17.3 Biological Pretreatment

275

17.3.3.1

Lignin Peroxidase

Lignin peroxidases (LiPs) (EC 1.11.1.4) were discovered upon culturing the white-rot

fungus P. chrysosporium in medium deficient in nitrogen. These are glycosylated

enzymes made up of about 360 amino acids with molecular mass ranging from 38

to 50 kDa coupled with two calcium ions and one heme group. Extraction of LiP has

been reported from different strains of white- and brown-rot fungi such as Phlebia

flavido-alba, and T. versicolor, Aspergillus species and bacterial strains of Strepto-

myces, Acinetobacter, and Calcaceticus. LiPs are present in the peripheral region of

the cell where they assist delignification occurring in the outer side by engaging in

substrate interaction at heme edge and glutamine 146 site and subsequently leading

to hydrolysis of the compound. This depolymerization is a H2O2-dependent process

involving multiple steps including formation of oxo-ferryl intermediate followed by

electron reduction of the intermediates and non-enzymatic hydrolysis of the radical

generated previously.

17.3.3.2

Manganese Peroxidase

Manganese peroxidases (MnPs) (EC 1.11.1.13) primarily oxidize the phenolic rings

of lignin in a manganese-dependent reaction. The reaction yields phenolic com-

pounds such as 2,6-dimethyloxyphenol syringol, 3-ethylthiazoline-6-sulfonate, and

guaiacol, as well as alcohol and other non-phenolic compounds. The enzyme itself is

composed of 350 amino acids with a molecular weight of 40 kDa. It is commercially

extracted from P. chrysosporium, Ceriporiopsis sp., Schizosporium sp., Lentinula edo-

des, Dichomatius squalens, and T. versicolor. The production of MnPs greatly varies

between microbial strains and species, nutritional source, and presence of the aro-

matic moiety. The enzyme oxidizes Mn²⁺ to Mn³⁺, which then reacts with phenol

rings, resulting in phenoxy radicals that degrade the compounds.

17.3.3.3

Laccases

Laccases (Lac) (EC 1.10.3.2) are benzenediol oxygen oxidoreductases. Chemically,

the enzyme is a glycoprotein with multiple copper (Cu) catalytic cores, with a

molecular weight between 60 and 80 kDa. The presence of Lac is widely studied in

both fungi and bacteria. Pycnoporus sp., Myceliopthora sp., Trametes sp., Pleurotus

sp., Bacillus sp., Haloferax sp., and Streptomyces sp. are few industrially used

microorganisms for the production of Lac. This enzyme facilitates oxidation of

phenolic compounds, where oxygen serves as an electron acceptor. The Cu atoms

are arranged in three different ways leading to the emergence of diverse groups:

blue copper core or type 1, normal Cu core or type 2, and binuclear copper core or

type 3. This organized structure participates in lignin degradation involving a series

of steps: (i) lignin oxidation by reduction of copper, (ii) electron released in step 1

is transferred to two groups of Cu atom, and (iii) oxygen is reduced to water at the

core of type 3 and type 2 Cu [33]. Laccases can potentially oxidize heterogeneous

substrates as well as aromatic diamines, polyphenols, and methoxy-substituted

compounds by creating a split between Cα–Cβ and alkyl–aryl bonds [8].