Petroleum degradation- Alkanes & Alicyclic Hydrocarbons

The most rapid and complete degradation of the majority of organic pollutants is brought about under aerobic conditions. The initial attack is an oxidative process catalyzed by oxygenases. Degradation pathways convert organic pollutants step by step into intermediates of metabolic pathway like, the tricarboxylic acid cycle. Biosynthesis of cell biomass occurs from the metabolites like acetyl-CoA, succinate, pyruvate.

Petroleum hydrocarbons

1)      Initial attack on alkanes by monoxygenases and dioxygenases

Commonly, the initial terminal methyl group is targeted to form primary alcohol by oxidation, followed by aldehyde and then fatty acid. Further metabolism by β oxidation

Alkane+O₂+NADPH₂à1^o alcohol +NADP+H₂O

2)      Alkane + O₂ +NADPH₂à labile hydroperoxide intermediate -à Alcohol +NADP + H₂O

1^o alcohol +O₂ à Aldehyde + O₂ à Fatty acid à β oxidation

3)      Occasionally, both the terminal methyl groups are oxidised to form a dicarboxylic acid- diterminal oxidation or ω (omega) oxidation. Helps in bypassing a block to β oxidation due to a branching of C atom. Fatty acid, once formed, further catabolism by β oxidation.

          Alkane+O₂àdicarboxylic acid          -  ω (omega) oxidation

        Fatty acids once formed are subjected to beta oxidation. Fatty acid reacts with Acyl CoA and is converted to its acyl coenzyme A form.Then, acted upon by a series of enzymes. 

(Acyl-CoA is a group of coenzymes that metabolize fatty acids. Acyl-CoA's are susceptible to beta oxidation, forming, ultimately, acetyl-CoA.)

Thus, at the end of beta oxidation, an acetyl CoA is liberated along with  a fatty acid is shortened by a two Carbon unit  (than of parent fatty acid). The sequence is repeated.

 The acetyl CoA units are converted to CO₂ through the TCA cycle. The end products of hydrocarbon degradation/mineralization are CO₂ and H₂O.

β oxidation 

(Steps include Hydration, dehydrogenation, oxidation and thyolisis)

β oxidation does not require molecular oxygen. After the initial oxygenation, fatty acid degradation can proceed under anaerobic conditions.

4) A direct dehyhrogenation of an intact hydrocarbon leading through 1-alkene, alcohol, aldehyde to  a fatty acid can happen as an anoxic biodegradation mechanism, in the presence of an electron acceptor such as nitrate.

Anoxic biodegradation

Though this is possible, it is not much demonstrated. Usually, under geologic and environmental conditions present, in the absence of molecular oxygen, hydrocarbons persist.

5) Alkanes are attacked sub terminally; oxygen is inserted on a carbon atom within the chain instead of at its end. Thus a secondary alcohol is formed first, which is further oxidised to acetone, and finally to an ester. The ester bond is cleaved yielding a primary alcohol and a fatty acid. The sum of carbon atoms of these two fragments is equal to that of the parent hydrocarbon.

            The alcohol is oxidised to aldehyde then fatty acid. Further metabolism by β oxidation

Alkane à 2 ^o alcohol à Acetone à  Ester (1)

Ester à1^o alcohol + Fatty acid (2)

1^o alcohol à Aldehyde à Fatty acid (3)

 

Alicyclic Hydrocarbons

Alicyclic Hydrocarbons with no terminal methyl groups. Eg., Cyclohexane 

Cyclohexane is acted upon by a monooxygenase forming an alicyclic alcohol, cyclohexanol, which is dehydrogenated to form cyclohexanone (ketone). Cyclohexanone is further oxidised to form a lactone. Lactone is oxidised to an alcohol, aldehyde and then a dicarboxylic acid. Further metabolism is by β oxidation.

Microorganisms are found to perform these reactions and grow on cyclohexane. More frequently however, microorganisms are unable to lactonize and open the ring. Commensalism and cometabolism thus play an important role in the biodegradation of Alicyclic Hydrocarbons