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Xenobiotic Compounds and their Biodegradation
Xenobiotic compounds are man-made chemicals that are present in the atmosphere at unusually high concentrations. Microorganisms are able to digest most of the naturally occurring xenobiotic compounds and this property is called as microbial infallibility. Those xenobiotic compounds that resist digestion from even microbes are called recalcitrant.

General Features of Xenobiotic Degradation:

Since xenobiotics consist of a wide variety of compounds, their degradation occurs via a large number of metabolic pathways.

Degradation of alkanes and aromatic hydrocarbons generally occurs as follows:
1. An oxygenase first introduces a hydroxyl group to make the compound reactive
2. The hydroxyl group is then oxidized to a carboxyl group
3. The ring structure is opened up in case of cyclic compounds
4. The linear molecule is degraded by beta oxidation to yield acetyl-CoA, which is then utilized in the usual manner to carbon dioxide.

Similarly, an alicyclic hydrocarbon e.g. cyclohexane is oxidized as follows:
1. First an oxygenase adds a –OH group in the ring
2. Then another oxygenase forms an ester in the form of a lacone
3. The lactone is then hydrolyzed to open the ring structure to give a linear molecule

In both these oxidations, mono-oxygenases are involved, which add oxygen to a single position in the molecule. In contrast, oxidation of benzene ring may involve a di-oxygenase, which adds oxygen at two positions in the molecule in a single step.

Both mono- and di-oxygenases are of a variety of types: some react best with short chain alkanes while others act on cyclic alkanes. But these enzymes are not very specific and each enzyme oxidizes a limited range of compounds. Thus xenobiotics are degraded by a wide variety of microorganisms, each of which degrades a small range of compounds. Frequently, oxidation of xenobiotic compounds involves cytochrome P450 or rubredoxin. In addition, the halogens and/or other substituent groups are either modified or removed usually as one of the initial reactions or sometimes it is achieved later in the process.
Chemicals that are foreign to the biosphere are known as xenobiotic compounds. Release of chemical substances due to rapid industrial progress has now become a serious problem causing environmental pollution. Pollutants resembling structural features of xenobiotics mostly include organic sulfonic acids, halogenated aliphatic and polycyclic aromatic hydrocarbons, s-triazines, nitroaromatic compounds, azo compounds and synthetic polymers. Over the years huge quantity of hazardous waste sites is being generated throughout the world due to accumulation of xenobiotic compounds in soil and water. Polycyclic aromatics, nitroaromatic compounds (NACs), and other hydrocarbons (PAHs) constituting crude oil, are among the diverse group of xenobiotic chemicals responsible for immense environmental pollution. The conventional physico-chemical strategies for remediation of xenobiotics to clean up contaminated sites are not quite costly and adequate. Therefore, research focused on biodegradation and elimination of these hazardous compounds are gaining importance. The process of biodegradation wherein Xenobiotic contaminated sites are remediated by means of bacteriological actions, utilizes the capability of microorganisms in reducing the toxicity and concentration of a large number of Xenobiotic pollutants. This is an ecofriendly and economical and efficient treatment technique that is developing quite rapidly in the field of environmental restoration. Bacteria and fungi can utilize the xenobiotic compounds as substrates, by mineralizing or converting them into less toxic products. Microorganisms gain access to xenobiotics pollutants depending on the availability of these compounds in air, water, soil various environmental compartments. Transformation and degradation of xenobiotic residues are mostly carried out by microorganisms. Bioremediation is a major mechanism in aquatic and terrestrial environments, and provides the very foundation of the modern wastewater treatment plants. Biodegradation performance in the natural habitat is affected and controlled by the physicochemical properties of the environment. Micropore entrapment as well as soil accumulation are the major causes xenobiotic persistence. Both aerobic and anaerobic bacteria, and fungi are effectively involved in the biodegradation of xenobiotics. These microbial transformations are sometimes fortuitous, a phenomenon quite common in microbiology. Xenobiotic compounds are often utilized as a source of energy, nitrogen, carbon, or sulfur by various microorganisms. But few xenobiotics are there that are resistant to microbial attack. Therefore, research should focus on understanding mechanism of the interaction between microorganisms and xenobiotic compounds in the environment that must include biochemical as well as genetic engineering areas. Such effective strategies then can play a pivotal role for successful environmental clean up by a diverse group of microorganisms for efficient remediation of toxic xenobiotic compounds.
1. Can any one tell what is the Status of recalcitrant/xenobiotic compounds in water bodies in India?
2. How to measure recalcitrant compounds?


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