Cometabolism

Cometabolism is defined as the simultaneous degradation of two compounds, in which the degradation of the second compound (the secondary substrate) depends on the presence of the first compound (the primary substrate).  This shouldn’t be confused with simultaneous catabolism, where each substrate is catabolized concomitantly by different enzymes. Cometabolism occurs when an enzyme produced by an organism to catalyze the degradation of it's growth-substrate to derive energy and carbon from it is also capable of degrading additional compounds. The fortituous degradation of these additional compounds does not support the growth of the bacteria, and some of these compounds can even be toxic in certain concentrations and limit the growth of the bacteria.

The first report of this phenomena was the degredation of ethene by the species Pseudomonas methanica. These bacteria degrade their growth-substrate methane with the enzyme methane monooxygenase(MMO). MMO was discovered to be capable of catalyzing the degradation ethene and propene, although the bacteria were unable to use these compounds as energy and carbon sources to grow. 

Another example is Mycobacterium vaccae, which uses an alkane monooxygenase enzyme to oxidize propane. Accidentally, this enzyme also oxidizes, at no additional cost for M. vaccae, cyclohexane into cyclohexanol. Thus, cyclohexane is co-metabolized in the presence of propane. This allows for the commensal growth of Pseudomonas on cyclohexane. The latter can metabolize cyclohexanol, but not cyclohexane. 

Cometabolism in Bioremediation

Some of the molecules that are cometabolically degraded by bacteria are xenobiotic, persistent compounds, such as PCE, TCE, and MTBE, that have harmful effects on several types of environments. Co-metabolism is thus used as an approach to biologically degrade hazardous solvents.

For instance, cometabolism can be used for the biodegradation of the pollutant methyl-tert-butyl ether (MTBE): a chemical synthesized by the use of fossil fuels and that is toxic to both ground and underground aqueous environments. Pseudomonas aeruginosa and Pseudomonas citronellolis were shown to be able to carry out the cometabolic degradation of MTBE and fully degrade it by using their enzymes that have a physiological role of oxidizing n-alkane (e.g. methane, propane) to utilize them as growth sources.

Additionally, a promising method of bioremediation of chlorinated solvents involves cometabolism of the contaminants by aerobic microorganisms in groundwater and soils. Several aerobic microorganisms have been demonstrated to be capable of doing this, including n-alkane, aromatic compound (e.g. toluene, phenol) and ammonium oxidizers.

One example is Pseudomonas stutzeri OX1, which can degrade a hazardous, and water-soluble compound tetrachloroethylene (PCE). PCE, one of the major underground water contaminants, was regarded as being undegradable under aerobic conditions and only degraded via reductive dehalogenation to be used as a growth-substrate by organisms. Reductive dehalogenation often results in the partial dechlorination of the PCE, which gives rise to toxic and carcinogenic compounds such as TCE, DCE, and vinyl chloride. Pseudomonas st. OX1 fully degrade PCE by using toluene-o-xylene monooxygenase (ToMO), an enzyme they produce to derive energy and carbon from toluene and several other aromatic compounds. Thus, cometabolism poses as an potential pathway to remove PCE from polluted sites.

The difficulties and high costs of maintaining the growth-substrates of the organisms capable of cometabolising these hazardous compounds and providing them an aerobic environment have led to the limited field-scale application of co-metabolism for pollutant solvent degradation. Recently, this method of remediation has been proposed to be improved by the substitution of the synthetic aromatic growth-substrates (e.g. toluene) of these bacteria with cheap, non-toxic plant secondary metabolites. This would allow the cometabolism of the pollutant cDCE (cis-1,2-dichloroethene) while taking away the requirement of adding toxic growth-substrates of the bacteria to the polluted site.