Xenobiotic compounds

Man-made chemicals present in the nature at high concentrations polluting the environment is known as Xenobiotic compounds. These compounds are not commonly produced by nature. Some microbes have been seen to be capable of breaking down of xenobiotics to some extent. But most of the xenobiotic compounds are non-degradable in nature. Such compounds are known to be recalcitrant in nature.

A commonly used example of this is the effect experienced by fish that live downstream from the outlet of a sewage treatment plant. Hormones produced by humans may be present, even in treated water, and these compounds are foreign as far as fish are concerned.

Types of Xenobiotic compounds
There are two types of xenobiotic compounds. They may be biodegradable or non-degradable (recalcitrant). Biodegradable xenobiotic compounds are those that get degraded by the action of microbes or other reactions while recalcitrant compounds are resistant to degradation by any reactions.
Types of Recalcitrant Xenobiotic Compounds
The recalcitrant xenobiotic compounds can be grouped into the following six types:
1)  halocarbons,
2)  polychlorinated biphenyls,
3) synthetic polymers,
4) alkylbenzyl sulphonate,
5) oil mixture and
6) others. 

Properties of xenobiotic

The properties of xenobiotic compounds attributing to its recalcitrant properties are:

(i) Non recognizable as substrate by microbes to act upon and degrade it.
(ii) It does not contain permease which is needed for transport into microbial cell.
(iii) Large molecular nature makes it difficult to enter microbial cell.
(iv) They are highly stable and insolubility to water adds to this property.
(v) Mostly toxic in nature.

Xenobiotic metabolism

Xenobiotic metabolism refers to the various chemical reactions, called metabolic pathways that a living organism uses to alter chemicals that are not normally found in an organism as part of its natural biochemistry. These chemicals, called xenobiotics, can include things such as poisons, drugs, and environmental pollutants. Xenobiotic metabolism is important for life, as it allows an organism to neutralize and eliminate foreign toxins that would otherwise interfere with the chemical processes that keep it alive. The xenobiotic metabolism of humans and many other forms of life is important in fields such as medicine, agriculture, and environmental science.

In the first stage of xenobiotic metabolism, the foreign substance is modified through chemical reactions that add polar or reactive groups to its molecules. This is most commonly done with enzymes that catalyze monooxygenase reactions with oxygen molecules, or O2, and hydrogen, adding one atom of oxygen from the O2 to the xenobiotic molecule and producing a molecule of water as a byproduct. The most prominent group of proteins involved in this stage is the cytochrome P450 family, which encompasses more than 11,500 different proteins and is present in all forms of life on Earth.

The modified xenobiotic is then detoxified through reactions with other molecules, combining with them to form molecules called xenobiotic conjugates. Chemicals commonly used in this phase include glycine (C2H5NO2), glutathione (C10H17N3O6S), and glucuronic acid (C6H10O7). These molecules are anionic, meaning that they contain more electrons than protons and so have a negative electrical charge. Depending on the substance involved, the resulting conjugates may undergo further chemical reactions in the course of detoxification.

Finally, the conjugate is excreted from the cell. Its negatively charged anionic groups allow it to bind with protein transporter molecules, which carry the conjugate across the cellular membrane and out of the cell. From there the xenobiotic may be further metabolized by extracellular biochemicals or expelled from the body entirely in sweat, urine, or feces.

Biodegradation:

Certain microbes on continuous exposure to xenobiotics develop the ability to degrade the same as a result of mutations. Mutations resulted in modification of gene of microbes so that the active site of enzymes is modified to show increased affinity to xenobiotics. Certain mutations also resulted in developing new enzymatic pathway for xenobiotic degradation. Use of mixed population of microbes is usually recommended as it has been seen to yield faster results as the two different microbes attack different parts through different mechanisms resulting in effective break down. It also creates a condition of co metabolism. The modification of certain genes of microbes to break down xenobiotics is also recommended and is seen to produce high level of accuracy.

Basis of biodegradation

Microbes play the most important role in the process of biodegradation. Certain abiotic mechanisms and photo-oxidation also play an important role in the degradation of certain organic chemicals but such transformations are generally incomplete because these processes cannot convert the compound into inorganic form

Before the industrialization, biosphere of earth remained constant because of more or less balanced biosynthesis and biodegradation reactions. During the course of evolution, a vast variety of chemical compounds are biosynthesized in nature and microbes were exposed to these compounds. For this millions of year's exposure, they have developed the capacity and mechanism to attack these compounds. Out of several chemical compounds synthesized by chemists, many have structural features and bonding similar to that of natural compounds, so can be biodegraded

Biodegradation of aromatic compounds

Sources of aromatic compounds in the environment include degradation of lignin in plants, use of detergents, pesticides, drugs and dyes etc. Several polycyclic aromatic hydrocarbons (PAH) released from industrial processes are carcinogenic. Variety of microbes including bacteria, fungi, yeast can act on these compounds and degrade them. Benzene, toluene, xylene, ethylbenzene are degraded by bacteria. Presence of methyl, chloro, nitro, amino and sulphonyl group in benzene ring cause recalcitrancy of the compound. Catechol and protocatechuate are the intermediates in the catabolism of benzene, phenol, vanillin, shikimate.

Both are degraded by two different mechanisms:-

1) Ortho cleavage (intradiol fission):- catechol is cleaved between two phenolic hydroxyl groups generating cis-cis muconic acid. This muconic acid is converted into oxo-adipic acid, which is finally degraded into acetyl CoA and succinate, which then enters into TCA cycle.

2) Meta fission (extradiol cleavage):- catechol is cleaved to 2-hydroxy-muconic semialdehyde which is converted to acetaldehyde and pyruvate via 4-hydroxy-2-oxo valerate.

Biodegradation of pesticides

In soil, herbicides and pesticides are degraded at different rates. As example,

1) Diquat and paraquat are photolysed to α-picolinate and N-methyl-isonicotinate, respectively, which are then degraded by microbes. Binding of herbicides to soil particles retard their biodegradation.

2) Parathion hydrolase from Pseudomonas diminuta is encoded by plasmids, which hydrolyse parathion to p-nitrophenol, the latter is then degraded by other microbes.

Biodegradation of chloroaromatic compounds

Chloroaromatic compounds are the pollutants of major concern and are toxic, resist to biodegradation. Chlorobenzene, dichlorobenzene and trichlorobenzene are not easily biodegraded in biological treatment system. However, they are degraded by some soil microorganisms and also co-metabolically by strains of Pseudomonas putida.

Chlorophenols and chlorocatechol are the intermediate compounds in the biodegradation of chlorobenzenes, various pesticides and other chloroaromatic compounds.

2, 4-dichlorophenol is readily degraded by many pseudomonas, achromobactor and arthrobactor. Polychlorinated phenols, especially pentachlorophenol (PCP) used in wood preservation, as fungicide and herbicide. PCP inhibits anaerobic digestion of sludge. Strains of pseudomonas, arthrobactor, flavobacterium use PCP as sol.

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