UC Irvine researchers report additional hydrocarbons produced from CO by vanadium/molybdenum nitrogenases; suggest there may be a basis for a new synthetic fuel production pathway
Vanadium-dependent nitrogenase has been shown to catalyze the reductive catenation—i.e., the ability to form a long chain via a series of covalent bonds—of carbon monoxide (CO) to the hydrocarbons ethylene, ethane, propylene, and propane. Now, a team of researchers from UC Irvine have identified the additional hydrocarbon products α-butylene, n-butane, and methane in a scaled-up reaction featuring the catalytic component of vanadium nitrogenase.
In a paper in the journal Science, they also report showing that the more common molybdenum-dependent nitrogenase can generate the same hydrocarbons from CO, although CH4 was not detected.
Nitrogenase enzymes catalyze the reduction of N2 to ammonia (NH3), a key step in the global nitrogen cycle. The molybdenum (Mo)– and vanadium (V)–dependent nitrogenases are two homologous members of this enzyme family. CO and N2 are similar molecules: diatomics with the same number of electrons and a very strong central triple bond.
The researchers suggested that the identification of CO as a substrate for both molybdenum- and vanadium-nitrogenases strengthens the hypothesis that CO reduction is an evolutionary relic of the function of the nitrogenase family.
The parallelism between the nitrogenase catalyzed CO and N2 reduction—demonstrated further by the ability of V nitrogenase to form CH4, an analogous product of NH3—strengthens the theory that the ancestral nitrogenase may represent an evolutionary link between carbon and nitrogen cycles on earth.
In a practical vein, the industrial Fischer-Tropsch process uses H2, a costly syngas component, for CO reduction and shows a tendency toward excessive formation of CH4, a low-value product, whereas the nitrogenase-based reaction uses H+ for hydrocarbon formation and favors the formation of C-C bond over the cleavage of C-O bond.
These features make nitrogenase an attractive template for cost-efficient hydrocarbon formation that is directed toward C-C coupling and carbon-chain extension. Borrowing a trick or two from this ancient enzyme family, perhaps an efficient strategy could be developed in the future for controlled fuel production from CO? Only time will tell.—Hu et al.
Yilin Hu, Chi Chung Lee, and Markus W. Ribbe (2011) Extending the Carbon Chain: Hydrocarbon Formation Catalyzed by Vanadium/Molybdenum Nitrogenases. Science 333 (6043), 753-755 doi: 10.1126/science.1206883