Anaerobic Initial Reaction of n -Alkanes in a Denitrifying Bacterium: Evidence for (1-Methylpentyl)succinate as Initial Product and for Involvement of an Organic Radical in n -Hexane Metabolism

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Abstract
A novel type of denitrifying bacterium (strain HxN1) with the capacity to oxidize n-alkanes anaerobically with nitrate as the electron acceptor to CO2 formed (1-methylpentyl)succinate (MPS) during growth on n-hexane as the only organic substrate under strict exclusion of air. Identification of MPS by gas chromatography-mass spectrometry was based on comparison with a synthetic standard. MPS was not formed during anaerobic growth on n-hexanoate. Anaerobic growth with [1-13C]n-hexane ord14-n-hexane led to a 1-methylpentyl side chain in MPS with one 13C atom or 13 deuterium atoms, respectively. This indicates that the 1-methylpentyl side chain originates directly from n-hexane. Electron paramagnetic resonance spectroscopy revealed the presence of an organic radical in n-hexane-grown cells but not inn-hexanoate-grown cells. Results point at a mechanistic similarity between the anaerobic initial reaction ofn-hexane and that of toluene, even thoughn-hexane is much less reactive; the described initial reaction of toluene in anaerobic bacteria is an addition to fumarate via a radical mechanism yielding benzylsuccinate. We conclude thatn-hexane is activated at its second carbon atom by a radical reaction and presumably added to fumarate as a cosubstrate, yielding MPS as the first stable product. When 2,3-d2-fumarate was added to cultures growing on unlabeled n-hexane, 3-d1-MPS rather than 2,3-d2-MPS was detected, indicating loss of one deuterium atom by an as yet unknown mechanism.