Microbial Biotransformation of xenobiotic compounds

Microbial transformation of xenobiotic compounds is discussed.

1. Microbial Biotransformation of Xenobiotic compounds

2. Xenobiotic Compounds The term Xenobiotic means a chemical substance that is not a natural component of a living organism exposed to it, i.e. a strange, exogenous substance or anthropogenic material. An important group of xenobiotics are chemical compounds produced by humans, with artificial chemical structure, to which organisms have not adjusted through prior evolution.

3. Xenobiotic compounds Man made chemicals, not produced naturally Present in the environment at unnaturally high concentrations Microbes have the capability to degrade all naturally occurring compounds Microbes can degrade many of the xenobiotic compounds, but some remain non-degradable – these are called Recalcitrant compounds

4. Recalcitrant Xenobiotic compounds Xenobiotics may be recalcitrant due to: 1. they are not recognized as substrates by the existing degradative enzymes 2. they are highly stable, chemically and biologically inert 3. they are insoluble in water/adsorbed to soil 4. they are highly toxic 5. their large molecular weight prevents entry into microbial cells

5. Types of recalcitrant Xenobiotic compounds Types of recalcitrant xenobiotic compounds: 1. Polychlorinated biphenyls 2. Synthetic polymers 3. Alkylbenzyl sulphonates 4. Oil mixtures 5. Others like Pesticides etc.

6. Halocarbons - PCBs Halocarbons – These compounds contain different number of halogens as substitutes. These are used as solvents, as propellants, sprays, cosmetics, paints in condensor units of cooling systems ( Freons ), Insecticides, Herbicides Polychlorinated Biphenyls (PCBs) – These compounds have two covalently linked benzene rings having halogens as substitites PCBs are used as plasticisers , insulator coolants.

7. Synthetic Polymers These are produced as plastics – polyethylene, polystyrene, polyvinyl chloride, nylon etc These are recalcitrant due to their insolubility and high molecular weight.

8. Other Xenobiotics A number of pesticides are based on aliphatic, cycling ring structures containing substitution of nitro, sulphonate , methoxy , amino and carbomyl groups

9. Hazards of Xenobiotics Toxicity – many xenobiotics are toxic to bacteria, eukaryotes, even humans Carcinogenicity Recalcitrant compounds accumulate in environment causing pollution Many compounds show bioaccumulation and Biomagnification

10. Transformation of Xenobiotics Biodegradation, mineralisation or transformation Mineralisation is the complete decomposition of an organic compound into inorganic elements. Biodegradation is a process of decomposition of organic compounds into inorganic elements taking place with the participation of living organisms with the simultaneous accretion of biomass. Biotransformation is the process leading to the change of the structure of the original chemical compound to such degree that its original characteristic properties change as well. Biotransformation modifies not only the physico -chemical properties of compounds, such as solubility or (bio)availability, but also the toxicity level of the given xenobiotic .

11. Possible ways of biological transformations of xenobiotics in the environment

12. Rate of transformation The scope and rate of all transformations of xenobiotics depends on: Chemical structure and concentration of the xenobiotic Type and number of microorganisms capable of degrading or transforming the xenobiotic The physico -chemical properties of the environment

13. Bioavailability of xenobiotics The bioavailability of a xenobiotic depends on its state (solid, liquid or gas) water solubility capability of adsorbing and adhering to solid particles of soil or sediment Water-dissolved fraction of the xenobiotic is available to the microorganisms Majority of xenobiotics exhibit significant hydrophobicity are immobilised on the solid particles of the matrix by sorption The process of desorption, that is, releasing the contamination, is the result of a collaboration between the physicochemical factors of the environment Passing the physiological barrier of cellular membranes of microorganisms is the key stage in the process of transformation of xenobiotics

14. Biodegradation of Xenobiotics General features: Since xenobiotics consist of a wide variety of compounds their degradation occurs via many metabolic pathways. Biodegradation involves conversion of complex molecules to simpler molecules or if possible to CO 2 and H 2 O. This is carried out via aerobic metabolism or anaerobic metabolism

15. Aerobic metabolism - Microbes use O 2 in their metabolism to degrade contaminants Anaerobic metabolism - Microbes substitute another chemical for O 2 to degrade contaminants Nitrate, iron, sulfate, carbon dioxide, uranium, technicium , perchlorate The microbes degrade oil or other organic contaminants like Polycyclic Aromatic Hydrocarbons (PAHs) either by aerobic biodegradation or anaerobic biodegradation.

16. 16 Biodegradation of Petroleum compounds H.C. (substrate) + O 2 H.C.-OH + H 2 O carboxylic acids H.C. (substrate) + O 2 H.C. OH OH monooxygenase dioxygenase

17. The ring structure is opened up in case of cyclic compounds. Linear molecule is degraded by β -oxidation to acetyl CoA which is further metabolized through TCA cycle. The complete biodegradation (mineralization) of hydrocarbons produces the non-toxic end products carbon dioxide and water, as well as cell biomass (largely protein) which can be safely assimilated into the food web (Atlas, 1995).

18. Alkanes are subsequently converted to carboxylic acids that are further biodegraded via ß-oxidation to yield acetyl CoA Biodegradation of Petroleum compounds

19. Biodegradation of Petroleum compounds Alicyclic hydrocarbons are subsequently converted to carboxylic acids that are further biodegraded via ß-oxidation to yield acetyl CoA

20. Biodegradation of Petroleum compounds Aromatic hydrocarbon rings generally are hydroxylated by Dioxygenases to form diols ; the rings are then cleaved with the formation of catechols Catechol is further metabolized either via Ortho ring cleavage or Meta ring clevage . Subsequently degraded to intermediates of the tricarboxylic acid cycle.

21. Degradation of Halogenated Xenobiotics Biodegradation of Halogenated compounds Involves – elimination of halogen group and degradation of non-halogenated product Removal of halogen molecule may involve removal of hydrogen halide or substitution of halide by H or OH or thio groups The aerobic degradation of halogenated aromatic compounds involves the following steps – 1. addition of OH group by a dioxygenase to yield chlorinated catechol 2. cleavage of ring by ortho pr meta cleavage 3. elimination of halogen from straight chain product 4. degradation of aliphatic hydrocarbon product

22. Degradation of Halogenated Xenobiotics

23. Microorganisms for the aerobic transformations of xenobiotics The diversity of microorganisms participating in the aerobic transformations of xenobiotics is vast . The majority of those microorganisms include the following species: Pseudomonas, Acinetobacter , Alcaligenes , Bacillus, Rhodoccocus , Micrococcus orStreptomyces

24. Decomposition of xenobiotics in anaerobic conditions The processes of xenobiotic transformations in the conditions of oxygen deficiency The strains isolated thus far, transforming the aromatic xenobiotics , belong mainly to bacteria reducing nitrate(V ), sulphate(VI ), ferrum (III), vanadium(V), chromium(VI) ions, as well as photosynthesising purple bacteria and fermentation bacteria . The central intermediate of those transformations is benzoyl - CoA which is produced by way of numerous transformations

25. Origin of Capacity to degrade Xenobiotics The continued exposure of microorganism to xenobiotics lead to evolution of partial or complete xenobiotic degradation capacities in microbes through metabolic pathways. The molecular mechanisms involves: 1. mutation 2. transfer of genes through plasmids eg . TOL plasmids, pAC21, pAC25