Cometabolism

Co-metabolism 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). For example, in the process of metabolizing methane, propane or simple sugars, some bacteria, such as Pseudomonas stutzeri OX1, can degrade hazardous chlorinated solvents, such as tetrachloroethylene and trichloroethylene, that they would otherwise be unable to attack. They do this by producing methane monooxygenase, an enzyme which is known to oxidize numerous compounds, including pollutants such as chlorinated solvents, via co-metabolism. Co-metabolism is thus used as an approach to biological degradation of hazardous solvents.

Another example is Mycobacterium vaccae, which uses an 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.^[1]

^[2]

Another promising method of bioremediation of chlorinated solvents involves co-metabolism of the contaminants by aerobic microorganisms in groundwater and soils. Several aerobic microorganisms have been demonstrated to be capable of doing this, including methane oxidizers, phenol-degraders, and toluene-degraders. Unlike reductive dechlorination, the chlorinated compounds are completely mineralized to CO2 and chloride with no intermediates making co-metabolism an attractive alternative where it can be sustained. However, the microorganisms gain no energy from these processes, limiting the ability of cells to co-metabolize chlorinated compounds. This, together with the difficulties and high costs of maintaining substrate and an oxic environment, have led to limited field-scale application of co-metabolism for solvent degradation

References

1. Beam and Perry (1974)
2. Ryoo, D., Shim, H., Canada, K., Barbieri, P., Wood T. K. (2000) Aerobic Degradation of Tetrachloroethylene by Toluene-o-xylene Monooxoygenase of Pseudomonas Stutzeri OX1, Nat Biotechnol, 18: 775-778.

Co-metabolism--the transformation of a non-growth substrate in the obligate presence of a growth substrate or another transformable compound. The term 'non-growth substrate' describes compounds that are unable to support cell replication as opposed to an increase in biomass. This definition was devised primarily as a result of non-growth substrate metabolism studies with methane-utilizing bacteria. (vinay)