08-18-2013, 04:49 AM
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.
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.
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