Degradation of Anthroquinone Dyes Stimulated by Fungi - Microbial Degradation of Synthetic Dyes in Wastewaters - page 333

(Aust 1995 ). During this reaction, H 2 O 2 is reduced to water. Subsequently, com-
pound I oxidizes substrates by one electron and gets reduced to compound II. In this
step, the porphyrin ring gains the electron. Thus, compound I has higher ability to
oxidize substrates with higher redox potential than compound II (Mester and Tien
2000 ). After this, compounds II reacts with reducing substrates by gaining one
electron and returns to resting enzyme. There is also possibility that RH of com-
pound I can react with compounds II to form radical R ￿ ., and compound II can also
react with H 2 O 2 leading to formation of compound III (Cai and Tien 1992 ), which
causes the inactivation of peroxidase (Torres et al. 2003 ). The inactivation of LiP
can be avoided by veratryl alcohol which completes the catalytic cycle of LiP by
reducing compound II to resting enzyme, as illustrated below:
Catalytic cycle of lignin-peroxidase (Adapted from Banci 1997 )

1. Enz heme½Fe II ðÞ P ðÞ þ H 2 O 2 ! Enz heme þ ð Þ ½O ¼ Fe I ðÞ Compound I

Þ þ H 2 O ð

2. Enz heme þ

Þ þ RH ! Enz heme½O ¼ Fe(IV) ð Compound II Þ þ H þ R

ð Þ ½O ¼ Fe I ðÞ Compound I

ð

H þ R 3. Enz heme ½ O ¼ Fe IV ðÞ ð Compound II Þ þ RH ! Enz heme ½ Fe III ðÞ P ðÞ þ

where PX = native or resting enzyme, Enz = enzyme

3.2 Manganese Peroxidases (MnP): (EC: 1.11.1.13)

MnP is also a heme-containing peroxidase having heme (ferric protoporphyrin) as a
prosthetic group and glycoprotein in nature (Zapanta and Tien 1997 ). This is the
most common ligninolytic peroxidase, produced by almost all white rot basidio-
mycetes (Glenn and Gold 1985 ; Hofrichter 2002 ; Wesenberg et al. 2003 ). Its
mechanistic properties are similar to LiP and it forms the oxidized intermediates,
compound I and compound II (Cai and Tien 1993 ; Zapanta and Tien 1997 ). H 2 O 2 is
essentially required by MnP for the oxidation of lignin and lignin-related com-
pounds (Mester and Tien 2000 ). MnP is dependent on Mn (II) as a substrate for the
formation of compound II (Wariishi et al. 1988 ). Mn (II) is also a preferred sub-
strate for compound I. During degradation of lignin and other substrates, MnP
oxidizes Mn (II) to Mn (III) and subsequently, Mn (III) oxidizes a variety of
compounds (Glenn et al. 1986 ; Mester and Tien 2000 ). It was also observed that the
chelation of Mn (II) and Mn (III) by organic acids, such as oxalate, was essential for
MnP activity (Zapanta and Tien 1997 ). Oxalate is an organic acid chelator produced
by white rot fungi at the same time when MnP is synthesized in the liquid cultures
of Phanerochaete chrysosporium (Wariishi et al. 1992 ; Kuan and Tien 1993 ;
Zapanta and Tien 1997 ).