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Amazing Catabolism

©2000 Timothy Paustian, University of Wisconsin-Madison

So far we have looked at forms of catabolism that many microorganisms possess. Now we take a walk on the wild side and explore an example of an unusual food source some microbes can utilize.

Nitroglycerin

Figure 1 - The Badger Ammunitions Plant.

From 1942-1976 the Badger Ammunitions Plant provided weapons for the military and handled large quantities of nitroglycerin (NG). Inevitably, some of this material found its way into the surrounding soil. Naturally occurring nitrated compounds are rare and have been found to exist in the environment for long periods of time, causing damage to many organisms that come in contact with them, including humans.

Figure 2 - The structure of nitroglycerin

In the past few years David Blehert of Professor Glenn Chambliss' lab has begun to look for organisms that were capable of degrading NG in the hope of using this information to help eliminate explosives from the environment. Several microbes were isolated that could metabolize NG and general pathways from Pseudomonas fluorescens and Pseudomonas putida were studied extensively. Below is a schematic of the pathway for NG degradation.

Figure 3 - Pathway of NG degradation.

It turns out that these Pseudomonads uses NG as a nitrogen source, but do not efficiently degrade NG to glycerol. They rapidly metabolize NG to mononitroglycerin and it is thought that other microbes in the soil will take the mononitroglycerin and degrade it to glycerol. In fact, Accashian and coworkers have recently demonstrated complete degradation of NG using a mixed bacterial culture. Glycerol can then be converted to glyceraldehyde-3-phosphate and metabolized by the Emden-Meyerhoff-Parnas pathway.

The steps in this degradation pathway in P. fluorescens and P. putidaare catalyzed by a single enzyme, xenobiotic reductase. Unlike the typical enzyme that is very selective for its substrate, xenobiotic reductase is nonspecific, recognizing many molecules with a nitro group and other electrophilic compounds. For example, the P. fluorescens xenobiotic reductase can also attack trinitrotoluene (TNT - the explosive) yielding several different products.

Figure 4 - Attack of TNT by xenobiotic reductase.

Having such a versatile enzyme allows P. fluorescens to attack many different molecules with just one enzyme instead of producing a separate enzyme for each potential compound. David has published some of his findings in the "Journal of Bacteriology".

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