Green amino acid production incorporates carbon dioxide

L-METHIONINE is an amino acid that is typically produced using toxic hydrogen cyanide. Researchers have now devised an environmentally friendly way to produce the amino acid, while also providing a new industrial use for CO₂.

L-methionine is an essential amino acid that is added to animal feedstocks and has a worldwide production of over 1m t/y. Currently the production of L-methionine is a challenging six-step process that requires the reactant hydrogen cyanide. Researchers at the Technical University of Munich (TUM) in Germany, working in partnership with Evonik, have now developed a new method for producing L-methionine that doesn’t require hydrogen cyanide. Evonik issued a call for proposals in 2013 for safer methods of producing L-methionine.  

In nature, L-methionine is used to produce methional in a series of steps including decarboxylation, which releases CO₂ as a by-product. The CO₂ gas gets diluted in the atmosphere which had led to the general assumption that decarboxylation reactions aren’t reversible in nature.

"Based on the idea that methionine in microorganisms is degraded by enzymes to methional with the release of CO2, we tried to reverse this process because every chemical reaction is in principle reversible, while often only with the extensive use of energy and pressure," said Arne Skerra from the department of biological chemistry at TUM.

The team found that is was possible to perform the reverse reaction, carboxylation of methional, using a decarboxylase enzyme from Lactococcus lactis bacteria. The process should have required extremely high pressure for the CO₂, so the researchers were surprised that the reaction worked with a CO₂ pressure around 2 bar. The requirement of gaseous CO₂ in the reaction also provides a new industrial use for the greenhouse gas.

They even found that their reaction is more efficient than photosynthesis, which also includes a carboxylation step. "Compared to the complex photosynthesis, in which nature also biocatalytically incorporates CO₂ into biomolecules as a building block, our process is highly elegant and simple," said Skerra. "Photosynthesis uses 14 enzymes and has a yield of only 20 percent, while our method requires just two enzymes."

The method can also be used to produce other amino acids without the need for hydrogen cyanide, thereby providing a greener route to creating valuable building blocks in the pharmaceutical and chemical industries.

Nature Catalysis http://doi.org/cs8z