The bacteria that degrade propylene were initially studied for their potential to degrade chlorinated alkenes and produce optically pure epoxides (Small, 1995). The first metabolite of the propylene degradation pathway is 1,2-Epoxypropane. This compound is an aliphatic epoxide with known stereochemistry of 95% (R)-1,2-Epoxypropane and 5% (S)-1,2-Epoxypropane. Aliphatic epoxides have been extensively studied because of their toxicity, mutagenicity and potential carcinogenicity (Wade, 1978). The study of propylene degradation will help to understand the way in which toxic alkene epoxides are degraded by bacteria. Propylene is also a source of carbon and energy for different bacteria such as Xanthobacter Py2.
The degradation of propylene leads two compounds that can go into intermediary metabolism, acetoacetone and actetone. The reaction that produces these intermediates is catalyzed by an interesting epoxide carboxylase. In the presence of carbon dioxide this enzyme converts propylene oxide into acetoacetate. When carbon dioxide is not present, there is an enzyme artifact that releases the intermediate from the enzyme active site as acetone.
Epoxide carboxylase is a four-component "multiprotein enzyme system". Each of the four enzyme component functions independently of the others: Component I is epoxyalkane:CoM transferase, Components III and IV are stereoselective 2-hydroxypropyl-CoM dehydrogenases, Component II is a dual function NADPH:2-ketopropyl-CoM oxidoreductase/carboxylase. In addition, Allen et al. (1999) reported that coenzyme M (2-mercaptoethanesulfonic acid), which serves as a C3 carrier during the reaction, is a central cofactor of propylene oxide carboxylation. Coenzyme M previously was only found in the methanogenic Archaea, where it plays a central role in the reductive formation of methane.
The following is a text-format propylene pathway map. This map is also available in graphic (8k) format . Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions.
Propylene Xanthobacter sp. Py2 | | | | / \ / \ / \ alkene monoxygenase / \ alkene monoxygenase / \ / \ / \ v v (R)-1,2-Epoxypropane (S)-1,2-Epoxypropane | | 2-hydroxypropyl | | 2-hydroxypropyl -CoM lyase | | -CoM lyase | | | | v v (R)-2-Hydroxy- (S)-2-Hydroxy- propyl-CoM propyl-CoM \ / \ / \ / 2-(R)-hydroxypropyl- \ / 2-(S)-hydroxypropyl- CoM dehydrogenase \ / CoM dehydrogenase \ / \ / | | | v 2-Oxypropyl-CoM | | 2-oxopropyl- | CoM reductase | v Acetoacetate | | | | v Intermediary Metabolism (KEGG)
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