Scientific, technological and industrial advances have dumped large amounts of sewage and nuclear waste into the ecosystem, causing serious problems for human beings to survive on earth. The traditional way to dispose of waste is to dig a hole to fill the waste. This method is difficult to maintain because there is no new filling place. New methods of treating waste use high temperature incineration and chemical decomposition, such as base catalyzed dechlorination and UV oxidation. Although these methods are very effective in dealing with various contaminants, these methods are complex and uneconomical, so bioremediation is now a suitable alternative.

Bioremediation is a microbial-mediated transformation or degradation of contaminants into nonhazardous or less-hazardous substances. Bacteria, fungi, algae and plants can be used for effective bioremediation of contaminants. The involvement of plants in the bioremediation of pollutants is called phytoremediation. The phytoremediation process is an emerging green technology that promotes the removal or degradation of toxic chemicals in soil, sediment, groundwater, surface water and air. The bioremediation process mainly relies on microorganisms that attack the contaminants by enzymes and convert them into innocuous products. Since bioremediation can be effective where environmental conditions permit microbial growth and activity, its application typically involves manipulation of environmental parameters to allow for microbial growth and faster degradation. Bioremediation is a very slow process. Only certain types of bacteria and fungi have an effective ability to degrade pollutants, relying on the involvement of different intracellular and extracellular enzymes to repair recalcitrant and lignin and organopollutants.

Microbial Oxidoreductases

The detoxification of various compounds by bacteria, fungi and higher plants by oxidative coupling is mediated by oxidoreductases. Microorganisms extract energy through these enzyme-mediated energizing biochemical reactions to cleave chemical bonds and help the transfer of electrons from the reduced organic substrate to another compound. During such reactions, the contaminants are eventually oxidized to harmless Compound. Oxidoreductase is involved in the humification of various phenolic substances produced by the decomposition of lignin in the soil. Similarly, oxidoreductases can also detoxify toxic xenobiotics (eg, phenolic or aniline compounds) by polymerization, copolymerization with other substrates, or binding to humic substances.

Many bacteria can reduce radioactive metals from soluble oxidized form to insoluble form. In the process of energy production, bacteria absorb electrons from organic compounds and use radioactive metals as final electron acceptors. Some bacteria indirectly reduce the radioactive metal with the help of an intermediate electron donor. The final precipitant can be regarded as the result of redox reaction of metal-reducing bacteria

Chlorinated phenolic compounds are one of the most abundant contaminants in the paper industry. These compounds are produced when the lignin is partially degraded during pulp bleaching. Many fungi are suitable for removing chlorinated phenolic compounds from environmental pollutants. The activity of fungi is mainly due to the action of extracellular oxidoreductases such as laccase, manganese peroxidase and lignin peroxidase. These enzymes are released from the fungal mycelium into the surrounding environment.

Water contaminated with phenolic compounds can be decontaminated by plants with enzymes secreted by their roots. Fabaceae, Gramineae and Solanaceae plants release oxidoreductases, which are involved in the oxidative degradation of certain soil components. The phytoremediation of organic pollutants is mainly concentrated in three types of compounds: chlorinated solvents, explosives and petroleum hydrocarbons.

Microbial Laccases

Laccase (p-diphenol: oxy-oxidoreductase) forms a family of multi-copper oxidases, mainly produced by plants, fungi, insects and bacteria, which catalyze the oxidation of various reduced phenolic and aromatic substrates, while molecular oxygen is reduced to water. Laccases exist in a variety of isoenzyme forms, each encoded by a separate gene, and in some cases the expression of these genes depends on the nature of the inducer. Many microorganisms produce intracellular and extracellular laccases that catalyze the oxidation of o- and p-diphenols, aminophenols, polyphenols, polyamines, lignin and aryl diamines, as well as certain inorganic ions. Laccase not only oxidizes phenols and methoxyphenolic acids, but also decarboxylates them and attacks their methoxy groups (demethylation). These enzymes are involved in the depolymerization of lignin to produce a variety of phenols. These compounds are used as nutrients for microorganisms or repolymerized to humic materials by laccase. In biologics, laccases represent a ubiquitous group of oxidoreductases that have great potential for biotechnology and bioremediation applications.

Microbial Peroxidases

Peroxidase (donor: hydrogen peroxide oxidoreductase) is a ubiquitous enzyme that catalyzes the oxidation of lignin and other phenolic compounds at the expense of hydrogen peroxide (H₂O₂) in the presence of a mediator. The peroxidase can be heme or non-heme protein. In mammals, they are involved in biological processes such as the immune system or hormonal regulation. In plants, they are involved in the metabolism of auxins, the formation of lignin and suberin, the cross-linking of cell wall components, defense against pathogens or cell elongation.

Heme peroxidase is divided into two groups, and the second group is subdivided into three categories. Class I is intracellular enzymes include yeast cytochrome c peroxidase, plant ascorbate peroxidase (APX) and bacterial gene-duplicated catalase peroxidase. Class II consists of secreted fungal peroxidases, for example, lignin peroxidase (LiP) and manganese peroxidase (Mnp) of Phanerochaete chrysosporium, Coprinus cinereus peroxidase or Arthromyces ramosus peroxidase (ARP). The main role of class II peroxidase is to degrade lignin in wood. Class III comprises the secreted plant peroxidase, such as those from horseradish (HRP), barley or soybean. These peroxidases are biosynthetic enzymes involved in processes such as plant cell wall formation and lignification. There is no evolutionary link between non-heme peroxidases, forming five separate families, including thiol peroxidase, alkyl hydroperoxidase, nonhaem haloperoxidase, manganese catalase and NADH peroxidase.

References

1. Karigar C S, Rao S S. Role of microbial enzymes in the bioremediation of pollutants. [J]. Enzyme Research, 2011.

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