Subterminal Oxidation of Aliphatic Hydrocarbons by Microorganisms

Differential Protein Expression During Growth on Medium Versus Long-Chain Alkanes in the Obligate Marine Hydrocarbon-Degrading Bacterium Thalassolituus oleivorans MIL-1

The marine obligate hydrocarbonoclastic bacterium Thalassolituus oleivorans MIL-1 metabolizes a broad range of aliphatic hydrocarbons almost exclusively as carbon and energy sources. We used LC-MS/MS shotgun proteomics to identify proteins involved in aerobic alkane degradation during growth on medium- (n-C₁₄) or long-chain (n-C₂₈) alkanes. During growth on n-C₁₄, T. oleivorans expresses an alkane monooxygenase system involved in terminal oxidation including two alkane 1-monooxygenases, a ferredoxin, a ferredoxin reductase and an aldehyde dehydrogenase. In contrast, during growth on long-chain alkanes (n-C₂₈), T. oleivorans may switch to a subterminal alkane oxidation pathway evidenced by significant upregulation of Baeyer-Villiger monooxygenase and an esterase, proteins catalyzing ketone and ester metabolism, respectively. The metabolite (primary alcohol) generated from terminal oxidation of an alkane was detected during growth on n-C₁₄ but not on n-C₂₈ also suggesting alternative metabolic pathways. Expression of both active and passive transport systems involved in uptake of long-chain alkanes was higher when compared to the non-hydrocarbon control, including a TonB-dependent receptor, a FadL homolog and a specialized porin. Also, an inner membrane transport protein involved in the export of an outer membrane protein was expressed. This study has demonstrated the substrate range of T. oleivorans is larger than previously reported with growth from n-C₁₀ up to n-C₃₂. It has also greatly enhanced our understanding of the fundamental physiology of T. oleivorans, a key bacterium that plays a significant role in natural attenuation of marine oil pollution, by identifying key enzymes expressed during the catabolism of n-alkanes.

Cloning and functional analysis of alkB genes in Alcanivorax borkumensis SK2

Alcanivorax is an alkane-degrading marine bacterium which propagates and becomes predominant in crude-oil-containing seawater when nitrogen and phosphorus nutrients are supplemented. To identify the genes responsible for alkane degradation in this organism, two putative genes for alkane hydroxylases were cloned from Alcanivorax borkumensis SK2. They were named alkB1 and alkB2. These genes were subsequently disrupted in A. borkumensis SK2, and the growth phenotypes of the disruptants were examined. The results indicate that the alkB1 gene is responsible for the degradation of short-chain n-alkanes. A double mutant defective in both alkB1 and alkB2 was still able to grow on medium-chain n-alkanes, indicating that genes other than alkB1 and alkB2 are also involved in n-alkane hydroxylation by A. borkumensis SK2.

Isolation and characterization of a novel oxygenase that catalyzes the first step of n-alkane oxidation in Acinetobacter sp. strain M-1

In the Finnerty pathway for n-alkane, oxidation in Acinetobacter sp., n-alkanes are postulated to be attacked by a dioxygenase and the product, n-alkyl hydroperoxide, is further metabolized to the corresponding aldehyde via the peroxy acid [W. R. Finnerty, P. 184-188, in A. H. Applewhite (ed.), Proceedings of the World Conference on Biotechnology for the Fats and Oil Industry, 1988]. However, no biochemical evidence regarding the first-step reaction is available. In this study, we found a novel n-alkane-oxidizing enzyme that requires only molecular oxygen, i.e., not NAD(P)H, in our isolate, Acinetobacter sp. strain M-1, and purified it to apparent homogeneity by gel electrophoresis. The purified enzyme is a homodimeric protein with a molecular mass of 134 kDa, contains 1 mol of flavin adenine dinucleotide per mol of subunit, and requires CU2+ for its activity. The enzyme uses n-alkanes ranging in length from 10 to 30 carbon atoms and is also active toward n-alkenes (C12 to C20) and some aromatic compounds with substituted alkyl groups but not toward a branched alkane, alcohol, or aldehyde. Transient accumulation of n-alkyl hydroperoxide was detected in the course of the reaction, and no oxygen radical scavengers affected the enzyme activity. From these properties, the enzyme is most probably a dioxygenase that catalyzes the introduction of two atoms of oxygen to the substrate, leading to the formation of the corresponding n-alkyl hydroperoxide. The enzymatic evidence strongly supports the existence of an n-alkane oxidation pathway, which is initiated by a dioxygenase reaction, in Acinetobacter spp.

Microbial growth on 2-bromobutane

A member of the genus Arthrobacter was isolated which grew at the expense of 2-bromobutane as sole source of carbon and energy. Evidence is presented which suggests that the initial conversion of 2-bromobutane to 2-butanol is a spontaneous chemical hydrolysis and not mediated by the organism. Further evidence from oxygen consumption experiments indicates that 2-bromobutane is oxidized through 2-butanol, methyl ethyl ketone, ethyl acetate to acetate and ethanol. Results of experiments with cells grown on pathway intermediates reveal that the enzymes necessary for the oxidation of 2-butanol, methyl ethyl ketone, ethyl acetate, ethanol and acetaldehyde are not coordinately, but individually induced by their respective substrates.

Metabolism of the alkane analogue n-dioctyl ether by Acinetobacter species

Metabolism of n-dioctyl ether by Acinetobacter species HO1-N resulted in formation of 8-n-octoxy-1-octanoic acid and 2-n-octoxy-1-acetic acid. The 16-carbon ether acid was incorporated into the cellular lipids, whereas the 10-carbon ether acid accumulated in the growth medium. Qualitative and quantitative characteristics of the cellular phospholipids were similar to hexadecane-grown cells. The growth of Acinetobacter on dioctyl ether occurred at the expense of six-carbon atoms of dioctyl ether.