Reductive Decolorization of Azo Dye by Bacteria - Microbial Degradation of Synthetic Dyes in Wastewaters - page 119

An increase of dye concentration generally resulted in decrease of decolorization
percentage (Saratale et al. 2011 ). However, when the speci c decolorization rate

was considered, the relationship between decolorization rate and dye concentration
could be well described with Michaelis-Menten equation. Therefore, before the
level off of the decolorization rate, it increases with the increase of dye concen-
tration. The kinetic parameters could be used to compare the decolorization
capacity of different bacterial strains (Liu et al. 2007b , 2013a , b ).

4 Involvement of Bacterial Azoreductase in Azo Dye
Decolorization

Azoreductases are enzymes catalyzing the reductive cleavage of azo bonds.
Enzymes with azo-bond reducing activities were initially found from rat or rabbit
liver (Autrup and Warwick 1975 ; Stoddart and Levine 1992 ). In early 1980s,
Zimmermann et al. ( 1982 , 1984 ) puri

ed and compared two azoreductases from
Pseudomonas KF46 (later known as Xenophilus azovorans KF46) and Pseudo-
monas K24 (later known as Pigmentiphaga kullae K24), which promoted great
interests in studies on bacterial azoreductase. The two azoreductases demonstrated
different substrate speci cities. The Organe II azoreductase from KF46 required the

presence of a hydroxyl group in the ortho-position of the aromatic ring of the dye.
In contrast, the Orange I azoreductase from K24 required a hydroxyl group in the
para-position of the aromatic ring of the dye (Zimmermann et al. 1982 , 1984 ).
During the past three decades, many azoreductases were puri

ed from different
bacterial strains (Table 2 ). Based on their functions, azoreductases are categorized
as avin (FMN)-dependent and avin-independent azoreductase. The former could

be further classi ed on the basis of its co-factor (NADH, NADPH) preference

(B rger and Stolz 2010 ). Azoreductases show low level of nucleotide or amino acid

sequence similarity. However, when their tertiary structures were superimposed and
compared, azoreductases could be classi

ü

ed into two families (Abraham and John
2007 ). The monomer of almost all the obtained avin-dependent azoreductase

possesses avodoxin-like structure (Liu et al. 2007a , 2008 , 2009 ).

The involvement of intracellular azoreductase in bacterial decolorization has
been in doubt recently. On one hand, azo dye molecules generally have high
molecular weight, high polarity and complex structure, which make it dif

cult for
them to enter the cells and react with intracellular enzymes. On the other hand, as
mentioned above, there is almost no azo compound in the natural environment,
which means intracellular proteins with azoreductase activity should be prepared
for other functions. To deduce the physiological role of azoreductase, we

rst
carried out the three-dimensional structure modeling studies of bacterial azore-
ductase. Based on structure analysis and experimental test, we found the nitrore-
ductase and quinone reductase activities of bacterial azoreductase, respectively (Liu
et al. 2007a , 2008 ). Then based on gene knock-out and mutant studies, we