Laccases in nature enzyme having the capability to oxidize

Laccases (benzenediol: oxygen oxidoreductase; EC are a ubiquitous in
nature enzyme having the capability to oxidize a wide range of recalcitrant phenolic
and nonphenolic compounds by converting oxygen molecule to reactive radicals and
water on concomitant four-electron reduction (Hakulinen and Rouvinen 2015). These
free electrons catalyze the oxidation of various aromatic and non-aromatic
compounds as well as phenolic ring-containing amines substituted with various
functional groups such as methoxy, amino, diamino and hydroxyindols and few
other metal compounds Mo(CN)84-, Fe(CN)64- and Os(CN)64- (Chandra and
Chowdhary 2015; Rezaei et al. 2017). Laccase was first reported from Japanese
laquer tree Rhus vernicifera in 1883(Yoshida). Laccases are the members
of multi-copper oxidases and contain histidine-rich copper binding domains participate
in (1) cross-linking of monomers, (2) degradation of polymers, and (3) ring
cleavage of aromatic compounds (Kawai et al. 1988). Laccases are found
glycoproteins, ranging from various fungi, bacteria, higher plants and some insects.
It is mainly produced from fungi, especially white rot, and has been widely exploited
for the application in industrial and biotechnology, for instance, in the
detoxification of chemicals in wastewater, degradation of lignin in pulp and
paper, degradation of inorganic compounds to soil organic matter and the
decolorization of dyes in textile due to their high redox potential (Niladevi and
Prema, 2008). However, the majority of the industrial processes are carried
out in extreme conditions, i.e., higher temperature and pH, and high salt
concentration, and fungal laccase generally fails to work in these extreme
environments (Du et al. 2015; Wang and Zhao 2016). In recent years, application
of bacterial laccases are growing rapidly due to their many remarkable features
in comparison to fungal laccase from the industrial point of view, such as work
in a broad range of temperature and pH with enormous stability against various
inhibitory agents (Guan et al. 2015).

        Bacterial laccase was first reported in
Azospirillum lipoferum (Givaudan et al., 1993); it plays a role in cell
pigmentation, oxidation of organic compounds (Faure et al., 1994, 1995) and/or
electron transport (Alexandre et al., 1999). Bacterial protein sequence studies
have indicated that the laccases are represented by high G+C Gram positive
bacteria and ?-, ?- and ?–proteobacteria (Alexandre and Zhulin, 2000); as have
been shown in Bacillus sp., Streptomyces sp., and a ?
proteobacterium (Bains et al., 2003; Sharma et al., 2007). Most bacterial
laccases are intracellular enzymes or periplasmic proteins as shown in A.
lipoferum and B. subtilis. Laccase like activity has also been found
in bacteria, for example, the copper efflux protein CueO from Escherichia
coli and the copper resistance protein CopA from Pseudomonas syringae
and Xanthomonas campestris. These were due to the dependence of the
2,6-dimethoxyphenol oxidation on Cu2+ addition (Solano et al.,
2001). Besides,
bacterial laccases have some additional advantages because of their practically
feasible, cost-effective
used in various industrial applications, which include broad substrate
specificity, enzyme production in a short time, and easiness to clone and express
in the host with suitable manipulation (Fernandes et al. 2014; Prins et al. 2015).
In the proposed review, comprehensive information of all laccase-producing
bacterial sources, characteristics of the enzyme, gene information and their
application have been summarized.

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