CO METABOLISM

Cometabolism is an approach to Biological degradation of hazardous solvents. Using methane as the primary energy source, some microbes release enzymes that degrade the chlorinated solvents.
Cometabolism is defined as the simultaneous metabolism 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 degrading methane, some bacteria can degrade hazardous chlorinated solvents that they would otherwise be unable to attack.




Co-metabolism can be defined as:
The transformation of an organic compound by a microorganism incapable of using the substrate as a source of energy or of one of its constituent elements
(Alexander, 1967. Agriculture and the Quality of Our Environment (N.C. Brady, Ed.) pp 331-342. Am. Soc. Adv. Sci. Washington D.C.)
What this means is that the organism in question derives no benefit from the co-metabolism of the compound. No energy is available, nor are any of the (C, N, P, S, etc) elements in the compound used as a significant source of material for biosynthetic activity.
The term "secondary substrate metabolism" is also in the literature - this means that the organism is actually growing on a second substrate and is transforming another substrate at the same without gaining benefit. This undoubtedly occurs, but is very difficult to detect and prove in natural environments, even though it can be demonstrated in pure cultures.
For this reason, co-metabolism is taken to be either case:
1. The organism in question is NOT growing on another substrate - it may not even be proliferating at all.
2. The organism IS growing on another substrate (known or unknown) but is also co-metabolizing the compound in question.
Mechanisms of co-metabolism
1. Initial enzyme or enzymes change the substrate to a product that is not further transformed by the other enzymes in the organism to anything that can be further metabolized. Do enzymes have many different substrates ?
2. The original substrate is transformed to a product that inhibits later enzymes in a metabolic sequence or .inhibits the growth of the organism.
3. The organism needs a second substrate to achieve some later reaction - that substrate is missing.
The first explanation is the most likely in most cases.
It implies that co-metabolism is "accidental" or "fortuitous". An enzyme can react with a compound, produce no energy for the cell, nor incorporate any of the elements of the compound into biosynthetic processes, but still achieve partial transformation of the compound. It may be that many of the compounds co-metabolized are similar to normal substrates of the cells, differing sufficiently so that their immediate products cannot be further metabolized.
For example:
Enzyme A ----------> Enzyme B -------------> Enzyme C
Substrate A ----------> Product B ------------> Product C
Substrate Ax-----------> Product Bx [not metabolized by enzyme C]
Substrate Ax is "sufficiently similar" to Substrate A that Enzyme A can transform it to Bx, but Bx is "sufficiently different" to B so as to prevent further metabolism by Enzyme C.