Pathways for degradation of longer chain alkanes are less clear, although a few
genes, namely ladA and almA have been identified. The ladA encodes a
monooxygenase that is responsible for terminal oxidation of alkanes >C16, first
identified in Geobacillus thermodentrificans (Feng et al. 2007). The almA encodes a
flavin-binding soluble monooxygenase that is responsible for degradation of C32
and longer alkanes (Li et al. 2008; Wentzel et al. 2007).
It has been reasonably well established that the alkane hydroxylase encoded by
alkB gene is a key player in hydrocarbon degradation pathways. AlkB is a
rubredoxin-dependent enzyme, and often both genes are found close together
when present. Both the alkB gene and the alcohol dehydrogenase have been reported
to be induced during hexadecane degradation in several bacterial species;
Rodococcus sp. NJ2 (Mishra and Singh 2012) and Geobacillus sp. (Tourova et al.
2018).
Analysis of the genomes of several Geobacillus strains using degenerate PCR
primers (Tourova et al. 2018) has shown the presence of multiple alkB genes that
encode the alkane-1 mono-oxygenase. The alkB genes in Geobacillus appear to be
located on a plasmid and are thought to have been transferred to Geobacilli from
Rhodococci or other related microbe (Tourova et al. 2018).
Another recently reported Pseudomonas aeruginosa strain (DN1) was found to
contain multiple alkane biodegradation systems, namely two homologs of alkB
(alkB 1 and alkB 2 ), a cyp153 homolog and two homologs of alm-like gene (almA 1
and almA 2 ). The strain demonstrated efficient (>85%) degradation of crude oil
containing alkanes ranging from C8 to C40. Contrary to current knowledge that
the alkB system is adapted for degradation of alkanes up to C16, in this strain the
alkB genes were found to be upregulated in the presence of longer alkanes, C20 and
C32 (Li et al. 2019).
The Dietzia sp. DQ12-45-1b has both alkB (coding for alkane monooxygenase)
and cyp153 genes (coding for P450 alkane hydroxylase of the cytochrome Cyp153
family), and their induction was detected. It was capable of utilizing a wide range of
n-alkanes (C6–C40), aromatic compounds, and crude oil as the sole carbon source
for growth (Wang et al. 2011).
Peroxygenase secreted by Agrocybe aegerita has been shown to catalyze with
high efficiency, the hydroxylation of linear alkanes at the 2-position and 3-position
using H 2 O 2 as a co-substrate, as well as the regioselective monooxygenation of
branched and cyclic alkanes. However, the peroxygenase appeared to lack activity
on long-chain alkanes (>C16) and highly branched alkanes (e.g.,
tetramethylpentane) (Peter et al. 2011).
Fungi (and some bacteria also) have been reported to use the cytochrome P450
family genes for initiating the degradation of petroleum hydrocarbons. Cytochrome
P450 protein isolated from microsomal membrane fractions of Candida maltosa has
been shown to be involved in the hydroxylation of hexadecane. Analysis of intermediates of n-hexadecane oxidation led to the conclusion that mono-terminal attack
was predominant, whereas di-terminal oxidation proceeded as a minor reaction
(Blasig et al. 1988).
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genes, namely ladA and almA have been identified. The ladA encodes a
monooxygenase that is responsible for terminal oxidation of alkanes >C16, first
identified in Geobacillus thermodentrificans (Feng et al. 2007). The almA encodes a
flavin-binding soluble monooxygenase that is responsible for degradation of C32
and longer alkanes (Li et al. 2008; Wentzel et al. 2007).
It has been reasonably well established that the alkane hydroxylase encoded by
alkB gene is a key player in hydrocarbon degradation pathways. AlkB is a
rubredoxin-dependent enzyme, and often both genes are found close together
when present. Both the alkB gene and the alcohol dehydrogenase have been reported
to be induced during hexadecane degradation in several bacterial species;
Rodococcus sp. NJ2 (Mishra and Singh 2012) and Geobacillus sp. (Tourova et al.
2018).
Analysis of the genomes of several Geobacillus strains using degenerate PCR
primers (Tourova et al. 2018) has shown the presence of multiple alkB genes that
encode the alkane-1 mono-oxygenase. The alkB genes in Geobacillus appear to be
located on a plasmid and are thought to have been transferred to Geobacilli from
Rhodococci or other related microbe (Tourova et al. 2018).
Another recently reported Pseudomonas aeruginosa strain (DN1) was found to
contain multiple alkane biodegradation systems, namely two homologs of alkB
(alkB 1 and alkB 2 ), a cyp153 homolog and two homologs of alm-like gene (almA 1
and almA 2 ). The strain demonstrated efficient (>85%) degradation of crude oil
containing alkanes ranging from C8 to C40. Contrary to current knowledge that
the alkB system is adapted for degradation of alkanes up to C16, in this strain the
alkB genes were found to be upregulated in the presence of longer alkanes, C20 and
C32 (Li et al. 2019).
The Dietzia sp. DQ12-45-1b has both alkB (coding for alkane monooxygenase)
and cyp153 genes (coding for P450 alkane hydroxylase of the cytochrome Cyp153
family), and their induction was detected. It was capable of utilizing a wide range of
n-alkanes (C6–C40), aromatic compounds, and crude oil as the sole carbon source
for growth (Wang et al. 2011).
Peroxygenase secreted by Agrocybe aegerita has been shown to catalyze with
high efficiency, the hydroxylation of linear alkanes at the 2-position and 3-position
using H 2 O 2 as a co-substrate, as well as the regioselective monooxygenation of
branched and cyclic alkanes. However, the peroxygenase appeared to lack activity
on long-chain alkanes (>C16) and highly branched alkanes (e.g.,
tetramethylpentane) (Peter et al. 2011).
Fungi (and some bacteria also) have been reported to use the cytochrome P450
family genes for initiating the degradation of petroleum hydrocarbons. Cytochrome
P450 protein isolated from microsomal membrane fractions of Candida maltosa has
been shown to be involved in the hydroxylation of hexadecane. Analysis of intermediates of n-hexadecane oxidation led to the conclusion that mono-terminal attack
was predominant, whereas di-terminal oxidation proceeded as a minor reaction
(Blasig et al. 1988).
278
S. Jayasena and M. Perera