Isooctane (2,2,4-trimethylpentane) is a common component of gasoline which serves as the 100 point on the octane rating scale (Solano-Serena et al., 2004). Along with other branched alkanes, it is used to replace aromatic compounds in gasoline formulations in response to regulations limiting the release of the latter compounds. However, isooctane is itself toxic: in particular, numerous studies have shown that it causes kidney and liver damage in rats (for instance, Fowlie et al., 1987).
Its quaternary carbon group makes isooctane one of the most recalcitrant constituents of gasoline. Nonetheless, Mycobacterium austroafricanum IFP 2173 is able to use it as a sole carbon source (Solano-Serena et al., 2000). In addition to isooctane, this strain can degrade a variety of other hydrocarbons, including various n-alkanes, multimethyl-substituted isoalkanes, and monoaromatic hydrocarbons, and some ethers.
Solano-Serena et al. (2004) have identified several of the intermediates in isooctane metabolism by M. austroafricanum IFP 2173. Isooctane is oxidized to 2,4,4-trimethylpentanoate, which is then likely ligated to coenzyme A and converted to 2,4,4-trimethyl-3-oxopentanoyl-CoA via beta-oxidation. Cleavage of propanoyl-CoA yields pivalyl-CoA, whose corresponding free form, pivalate, was observed. Pivalate is mineralized by M. austroafricanum IFP 2173, but the mechanism of its degradation is not well understood, and is not shown here. The authors suggest a pathway producing isobutyrate followed by methylmalonate, both of which are mineralized by isooctane-grown cells, as a reasonable hypothesis.
Solano-Serena et al. (2004) also detected small amounts of 2,2-dimethyl-3-pentanone and, in some cultures, 3,3-dimethyl-2-butanone produced during isooctane degradation by M. austroafricanum IFP 2173. These compounds are probably products of a minor, dead-end pathway beginning with decarboxylation of 2,4,4-trimethyl-3-oxopentanoate.
The following is a text-format isooctane pathway map. An organism which can initiate the pathway is 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 (22k) format.
Isooctane Mycobacterium austroafricanum IFP 2173 | | | isooctane | monooxygenase | | v 2,4,4-Trimethyl-1-pentanol | | | 2,4,4-trimethyl-1-pentanol | dehydrogenase | | v 2,4,4-Trimethylpentanal | | | 2,4,4-trimethylpentanal | dehydrogenase | | v 2,4,4-Trimethylpentanoate | | | 2,4,4-trimethylpentanoate- | CoA ligase | | v 2,4,4-Trimethylpentanoyl-CoA | | | 2,4,4-trimethylpentanoyl- | CoA dehydrogenase | | v 2,4,4-Trimethylpent-2-enoyl-CoA | | | 2,4,4-trimethylpent-2-enoyl- | CoA hydratase | | v 2,4,4-Trimethyl-3-hydroxypentanoyl-CoA | | | 2,4,4-trimethyl-3-hydroxy- | pentanoyl-CoA dehydrogenase | | v 2,4,4-Trimethyl-3-oxopentanoyl-CoA ----------------------+ | | | | | 2,4,4-trimethyl- | 2,4,4-trimethyl- | 3-oxopentanoyl-CoA | 3-oxopentanoyl-CoA | 2-C-propanoyl | thioesterase | transferase | | | | | v v Pivalyl-CoA + Propanoyl-CoA 2,4,4-Trimethyl-3-oxopentanoate | | | | | | | | | 2,4,4-trimethyl- | | | 3-oxopentanoate v v | decarboxylase to the Intermediary | Pivalic Acid Metabolism | Pathway (KEGG) v 2,2-Dimethyl-3-pentanone | | | pinacolone | 5-monooxygenase | | v 1-Hydroxy-4,4-dimethylpentan-3-one | | | 1-hydroxy-4,4-dimethyl | pentan-3-one | dehydrogenase | | v 4,4-Dimethyl-3-oxopentanal | | | 4,4-dimethyl- | 3-oxopentanal | dehydrogenase | | v 4,4-Dimethyl-3-oxopentanoate | | | 4,4-dimethyl- | 3-oxpentanoate | decarboxylase | | v 3,3-Dimethyl-2-butanone
Page Author: Daniel Baron, Michael Turnbull, and Carla Essenberg
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