Bacterial Degradation of Azo Dye Containing Wastes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 71

5.5 Azo Reductases

Azo reductases which are reducing enzymes widely applied in eld of dye deg-

radation, catalyse reductive cleavage of electrophilic azo groups ( ) and other

compounds containing azo bond to produce aromatic amines. Many bacterial
strains possess unspeci

- N=N -

c cytoplasmic enzymes, which act as “ azo reductases ” . Azo

reductases reduce azo bond by transferring electrons via soluble avins to azo dyes.

First report of presence of azo reductase in anaerobic bacteria was given by Rai
et al. ( 1990 ). They reported extracellular oxygen sensitive azo reductase from
Clostridium and Eubacterium for azo dye degradation. Apart from bacteria, algae
and yeast, azo reductases have also been detected in liver cells (Solis et al. 2012 ;
Chengalroyen and Dabbs 2013 ).
Classi

cation of azo reductases is broadly based on their oxygen requirement
and structure. But due to low level of similarity in nucleotide and amino acid
sequence of azo reductases, they are mainly classi ed based on secondary and

tertiary structure. Further based on function they are classi ed into two groups,

avin dependent azo reductases are
categorized according to co-enzymes required, NADH, NADPH or both. Recently,
three group classi

avin dependent and avin independent. Again

cation systems have been proposed. First group is consisting of
FMN dependent enzymes utilizing NADH, second NADPH utilizing enzymes
and third group is avin free reductases (Saratale et al. 2011 ; Solis et al. 2012 ;

Chengalroyen and Dabbs 2013 ). Structurally azo reductases are mainly monomeric,
but a few are also reported as dimeric and tetrameric in nature (Bafana and
Chakrabarti 2008 ) The optimum temperature range of bacterial azo reductases is
between 25 and 45

C and pH of 7.0. Azo reductase activity is not dependent on the
intracellular uptake of dye, as high molecular weight azo dyes are unlikely to pass
through cell membrane of bacterial cells (Chengalroyen and Dabbs 2013 ).
Azo reductases are localized at intracellular or extracellular site of the bacterial
cell membrane. These azo reductases required NADH or NADPH or FADH as an
electron donor for the reduction of an azo bond (Russ et al. 2000 ). Azo reductase
activities have been also observed in cell extracts. As co-factors, FADH 2 , NADH,
NADPH, FMNH 2 and their reducing enzymes are located in the cytoplasm, lysis of
cells releases these co-factors and enzymes in extracellular environment. In case of
intact cells, membrane transport system may be a prerequisite for azo dye reduction.
Ribo

°

avin can pass through cell membranes, but FAD and FMN cannot easily pass
through cell walls. Similarly, many azo dyes cannot pass through cell membranes
due to complex structure and high polarities, while azo reductases are found
intracellular in many bacteria. Thus, cell extracts of lysed cells often show higher
reductase activity of dye reduction as compared to intact cells. Many researchers
have observed lack of speci city in the azo reductase system and showed the

substrate speci city of azo reductases depends on the functional group present near

azo bond (Saratale et al. 2011 ; Solis et al. 2012 ; Chengalroyen and Dabbs 2013 ).
Azo reductases of bacteria showed very less similarity with other reported
reductases and hence represent novel families. Puri cation and biochemical