Degradation of Azo Dyes by White-Rot Fungi - Microbial Degradation of Synthetic Dyes in Wastewaters - page 318

reactivity with H 2 O 2 , and in the presence of a poor substrate and excess H 2 O 2 ,itis
instead converted to an inactive form of the enzyme (compound III) (Gold et al.
1989 ; Wariishi and Gold 1989 ). Compounds, such as veratryl alcohol and trypto-
phan, exert a protective effect against the enzyme inactivation by excess of H 2 O 2
(Collins et al. 1997 ) and when present, they convert compound II into the resting
enzyme, completing the catalytic cycle.
Manganese-dependent peroxidase (MnP, E.C. 1.11.1.13) is an extracellular
glycosylated heme-containing peroxidase that catalyses H 2 O 2 -dependent oxidation
of Mn 2+ to highly reactive Mn 3+ . It is often produced in multiple isoforms with
molecular masses between 32 and 62.5 kDa, optimum pH of 4

7 and optimum

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temperature of 40 et al. 2006 ). MnP

oxidises the phenolic units of lignin producing free radicals. The catalytic cycle of
MnP (Fig. 6 ) is similar to that of the LiP. It includes the native ferric enzyme as well
as the reactive intermediates (compound I and compound II) (Hofrichter 2002 ). In
contrast to other peroxidases, MnP uses Mn 2+ as the preferred substrate. The cycle
starts by binding of H 2 O 2 to the native ferric enzyme and the formation of an iron
peroxide complex (Hofrichter 2002 ). The subsequent cleavage of the peroxide
oxygen-oxygen bond requires a two-electron transfer from the heme, resulting in
the formation of MnP-compound I. Afterwards, one molecule of water is released.
A subsequent reduction proceeds through MnP-compound II. A mono-chelated
Mn 2+ ion acts as the one-electron donor for this enzyme intermediate and is oxi-
dised to Mn 3+ , leading to the generation of the native enzyme and the release of a
second molecule of water (Hofrichter 2002 ).
Laccase (E.C. 1.10.3.2; benzendiol-oxygen oxidoreductase) belongs to a group
of polyphenol oxidases containing copper atoms in the catalytic centre, usually
called multi-copper oxidases. Fungal laccases are produced mainly extracellularly
in multiple isoforms with a typical molecular mass of 60

60 ° C( Ü rek and Pazarlioglu 2004 ; Baborov á

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80 kDa and an acidic

-

pI value of 3 6 (Thurston 1994 ; Baldrian 2006 ). In addition to lignin degradation,

fungal laccases are also involved in the formation of fruiting bodies, synthesis
of melanin and other pigments, sporulation, conidiation and plant pathogenesis
(Alexandre and Zhulin 2000 ; Mayer and Staples 2002 ).

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Fig. 6 The catalytic cycle of
manganese-dependent
peroxidase (MnP) (Hofrichter
2002 ; reprinted with
permission from Elsevier
Ltd., UK.)