Bacterial Enzymes and Their Role in Decolorization of Azo Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 152

Reduced laccase Peroxide level intermediate

Native intermediate

Resting enzyme

Fig. 5 Catalytic cycle of laccases

thereby avoiding the formation of toxic aromatic amines. The puri ed laccase from

Pseudomonas desmolyticum NCIM 2112 and Bacillus sp. ADR have ability to
decolorize the several textile dyes withdecolorization ef ciency varying from 65 to

90 % (Kalme et al. 2009 ; Telke et al. 2011 ).

6.1.5 Polyphenol Oxidase (Tyrosinase)

Polyphenol oxidases (PPOs) were catalyze the o-hydroxylation of monophenols
(phenol molecules in which the benzene ring contains a single hydroxyl substituent)
to o-diphenols (phenol molecules containing two hydroxyl substituents). They can
also further catalyze the oxidation of o-diphenols to produce o-quinones. The amino
acid tyrosine contains a single phenolic ring that may be oxidised by the action of
PPOs to form o-quinone (Fig. 7 ). Hence, PPOs may also be referred as tyrosinases.
Polyphenol oxidases are enzymes that catalyze oxidation of certain phenolic sub-
strates to quinones in the presence of molecular oxygen. Polyphenol oxidases have
been reported in the bacteria viz. Streptomyces glaucescens, Streptomyces antibi-
oticus, Bacillus licheniformis, Bacillus natto and Bacillus sphaericus (Whitaker
1994 ; Echiqo and Ritsuko 2001 ). The puri ed polyphenol oxidase was used for the

oxidation of colored and phenolic substances.