Abstract
The microbial degradation pathways of bicyclic monoterpenes contain unknown enzymes for carbon–carbon cleavages. Such enzymes may also be present in the betaproteobacterium Castellaniella defragrans, a model organism to study the anaerobic monoterpene degradation. In this study, a deletion mutant strain missing the first enzyme of the monocyclic monoterpene pathway transformed cometabolically the bicyclics sabinene, 3-carene and α-pinene into several monocyclic monoterpenes and traces of cyclic monoterpene alcohols. Proteomes of cells grown on bicyclic monoterpenes resembled the proteomes of cells grown on monocyclic monoterpenes. Many transposon mutants unable to grow on bicyclic monoterpenes contained inactivated genes of the monocyclic monoterpene pathway. These observations suggest that the monocyclic degradation pathway is used to metabolize bicyclic monoterpenes. The initial step in the degradation is a decyclization (ring-opening) reaction yielding monocyclic monoterpenes, which can be considered as a reverse reaction of the olefin cyclization of polyenes.
Highlights
Monoterpenes are a diverse family of biogenic (C10) hydrocarbons
In order to identify possible intermediates in the metabolism of the bicyclic monoterpenes, cultures of C. defragrans strains 65Phen and 65Phen ∆ctmAB were grown in liquid artificial freshwater (AFW) medium with 20 mM acetate and 3 mM of either sabinene, 3-carene or α-pinene as carbon sources and 10 mM nitrate as electron acceptor
Cultures were harvested in early stationary phase, and hydrophobic substances were extracted and analyzed by gas chromatography with flame ionization detection (GC-flame ionization detector (FID)) for quantification and with mass spectrum detection (GC-MS) for identification
Summary
Monoterpenes are a diverse family of biogenic (C10) hydrocarbons. They are synthesized by the condensation of two activated isoprene (C5) units [1,2], primarily as secondary metabolites in higher plants. C. defragrans metabolizes more than a dozen structurally diverse monoterpenes to carbon dioxide [15], while most studied aerobic bacteria metabolize one or two closely related substrates [17]. For this reason, over the years, C. defragrans ( strain 65Phen) has become a model organism to study the anaerobic mineralization of monoterpenes. Metabolites 2018, 8, 12 to (S)-linalool, an oxygen-independent reaction catalyzed by the linalool dehydratase/isomerase, has raised interest for its industrial potential in the production of alkenes [18,22,23] Another novelty is the anaerobic hydroxylation of limonene to perillyl alcohol, the first step in the oxidation of monocyclic monoterpenes [19,20]. We studied the transformation of the bicyclic monoterpenes α-pinene, sabinene and 3-carene, using several experimental approaches including the metabolite accumulation in mutants with a defect in the degradation pathway for monocyclic monoterpenes
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