevertheless, the breakdown of cellulose and ß-D-glucan is widespread among both fungi and bacteria. The enzymes involved in the breakdown of cellulose can be divided into two major groups: endoglucanases, which hydrolyze intramolecular bonds, and exoglucanases, which cleave glucose or cellobiose from the end of the molecule, thus shortening the polymer molecule.
The major enzymes that break down cellulose and ß-D-glucan
Enzymes EC number Reaction Cellulase. Endo-1,4-ß-glucanase EC 3.2.1.4 It hydrolyzes 1,4-ß-D-glucosidic bonds in the cellulose molecule (it also hydrolyzes similar bonds in ß-D-glucan). Endo-1,3-(1,4)-ß-glucanase. Laminarinase. EC 3.2.1.6 It hydrolyzes 1,3-ß-D- and 1,4-ß-D-glucosidic bonds in ß-D-glucan when the 3rd carbon atom of the reducing end β-D-glucose is substituted. (The enzyme is also a substrate for laminarin, lichenin, and grain ß-D-glucan). Glucan endo-1,3-ß-D-glucosidase. Endo-1,3-ß-glucanase. EC 3.2.1.39 It hydrolyzes 1,3-ß-D-glucosidic bonds in ß-D-glucan but is not a substrate for 1,3- -1,4-ß-D-glucan and thus differs from EC 3.2.1.6. Glucan 1,3-ß-glucosidase. Exo-1,3-ß-glucosidase. EC 3.2.1.58 It cleaves glucose by hydrolyzing 1,3-ß-D bonds from the non-reducing end of ß-D-glucan. Glucan endo-1,2-ß-glucosidase. Endo-1,2-ß-glucanase. EC 3.2.1.71 Hydrolyzes 1,2-ß-D-glucosidic bonds in ß-D-glucan. Licheninase. Endo-1,3-1,4-ß-glucanase. EC 3.2.1.73 Hydrolyzes 1,3-ß-D- and 1,4-ß-D-glucosidic bonds in ß-D-glucan when both types of bonds are present. (The enzyme substrate is lichenin and grain ß-D-glucan, but does not break down glucan containing only 1,3- or only 1,4-ß-D bonds). Glucan 1,4-ß-glucosidase. Exo-1,4-ß-glucosidase. EC 3.2.1.74 It cleaves glucose units by hydrolyzing the 1,4-ß-D bonds from the non-reducing end of cellulose (ß-D-glucan). It breaks down cellobiose very slowly. Glucan endo-1,6-ß-glucosidase. Endo-1,6-ß-glucanase. EC 3.2.1.75 Hydrolyzes 1,6-ß-D-glucosidic bonds in ß-D-glucan. Cellulose 1,4-ß-cellobiosidase. Exocellobiohydrolase. EC 3.2.1.91 It hydrolyzes cellobiose units from the non-reducing end of cellulose (ß-D-glucan).
Among hemicelluloses, xylans are composed of ß-D-xylose primarily by the formation of 1,4-ß-D-xylosidic bonds, and less frequently by ß-D-xylose branches linked by 1,3-ß-D-xylosidic bonds. Pure ß-1.3 binding was found only in marine algae. Xylan makes up 10-35% of hardwood and 10-15% of softwood. Xylan is treated with the glucuronic acid or with methylglucuronic acid as xyloglucan, alpha-L-arabinofuranose linked by alpha-1.3 and alpha-1.5 bonds as arabinoxylan (mainly in softwoods and grasses) is involved in the structure of the primary cell wall. In xylans, the second hydroxyl group of xylose is 7070% esterified with acetyl.
Major enzymes that break down xylan and its derivatives
Enzymes EC number Reaction Endo-1,4-ß-xylanase. 1,4-ß-D-xylan xylanohydrolase. EC 3.2.1.8 It hydrolyzes the 1,4-ß-D-xylosidic bonds of 1,4-ß-D-xylan. Xylan endo-1,3-ß-xylosidase. Endo-1,3-ß-xylanase. EC 3.2.1.32 1,3-ß-D-xylan hydrolyzes 1,3-ß-D-xylosidic bonds. Xylan 1,4-ß-xylosidase. Exo-1,4-ß-xylosidase. EC 3.2.1.37 1,4-β-D-xylan hydrolyzes D-xylose from the non-reducing end. alfa-N-arabinofuranosidase. Alfa-L-arabinofuranosidase. EC 3.2.1.55 The -L-arabinofuranoside ends attached to the (1,3) and / or (1,5) bonds are hydrolyzed in arabinoxylan and arabinogalactan. Xylan 1,3-ß-xylosidase. Exo-1,3-ß-xylosidase. EC 3.2.1.72 1,3-β-D-xylan hydrolyzes D-xylose from the non-reducing end. Acetylesterase. C-esterase EC 3.1.1.6 Acetyl ester + H2O = alcohol + acetic acid.
The other important hemicellulose in the structure of mannan is 1,4-ß-D-mannan, which is composed mainly of ß-D-mannose units with 1,4-ß-D-mannoside bonds. Rarely, mannose can also be attached to mannan via a 1,6-ß-D-mannoside bond. Approx. Glucose, which is called glucomannan and is found primarily in hardwood, can also be bound by 25% ß-1,4 binding. It forms galactomannan with a ß-1,4 bond to galactose. Xylose and arabinose ß-1,6 resp. may also be attached to mannan via alpha-1,3 bonds.
The major enzymes that break down mannan
Enzim neve EC szám Katalizált reakció Mannan endo-1,4-ß-mannosidase. Endo-1,4-mannanase. EC 3.2.1.78 Hydrolyzes 1,4-ß-D-mannoside bonds in mannan, galactomannan, and glucomannan. Mannan 1,4-ß-mannobiosidase. Exo-1,4-ß-mannobiohydrolase. EC 3.2.1.100 It hydrolyzes 1,4-ß-D-mannoside bonds in 1,4-ß-D-mannan by cleaving mannobiose units from the non-reducing end. Mannan endo-1,6-ß-mannosidase. Endo-1,6-ß-mannanase. EC 3.2.1.101 Hydrolyzes 1,6-ß-D-mannoside bonds in 1,6-ß-D-mannan.
Among the hemicelluloses, pectin should also be mentioned. The two main constituents of pectin are polygalacturonic acid and rhamnogalacturonane. Polygalacturonic acid is a helical homopolymer composed of alpha-D-galacturonic acid with alpha-1.4 bonds. The antiparallel lysates are stabilized by Ca2+ ions. Polygalacturonic acid is approx. It forms a helical moiety containing 200 galacturonic acid units. Rhamnogalacturonan is a rod-shaped heteropolymer of 1,2-β-L-rhamnosyl-1,4-β-D-galacturonic acid disaccharide units. Rhamnogalacturonane occasionally breaks the long chain of polygalacturonic acid. For the 4-OH group of the rhamnosyl moieties, arabinan, galactan and arabinogalactan.
Major enzymes that break down pectin and its derivatives
Enzymes EC number Reaction Pectinesterase. Pectin methylesterase. EC 3.1.1.11 It catalyzes the following reaction: pectin + n*H2O = n*methanol + pectic acid. Polygalacturonase. Pectinase. EC 3.2.1.15 It hydrolyzes 1,4-alpha-D-galacturonic acid bonds in polygalacturonic acid and other galacturonanes. Galacturan 1,4-alfa-galacturonidase. Exopolygalacturonase. EC 3.2.1.67 It is catalyzed by n*(1,4-alpha-D-galacturonic acid) + H2O = (n-1)*(1,4-alpha-D-galacturonide) + D-galacturonic acid. Exo-poly-alfa-galacturonosidase. EC 3.2.1.82 It hydrolyzes polygalacturonic acid by cleaving digalacturonic acid units from the non-reducing end. Pectate lyase. Pectate transeliminase. EC 4.2.2.2 It cleaves pectic acid from the non-reducing end by an elimination reaction. Pectin is not a substrate for the enzyme. Pectin lyase. EC 4.2.2.10 It cleaves pectin by an elimination reaction. Deesterified pectin is not a substrate for the enzyme.
Lignin (Latin lignum = wood) is a component of the woody tissues of plants. Three cinnamon alcohol derivatives, p-coumaric alcohol, coniferyl alcohol and synapyl alcohol, are involved in the structure of lignin and are formed by the enzyme dehydrogenase to form macromolecular lignin. The lignin of plants with different woody structures is not the same, and even the lignin of the same plant does not consist of polymer homologous molecules with a uniform structure, but the order of the structural details in each molecule may be different.
