This pathway was contributed by Charlotte Rosendahl Pedersen and updated by Carla Essenberg and Michael Turnbull, University of Minnesota.
Methyl tert-butyl ether (MTBE), an additive to gasoline, is used to replace other toxic components such as lead. This additive reduces the emissions of airborn toxics, carbon monoxide and volatile organic compounds. This compound was first used in the US in the 70's, but since the 1990 Clean Air Act, its use has gone up immensely. It is now the second most produced chemical in the US with more than 24 billion pounds used per year. There are problems with the storage of MTBE; it is now the second most common contaminant of urban aquifers in the US. MTBE is becoming a problem in drinking water too, and the toxicity of MTBE is not very well known. Some studies show that, in humans exposed to high amounts of MTBE in drinking water, it might induce programmed cell death.
While a number of microorganisms are known to metabolize MTBE, not much is known about the mechanism of degradation. The following pathway map is based primarily on Steffan et al. (1997), which examined the mechanism of MTBE degradation in Nocardia sp. ENV425. More recently, Francois et al. (2002) and Smith et al. (2003) have observed that Mycobacterium austroafricanum IFP 2012 and Mycobacterium vaccae JOB5 follow a slightly different pathway in the initial steps of MTBE degradation, converting MTBE to tert-butyl formate, which is then hydrolysed to tert-butyl alcohol and formate. The enzymes shown in these initial steps are those suggested by Smith et al. (2003).
Ferreira et al. (2006) reported the cloning and characterization of several genes from M. austroafricanum IFP 2012 involved in the lower portion of the MTBE pathway (from 2-methyl-2-hydroxy-1-propanol to 2-hydroxyisobutyrate). The exact route of 2-hydroisobutyrate metabolism is not certain; methacrylate, 2-propanol, and 2,3-dihydroxy-2-methyl propionate are all possible next products (Steffan et al., 1997). Wolinella succinogenes can anaerobically reduce methacrylate to isobutyrate (Gross et al., 2001). One strain of beta-Proteobacteria metabolizes MTBE producing tert-butyl alcohol (as above), 2-propanol, and acetone (Zhong et al., 2006).
The following is a text-format methyl-tert-butyl ether degradation pathway map. 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. This map is also available in graphic(15K) format.
Methyl tert-butyl ether Methyl tert-butyl ether Mycobacterium vaccae JOB5 Norcardia sp. ENV425 Mycobacterium austroafricanum IFP 2012 | | | | | | alkane | unspecific | 1-monooxygenase | monooxygenase | | v v [Hydroxymethyl tert-butyl ether] [Hydroxymethyl tert-butyl ether] | | | | | +--> Formaldehyde --> C1 Metabolic | | Cycle v carboxylesterase v tert-Butyl formate --------------+-------------> tert-Butyl alcohol | | v | Formate | alkane | | 1-monooxygenase v | C1 Metabolic | Cycle v 2-Methyl-2-hydroxy-1-propanol | | | 2-methyl-1,2-propanediol | dehydrogenase | v 1-Methyl-2-hydroxypropanal | | | hydroxyisobutyraldehyde | dehydrogenase | v +--------- 2-Hydroxyisobutyrate----------+ | beta-Proteobacteria | | | | | 2-hydroxy- | 2-hydroxy- | 2-hydroxy- | isobutyrate | isobutyrate | isobutyrate | dehydratase | decarboxylase | 3-monooxygenase | | | v v v [Methacrylate] 2-Propanol [2,3-Dihydroxy-2-methyl] Wolinella succinogenes | propionate] | | | | methacrylate | | | reductase | v C | | | | | | v v v Isobutyrate Intermediary 2-Hydroxy-2-methyl- Metabolism (KEGG) 1,3-dicarbonate | | | 2-Hydroxy-2-methyl- | 1,3-dicarbonate | decarboxylase | v Lactate | | | v Intermediary Metabolism (KEGG)
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