Xenobiotic compound Defination,Classification

What Is Xenobiotic Compound?

Xenobiotic compounds are man-made chemicals that are present in the environment at unnaturally high concentrations. The xenobiotic compounds are either not produced naturally, or are produced at much lower concentrations than man. Microorganism have the capability of degrading all naturally occurring compounds; this is known as the principle of microbial infallibility proposed by Alexander in 1965.
Microorganisms are also able to degrade many of the xenobiotic compounds, but they are unable to degrade many others. The compounds that resist biodegradation and thereby persists in the environment are called recalcitrant
The evolution of microbial catabolic enzymes cannot keep pace with the rapid introduction of novel compounds into the environment. These new synthetic compounds that are slowly biodegradable or non-biodegradable are known as recalcitrant compounds, and range from simple halogenated hydrocarbons to complex polymers. Recalcitrant compounds can be made biodegradable by developing microorganisms capable of degrading the compound and by treating the compound to make it more conducive to mirobial attack.

Why the xenobiotic compounds are called Recalcitrant?

  1. They are not recognised as substrate by the existing degradative enzymes
  2. Theye Highly Stable
  3. They are chemically and physically Inert due to the presence of substitution groups like halogens, nitro-, sulphonate, amino-, methoxy- and carbamyl groups.
  4. Highly Toxic
  5. Insoluble in water

Write the types of recalcitrant xenobiotic compounds?

The xenobiotic compounds are classifed into six group:
(i) Halocarbons:
These compounds contain different numbers of halogen (e.g., CI, Br, F (fluorine), I) atoms in the place of H atoms. They are used as solvents (chloroform, CHCI3), as propellants in spray cans of cosmetics, paints etc., in condenser units of cooling systems (Freons, CCI3F, CCl2F2, CClF3, CF4), and as insecticides (DDT, BHC, lindane etc.) and herbicides (dalapon, 2, 4-D, 2, 4, 5-T etc.).
The C1-C2 haloalkanes like chloroform, freons etc. are volatile and escape into the atmosphere where they destroy the protective ozone (O3) layer leading to increased UV radiation. Pesticides (herbicides, fungicides and insecticides) are applied to crops from where they leach into water bodies; many of them are subject to bio-magnification.
(ii) Poly chlorinated Biphenyls (PCB’s):
These compounds have two covalently linked benzene rings having halogens substituting for H. PCB’s are used as plasticisers, insulator coolants in transformers and as heat exchange fluids. They are both biologically and chemically inert to various degrees, which increases with the number of chlorine atoms present in the molecule.
The recalcitrant nature of the above two groups of compounds is due to their halogenation and as well their cyclic structure (PCB’s).
(iii) Synthetic Polymers:
These compounds are produced as plastics, e.g., polyethylene, polystyrene, polyvinyl chloride etc., and nylons which are used as garments, wrapping materials etc. They are recalcitrant mainly due to their insolubility in water and molecular size.
(iv) Alkylbenzyl Sulfonates:
These are surface-active detergents superior to soaps. The sulphonate (— SO3–) group present at one end resists microbial degradation, while the other end (non-polar alkyl end) becomes recalcitrant if its is branched, (resistance increases with the degree of branching). At present, alkylbenzyl sulphonates having non-branched alkyl ends are used; these are biodegraded by β-oxidation from their alkyl ends.
(v) Oil Mixtures:
Oil is a natural product, has many components and is biodegradable, the different components being degraded at different rates. Biodegradation is able to handle small oil seepages. But when large spills occur the problem of pollution becomes acute. Oil is recalcitrant mainly because of its insolubility in water and due to the toxicity of some of its components.
(vi) Other Xenobiotic Compounds:
A number of pesticides are based on aliphatic, cyclic ring structures containing substitution of nitro-, sulphonate, methoxy-, amino- and carbomyl groups; in addition, they also contain halogens. These substitutions make them recalcitrant.

Hazards from Xenobiotic Compounds:

The xenobiotics present a number of potential hazards to man and the environment which are briefly listed below.
(i) Toxicity:
Many xenobiotics like halogenated and aromatic hycrocarbons are toxic to bacteria, lower eukaryotes and even humans. At low concentrations they may cause various skin problems and reduce reproductive potential.
(ii) Carcinogenicity:
Certain halogenated hydrocarbons have been shown to be carcinogenic.
(iii) Many xenobiotics are recalcitrant and persist in the environment so that there is a build up in their concentration with time.
(iv) Many xenobiotics including DDT and PCB’s are recalcitrant and lipophilic; as a consequence they show bioaccumulation or bio-magnification often by a factor of 104 – 106.

Refference

  • http  ://www.biologydiscussion.com/microbiology-2/bioremediation/xenobiotic-compounds-meaning-hazards-and-biodegradation/55625
  • http  s://www.sciencedirect.com/science/article/pii/0141022979900413 �ƎU���u

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