14
microorganisms that produce lignin-degrading enzymes, white rot fungi are considered to be the most efficient producers. Most of the current understanding on lignindegrading enzymes has emerged from studies on the enzymes of white rot fungi. In
these organisms the ligninolytic enzyme system consists of three major classes,
namely, laccases, manganese peroxidases, and lignin peroxidases.
Laccases (EC 1.10.3.2) or benzene diol oxygen oxidoreductases are oxidoreductases which enjoy wide distribution among microorganisms. These are enzymes
having copper in their active site and are generally classified as multicopper oxidases or blue multicopper oxidases (Rodríguez Couto and Toca Herrera 2006).
They employ oxygen as an oxidizing agent and cofactor. Laccases have low substrate specificity and therefore can degrade several compounds having phenolic
structure (Placido and Capareda 2015). Other major enzymes that act on lignin –
lignin peroxidase (LiP) and manganese peroxidase (MnP) – are heme peroxidases
having protoporphyrin IX as the prosthetic group. Lignin peroxidases (EC1.11.1.14)
are capable of oxidizing sites of very high redox potential including moderately
activated aromatic rings of nonphenolic model lignin compounds. Manganese peroxidase (EC 1.11.1.13) on the other hand cannot oxidize nonphenolic lignin model
compounds and depend on the generation of Mn3+ as a diffusible charge transfer
mediator. Yet another peroxidase is the versatile peroxidase described in the fungus
Phanerochaete chrysosporium and capable of both LiP and MnP activities (Fischer
and Fong 2014).
Direct use of lignin-degrading enzymes in biomass hydrolysis is not practiced
and often the applications of these enzymes are in the delignification of biomass as
a pretreatment step. Here again, the pretreatment is more often accomplished by
whole microorganisms elaborating ligninases rather than use of their enzymes in
isolation. Nevertheless, there are several successful reports on the use of enzymes in
isolation for delignification (Gutiérrez et al. 2012; Wang et al. 2013). Mostly laccases are employed for such applications, though MnP, LiP, or combinations of
these enzymes may be employed. Another major application of ligninases is the
detoxification of the biomass hydrolysates. Several of the conventional pretreatment
processes generate toxic compounds classified as furan derivatives, sugar degradation products, weak acids, and phenolic compounds from lignin. These compounds
can affect the growth and ethanol production by yeasts or other microbes used for
bioethanol production and are sometimes removed prior to fermentation so that the
ethanol yields are improved. Ligninolytic enzymes are an efficient means of degrading these inhibitors and offer the advantages of reduced or no sugar loss, ambient
conditions of operation. Mostly phenolic compounds are removed, while lesser
known ligninolytic enzymes like aryl-alcohol oxidases (AAOs) are being investigated for removal of furan derivatives (Carro et al. 2015). Detailed reviews on the
ligninolytic enzymes and their applications for biofuels may be found in Placido
and Capareda (2015) and Fisher and Fong (2014).
R.K. Sukumaran et al.
microorganisms that produce lignin-degrading enzymes, white rot fungi are considered to be the most efficient producers. Most of the current understanding on lignindegrading enzymes has emerged from studies on the enzymes of white rot fungi. In
these organisms the ligninolytic enzyme system consists of three major classes,
namely, laccases, manganese peroxidases, and lignin peroxidases.
Laccases (EC 1.10.3.2) or benzene diol oxygen oxidoreductases are oxidoreductases which enjoy wide distribution among microorganisms. These are enzymes
having copper in their active site and are generally classified as multicopper oxidases or blue multicopper oxidases (Rodríguez Couto and Toca Herrera 2006).
They employ oxygen as an oxidizing agent and cofactor. Laccases have low substrate specificity and therefore can degrade several compounds having phenolic
structure (Placido and Capareda 2015). Other major enzymes that act on lignin –
lignin peroxidase (LiP) and manganese peroxidase (MnP) – are heme peroxidases
having protoporphyrin IX as the prosthetic group. Lignin peroxidases (EC1.11.1.14)
are capable of oxidizing sites of very high redox potential including moderately
activated aromatic rings of nonphenolic model lignin compounds. Manganese peroxidase (EC 1.11.1.13) on the other hand cannot oxidize nonphenolic lignin model
compounds and depend on the generation of Mn3+ as a diffusible charge transfer
mediator. Yet another peroxidase is the versatile peroxidase described in the fungus
Phanerochaete chrysosporium and capable of both LiP and MnP activities (Fischer
and Fong 2014).
Direct use of lignin-degrading enzymes in biomass hydrolysis is not practiced
and often the applications of these enzymes are in the delignification of biomass as
a pretreatment step. Here again, the pretreatment is more often accomplished by
whole microorganisms elaborating ligninases rather than use of their enzymes in
isolation. Nevertheless, there are several successful reports on the use of enzymes in
isolation for delignification (Gutiérrez et al. 2012; Wang et al. 2013). Mostly laccases are employed for such applications, though MnP, LiP, or combinations of
these enzymes may be employed. Another major application of ligninases is the
detoxification of the biomass hydrolysates. Several of the conventional pretreatment
processes generate toxic compounds classified as furan derivatives, sugar degradation products, weak acids, and phenolic compounds from lignin. These compounds
can affect the growth and ethanol production by yeasts or other microbes used for
bioethanol production and are sometimes removed prior to fermentation so that the
ethanol yields are improved. Ligninolytic enzymes are an efficient means of degrading these inhibitors and offer the advantages of reduced or no sugar loss, ambient
conditions of operation. Mostly phenolic compounds are removed, while lesser
known ligninolytic enzymes like aryl-alcohol oxidases (AAOs) are being investigated for removal of furan derivatives (Carro et al. 2015). Detailed reviews on the
ligninolytic enzymes and their applications for biofuels may be found in Placido
and Capareda (2015) and Fisher and Fong (2014).
R.K. Sukumaran et al.