Bacterial Degradation of Azo Dye Containing Wastes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 69

5.1 Tyrosinases

Tyrosinases (E.C. 1.14.18.1) are also known as monophenol mono-oxygenase
found in the variety of organisms, including bacteria, fungi, plants, insects,
amphibians and mammals. They are copper-containing enzymes and catalyze two
type of reactions, the o-hydroxylation of some monophenols (monophenolase,
cresolase) and the oxidation of o-diphenols to o-quinones (diphenolase, catecho-
lase) using molecular oxygen (Chen and Flurkey 2002 ). Tyrosine and catechol are
commonly used as substrates of the enzyme tyrosinase that produces dopaquinone
and o-benzoquinone, respectively. This enzyme acts as marker of the oxidative
enzymes involved in azo dye degradation. Signi cant induction of tyrosinase is

reported during the decolorization of azo dyes by bacteria. Reports on tyrosinase
induction after exposure to azo dyes are available from Alcaligenes faecalis PMS-1,
B. laterosporus MTCC 2298 and consortium consisting of Galactomyces geotri-
chum MTCC 1360 and Bacillus sp. VUS (Saratale et al. 2011 ; Shah et al. 2012 ).

5.2 Laccases

Laccases (E.C.1.10.3.2) are copper-containing oxidoreductases and also known as
phenol oxidases. Laccases have a high potential for biodegradation of dyes due to
their wide reaction capabilities as well as broad substrate speci

city along with no
requirement of co-factors. The molecular structure exhibits four histidine rich copper
containing binding domains, which are classi ed into three types: copper I, II, and III

which are differentiated by their spectroscopic properties. They are mainly classi ed

into two categories viz. the blue laccases which contain type 1 copper site and
second one which lacks the type 1 copper site. The molecular weight of laccase was
reported between 60 and 390 kDa. The mechanism of reaction involves the oxidation
of substituted phenolic and non-phenolic compounds in the presence of oxygen as an
electron acceptor to form free radicals which further undergo depolymerization,
repolymerization, demethylation or quinone formation. Laccases in immobilized
phase are also studied for degradation of azo dyes. This enzyme showed optimum
pH 4.0 and temperature 60 ° C for its activity with maximum substrate speci city for

2, 2 -azinobis, 3-ethylbenzothiazoline-6-sulfonic acid. In case of laccase mediated

degradation of azo dyes, there is no formation of aromatic amines after azo bond
cleavage because laccases directly cleave the azo group through non-speci

′

c free
radical mechanism. The role of laccase in asymmetric cleavage of azo dye was well
documented in Remazol Red degradation by Pseudomonas aeruginosa BCH. The
laccases from Bacillus sp. ADR and Pseudomonas desmolyticum NCIM 2112 which
are involved in degradation study of various azo dyes were puri

ed and character-

ized (Saratale et al. 2011 ).