m-monooxygenation X) protein from Ralstonia bickettli (Hearn et al. 2008) have
been shown to facilitate the diffusion of small hydrocarbons.
Virtuallly all outermembrane proteins involved in such transport are beta barrel
proteins having an even number (between 8 and 24) of beta strands and have been
classified as porins (Hua and Wang 2014).
Microorganisms have been shown to store the transported hydrocarbon in intracellular inclusion bodies (Mishra and Singh 2012).
11.6 Metabolic Pathways and Molecular Basis
of Hydrocarbon Degradation
Several pathways for degradation and utilization of petroleum carbon energy by
microbes have been identified, both aerobic and anaerobic. For bacteria, many of the
metabolic pathways have been elucidated and commonly involves oxidation, reduction, hydroxylation, and dehydrogenation (Varjani 2017).
Aerobic biodegradation represents the more commonly utilized method for degradation of hydrocarbons by microorganisms and has been widely investigated. The
microbes overcome the low reactivity of n-alkanes by an initial oxidation reaction
using molecular oxygen. Three possible peripheral pathways have been identified;
terminal oxidation, which is probably the most commonly used, subterminal oxidation, and ω-oxidation. Oxidation of the n-alkane via monooxygenases converts the
alkane into its respective fatty alcohol. This is then further oxidized to the
corresponding aldehyde using alcohol dehydrogenase and aldehyde dehydrogenase,
and then to a fatty acid. The fatty acids are then conjugated to Coenzyme A and then
enter the beta oxidation pathway, finally forming acetyl CoA, which is then used for
intermediary metabolism by the organism (Wentzel et al. 2007).
Degradation of aromatic pathways require different metabolic pathways. The
saturated aromatic ring is cleaved through hydroxylation. As their molecular weight
increases, along with loss of aqueous solubility, they become increasingly
re-calcitrant. These, when degraded to smaller units, are completely oxidized via
the TCA cycle.
Linear alkanes are degraded via several enzyme types, among which the alkane
hydroxylases play a prominent role. Several classes of alkane hydroxylases have
been found in microorganisms (Wang et al. 2011).
1. Soluble non-heme di-iron monooxygenase (degradation of C1–C5 n-alkanes)
2. Membrane-bound particulate copper-containing enzyme (degradation of C1–C5
n-alkanes)
3. Membrane-bound n-alkane hydroxylases (AlkB) (degradation of C6–C16 nalkanes)
4. Membrane-bound cytochrome P450 enzymes (e.g.: Cyp52, Cyp153) (degradation of C6–C16 n-alkanes)
11 Microbial Bioremediation of Petroleum Hydrocarbons
277
been shown to facilitate the diffusion of small hydrocarbons.
Virtuallly all outermembrane proteins involved in such transport are beta barrel
proteins having an even number (between 8 and 24) of beta strands and have been
classified as porins (Hua and Wang 2014).
Microorganisms have been shown to store the transported hydrocarbon in intracellular inclusion bodies (Mishra and Singh 2012).
11.6 Metabolic Pathways and Molecular Basis
of Hydrocarbon Degradation
Several pathways for degradation and utilization of petroleum carbon energy by
microbes have been identified, both aerobic and anaerobic. For bacteria, many of the
metabolic pathways have been elucidated and commonly involves oxidation, reduction, hydroxylation, and dehydrogenation (Varjani 2017).
Aerobic biodegradation represents the more commonly utilized method for degradation of hydrocarbons by microorganisms and has been widely investigated. The
microbes overcome the low reactivity of n-alkanes by an initial oxidation reaction
using molecular oxygen. Three possible peripheral pathways have been identified;
terminal oxidation, which is probably the most commonly used, subterminal oxidation, and ω-oxidation. Oxidation of the n-alkane via monooxygenases converts the
alkane into its respective fatty alcohol. This is then further oxidized to the
corresponding aldehyde using alcohol dehydrogenase and aldehyde dehydrogenase,
and then to a fatty acid. The fatty acids are then conjugated to Coenzyme A and then
enter the beta oxidation pathway, finally forming acetyl CoA, which is then used for
intermediary metabolism by the organism (Wentzel et al. 2007).
Degradation of aromatic pathways require different metabolic pathways. The
saturated aromatic ring is cleaved through hydroxylation. As their molecular weight
increases, along with loss of aqueous solubility, they become increasingly
re-calcitrant. These, when degraded to smaller units, are completely oxidized via
the TCA cycle.
Linear alkanes are degraded via several enzyme types, among which the alkane
hydroxylases play a prominent role. Several classes of alkane hydroxylases have
been found in microorganisms (Wang et al. 2011).
1. Soluble non-heme di-iron monooxygenase (degradation of C1–C5 n-alkanes)
2. Membrane-bound particulate copper-containing enzyme (degradation of C1–C5
n-alkanes)
3. Membrane-bound n-alkane hydroxylases (AlkB) (degradation of C6–C16 nalkanes)
4. Membrane-bound cytochrome P450 enzymes (e.g.: Cyp52, Cyp153) (degradation of C6–C16 n-alkanes)
11 Microbial Bioremediation of Petroleum Hydrocarbons
277