Researchers at Utah State University report on an engineered bacterial enzyme—a molybdenum (Mo) nitrogenase—capable of converting carbon monoxide into usable hydrocarbons in a reaction similar to Fischer-Tropsch synthesis. A open access paper on their work appears in the Journal of Biological Chemistry.
Nitrogenase is the bacterial enzyme responsible for the biological reduction of N2 to ammonia (NH3), accounting for more than half of the input of fixed nitrogen into the biogeochemical N cycle, the authors note. Three different nitrogenases have been identified, with each being coded for by unique sets of genes; the nitrogenase system containing Mo is the most widely occurring and is preferentially expressed if sufficient Mo is present in the cell growth medium.
The Mo-dependent nitrogenase catalyzes the multi-electron reduction of protons and N2 to yield H2 and 2NH3. It also catalyzes the reduction of a number of non-physiological doubly and triply bonded small molecules (e.g., C2H2, N2O). Carbon monoxide (CO) is not reduced by the wild-type Mo-nitrogenase, but instead inhibits the reduction of all substrates catalyzed by nitrogenase except protons.
Here, we report that when the nitrogenase MoFe protein a-70 Val residue is substituted by alanine or glycine, the resulting variant proteins will catalyze the reduction and coupling of CO to form methane (CH4), ethane (C2H6), ethylene (C2H4), propene (C3H6), and propane (C3H8). The rates and ratios of hydrocarbon production from CO can be adjusted by changing the flux of electrons through nitrogenase, by substitution of other amino acids located near FeMo-cofactor, or by changing the partial pressure of CO. Increasing the partial pressure of CO shifted the product ratio in favor of the longer chain alkanes and alkenes.—Yang et al.
The highest yielding product was ethylene, followed by propene, ethane, and propane.
This is pretty profound. Understanding this process paves the way for developing better ways of converting carbon monoxide, a toxic waste product of combustion, into transportation fuel and precursors for plastics—without the time and energy required for conventional extraction of fossil fuels.—Lance Seefeldt, professor in USU’s Department of Chemistry and Biochemistry
The researchers also found that introduction of additional amino acid substitutions near FeMo-cofactor, along with an increased CO concentration, can significantly shift the product profile of nitrogenase CO reduction and condensation in favor of production of longer chain hydrocarbons. Thus, the team concludes, the Mo-nitrogenase offers an experimentally tractable system for examining mechanistic features that favor the production of longer chain hydrocarbons from CO that might be translated to the development of small molecule metal complexes that catalyze such reactions.
Like many waste-to-energy processes, we’ve found we can produce such hydrocarbons as propane and butane from carbon monoxide. But using this process, we may have the potential to produce such transportation fuels as diesel and gasoline that are readily adaptable to today’s vehicles.— Zhi-Yong Yang
Zhi-Yong Yang, Dennis R. Dean, and Lance C. Seefeldt (2011) Molybdenum nitrogenase catalyzes the reduction and coupling of CO to form hydrocarbons J. Biol. Chem. doi: 10.1074/jbc.M111.229344