Alkane chain hydrocarbons comprise the bulk of useful petroleum constituents. They include propane, gasoline, diesel oil, jet fuel, and many other fuels, lubricants, and reaction precursors. Industrial scale techniques for producing these compounds (rather than refining them from pre-existing petroleum) depend on large energy input and capital investment and the use of toxic compounds. In contrast, certain strains of cyanobacteria, like Synechococcus elongatus, can produce long-chain alkanes and related alkenes from simple fatty acid precursors using a pair of enzymes. Remarkably, more than 80% of the hydrocarbon product leaves the cell after synthesis, which suggests a possible carbon-sink role for the process.
“Here we describe the discovery of an alkane biosynthesis pathway from cyanobacteria. The pathway consists of an acyl–acyl carrier protein reductase and an aldehyde decarbonylase, which together convert intermediates of fatty acid metabolism to alkanes and alkenes…production and secretion of C13 to C17 mixtures of alkanes and alkenes…Heptadecane is the most abundant alkane reported in these photoautotrophic bacteria, an observation consistent with the ‘n – 1’ rule for alkanes, resulting from decarbonylation of typically even-numbered fatty aldehydes…Ten of these strains produced alkanes, mainly heptadecane and pentadecane, along with alkenes, presumably derived from unsaturated fatty aldehydes.” (Schirmer et al. 2010:559).
“Two of the…hypothetical proteins stood out as likely candidates for alkane biosynthesis…orf1593 and orf1594 from S. elongatus PCC7942.” (Schirmer et al. 2010:560).
“PCC7942_orf1594-expressing cells contained substantial quantities of even- chain fatty aldehydes…coexpression of both PCC7942_orf1593 and orf1594 resulted in the production of odd-chain alkanes and alkenes…Thus, PCC7942_orf1593 and orf1594 are sufficient for in vivo alkane biosynthesis, and fatty aldehydes are likely the biosynthetic intermediates…Alkane profiles of selected strains…show that the ‘recombinant hydrocarbon’ mixtures are primarily made up of pentadecane and heptadecene. Strains with the highest titers…produced a mixture of tridecane, pentadecene, pentadecane, and heptadecene, typically at a ratio of 10:10:40:40. Alkane titers were over 300 mg/liter when a modified mineral medium was used, and more than 80% of the hydrocarbons were found outside the cells…Acyl-acyl carrier protein (acyl-ACP) and acyl- coenzyme A (acyl-CoA), the major activated forms of fatty acids in bacteria… When incubated with acyl-ACP or acyl-CoA…PCC7942_orf1594 catalyzed the reduced nicotinamide adenine dinucleotide phosphate (NADP+) (NADPH)–dependent reduction to the corresponding fatty aldehyde…and required divalent cations, such as magnesium for catalysis…we named this enzyme acyl-ACP reductase (AAR), because it appears that acyl ACP is kinetically preferred and is likely the in vivo substrate…bacterial aldehyde decarbonylases are members of the ferritin-like or ribonucleotide reductase–like family of nonheme diiron enzymes.” (Schirmer et al. 2010:561).
“The three-dimensional structure [of PCC7942_orf1593] shows similarities to the 8 α-helical bundle of the second subunit of E. [Escherichia] coli ribonucleotide reductase (R2). In the solved structures, both proteins have two irons coordinated to histidine and aspartate or glutamate residues…[but] decarbonylation proceeds through a mechanism different from ribonucleotide reductase R2…At present, we cannot exclude that active aldehyde decarbonylases are manganese/ iron proteins…in vitro decarbonylation of octadecanal to heptadecane was only observed in the presence of ferredoxin, ferredoxin reductase, and NADPH.” (Schirmer et al. 2010:562).