7 Mechanism of Microbial Degradation of Melanoidins
The mechanism involved in melanoidins degradation through the action of enzymes
is not still completely revealed, but it seems to be connected with that of the fungal
ligninolytic pathway. The most studied enzymes, which are capable of breaking a
large number of different chemical bonds present in phenolic and non-phenolic
recalcitrant compounds present in distillery effluent, include MnP, LiP, and Lac
(Gonzalez et al. 2008; Miyata et al. 1998a, b; Chandra et al. 2017; Pant and
Adholeya 2009b). The decolorization of distillery effluent and the decrease in the
MW of melanoidins are due to cleavage of C¼O, C¼C, and CN bonds in their
molecules by microbial treatment, since the role of enzymes other than Lac or MnP
in the degradation and decolorization of melanoidins by Coriolus (Trametes) strains
was reported during the 1980s. Several scientific studies claimed that intracellular
sugar-oxidase-type enzymes (glucose oxidase or sorbose oxidase) had melanoidindecolorizing activities. It was suggested that melanoidins were degraded and decolorized by the active oxygen species (O 2 ; H 2 O 2 ) formed by the reaction with glucose
oxidase and/or sorbose oxidase. In addition, melanoidin-decolorizing activity in
Coriolus versicolor Ps4a was attributed to intracellular enzymes composed of two
types: sugar-independent and sugar-dependent activities. Hayase et al. (1984)
reported that melanoidins were predominantly decolorized by glucose oxidase, but
when glucose as a substrate of glucose oxidase was not used, melanoidins were not
decolorized. Decolorization of melanoidins without supplementation of the carbon
source is not possible (Ghosh et al. 2004). Several authors evaluated the decolorization of melanoidins in the presence of different carbon sources such as glucose,
fructose, galactose, sucrose, maltose, etc., and they observed that melanoidin decolorization was achieved maximum with supplementation of glucose. They concluded
that degradation and decolorization of melanoidin polymer were the results of
co-metabolism of microorganism in the existence of an adequate concentration of
glucose. This indicated that carbon bounds with melanoidins are not bioavailable to
growing microorganisms. Therefore, decolorization of melanoidins without supplementation of the carbon source is not observed. Carbon source provided energy in
form of ATP for growth and survival of microorganisms, which are necessary for the
cleavage of conjugated C¼O, C¼C, and C N bonds of melanoidins by the active
oxygen such as H 2 O 2 which is secondarily produced by the enzymatic oxidation
reaction from glucose. Generally, H 2 O 2 reacts with the OH- to give mainly
perhydroxyl anion (HOO-) which has a strong nucleophilic activity. The HOO- is
considered to nucleophilically attack the carbonyl groups of melanoidins. The
finding suggested that both decolorization and degradation of melanoidins occur
markedly in the alkaline medium due to the abundant presence of the HOO anion.
The presence of HM ions such as Fe, Cu, etc. can induce an autodegradation of H 2 O 2
to form the free radicals, viz., OH, ÁOOH, and ÁOO which may positively react with
melanoidins (Miyata et al. 1998a, b, 2000; Kalavathi et al. 2001). Miyata and
co-workers in 1998a, b studied the role of enzymes in melanoidin degradation and
decolorization. Miyata et al. (1998a, b) demonstrated that synthetic melanoidin is
520
V. Kumar and R. Chandra
The mechanism involved in melanoidins degradation through the action of enzymes
is not still completely revealed, but it seems to be connected with that of the fungal
ligninolytic pathway. The most studied enzymes, which are capable of breaking a
large number of different chemical bonds present in phenolic and non-phenolic
recalcitrant compounds present in distillery effluent, include MnP, LiP, and Lac
(Gonzalez et al. 2008; Miyata et al. 1998a, b; Chandra et al. 2017; Pant and
Adholeya 2009b). The decolorization of distillery effluent and the decrease in the
MW of melanoidins are due to cleavage of C¼O, C¼C, and CN bonds in their
molecules by microbial treatment, since the role of enzymes other than Lac or MnP
in the degradation and decolorization of melanoidins by Coriolus (Trametes) strains
was reported during the 1980s. Several scientific studies claimed that intracellular
sugar-oxidase-type enzymes (glucose oxidase or sorbose oxidase) had melanoidindecolorizing activities. It was suggested that melanoidins were degraded and decolorized by the active oxygen species (O 2 ; H 2 O 2 ) formed by the reaction with glucose
oxidase and/or sorbose oxidase. In addition, melanoidin-decolorizing activity in
Coriolus versicolor Ps4a was attributed to intracellular enzymes composed of two
types: sugar-independent and sugar-dependent activities. Hayase et al. (1984)
reported that melanoidins were predominantly decolorized by glucose oxidase, but
when glucose as a substrate of glucose oxidase was not used, melanoidins were not
decolorized. Decolorization of melanoidins without supplementation of the carbon
source is not possible (Ghosh et al. 2004). Several authors evaluated the decolorization of melanoidins in the presence of different carbon sources such as glucose,
fructose, galactose, sucrose, maltose, etc., and they observed that melanoidin decolorization was achieved maximum with supplementation of glucose. They concluded
that degradation and decolorization of melanoidin polymer were the results of
co-metabolism of microorganism in the existence of an adequate concentration of
glucose. This indicated that carbon bounds with melanoidins are not bioavailable to
growing microorganisms. Therefore, decolorization of melanoidins without supplementation of the carbon source is not observed. Carbon source provided energy in
form of ATP for growth and survival of microorganisms, which are necessary for the
cleavage of conjugated C¼O, C¼C, and C N bonds of melanoidins by the active
oxygen such as H 2 O 2 which is secondarily produced by the enzymatic oxidation
reaction from glucose. Generally, H 2 O 2 reacts with the OH- to give mainly
perhydroxyl anion (HOO-) which has a strong nucleophilic activity. The HOO- is
considered to nucleophilically attack the carbonyl groups of melanoidins. The
finding suggested that both decolorization and degradation of melanoidins occur
markedly in the alkaline medium due to the abundant presence of the HOO anion.
The presence of HM ions such as Fe, Cu, etc. can induce an autodegradation of H 2 O 2
to form the free radicals, viz., OH, ÁOOH, and ÁOO which may positively react with
melanoidins (Miyata et al. 1998a, b, 2000; Kalavathi et al. 2001). Miyata and
co-workers in 1998a, b studied the role of enzymes in melanoidin degradation and
decolorization. Miyata et al. (1998a, b) demonstrated that synthetic melanoidin is
520
V. Kumar and R. Chandra