10.2 Enzymes Required for the Degradation of Lignocellulosic Waste

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heme peroxidases, whereas laccases comes under phenol oxidases. The peroxidase

enzymes contain heme molecule and require hydrogen peroxide for its oxidative

activity. Accessory oxidases provide H2O2 which will be used by peroxidase, whereas

laccases contain copper molecules and catalyze oxidation–reduction reactions. LiP,

MnP, versatile peroxidases, and laccases are the major ligninases.

LiP removes one electron from the C—C bonds (non-phenolic part) of lignin and

releases cation radicals that degrade chemically. However, MnP cannot directly

remove electron from the non-phenolic part of lignin. It has to first transfer its

oxidizing power to Mn³⁺ (which is a product of MnP reaction), and then, this Mn³⁺

enters the lignin structure and catalyzes the oxidative reactions [36].

The extracellular oxidases/accessory oxidases synergistically oxidize a cosubstrate

and reduce O2 to H2O2. This H2O2 is then utilized by peroxidases (i.e. LiP and MnP)

for their action. Aryl alcohol oxidase and glyoxal oxidase are the most significant

accessory oxidase enzymes used in ligninolysis [3].

Versatile peroxidase (VP) like LiP and MnP also contains a heme protein. In addi-

tion to the oxidative catalytic activity, VP also shows dye decoloration activity in the

presence of Mn(II). Laccase oxidizes the phenolic units of lignin and breaks down

into sinapyl alcohol [37]. It also acts as a catalyst in the oxidation of many aromatic

substrates and produces water as a byproduct.

10.2.4

Degradation of Pectin

Hydrolytic enzymes viz., GHs and polysaccharide lyases (PLs) are used in pectin

degradation.

Endo- and exo-polygalacturonases of the GH family break down the pectin

backbone. They do so by cleaving the α-1,4-glycosidic linkages of α-galacturonic

acids. Other enzymes of the GH family viz., endo- and exo-rhamnogalacturonases,

α-rhamnosidases, xylogalacturonases, unsaturated rhamnogalacturonan hydro-

lases, and unsaturated glucuronyl hydrolases are involved in the degradation of the

“hairy” region of pectin [26]. D-galacturonic acid and L-rhamnose residues of the

pectin backbone are present in the “hairy” region of pectin. Rhamnogalacturonases

acts upon the α-1,2-glycosidic bonds between the two residues [3]. The xylose

residue on the galacturonic acid backbone is cleaved by an endo-xylogalacturonase

from the fungus Aspergillus tubingensis [38].

The PLs viz., pectin lyase and pectate lyase cleave the α-1,4-linked D-galacturonic

acid residues of pectin backbone [39]. The two PLs have different preference for sub-

strate esterification. Pectin lyases prefer high degree of esterification, while pectate

lyases prefer low degree. Also, pectate lyases act in the presence of Ca²⁺ ions, while

pectin lyases do not have any such requirement. Another enzyme of the PL family,

rhamnogalacturonan lyase shows catalytic activity in the “hairy” region of pectin. A

set of accessory enzymes are required to cleave the substituted chains of pectin back-

bone to make way for the main pectinolytic enzymes. Some of the accessory enzymes

for pectin degradation are β-endogalactanases, endo- and exoarabinases [4].