Engineered Bacteria Produce Aromatic Aldehydes

Synthetic biologists have engineered bacteria to prevent the microbes from reducing aromatic aldehydes, allowing the cells to synthesize flavorants such as vanillin and a precursor to the pharmaceutical ephedrine (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja506664a). These bacterial strains could be used to expand the types of chemicals produced by microbes, the researchers say.

Some chemists want to use microbes instead of feedstocks from petroleum to synthesize commercially important molecules. But it’s difficult to produce aldehydes in Escherichia coli because the bacteria have many enzymes that reduce aldehydes to alcohols. Researchers previously have made aliphatic aldehydes by removing some of the genes for the troublesome enzymes. Another approach is to use cells with lowered metabolism rates, because they reduce aldehydes slowly.

Kristala L. J. Prather of Massachusetts Institute of Technology and her colleagues wanted to engineer E. coli that produced aromatic aldehydes under normal metabolism. They engineered the bacteria in two main steps. First, they added a gene for a protein that converts benzoate to benzaldehyde, which is used as an almond flavorant. Then the researchers deleted six genes for enzymes that can reduce benzaldehyde to benzyl alcohol.

The researchers fed benzoate to the engineered bacteria, and within 24 hours, the microbes had converted 66% of the starting material to benzaldehyde, with less than 12% of it going to benzyl alcohol. Wild-type bacteria converted all the benzoate to benzyl alcohol over the same time.

Then the researchers wondered if the microbes could use the accumulated benzaldehyde to synthesize other useful chemicals. In one experiment, they tried to make L-phenylacetylcarbinol (L-PAC), which is a precursor to the pharmaceutical ephedrine. The scientists added another gene to their engineered strain, one for a mutated protein that synthesizes L-PAC from benzaldehyde and pyruvate. In 24 hours, bacteria-fed benzoate converted about 50% of it to L-PAC. Again, wild-type bacteria only produced benzyl alcohol.

Prather says that the L-PAC synthesis indicates that these strains could be further engineered to take aromatic aldehydes and convert them into chiral amines for other types of molecules.

To show that their engineered strains could transform other starting materials, the researchers also fed them vanillate. The microbes produced the vanilla flavorant vanillin, without reducing it to vanillyl alcohol. The researchers also added genes to this strain to re-create a known biochemical pathway that converts glucose into vanillin. The engineered microbes produced 5,000% more vanillin than bacteria containing the aldehyde-reducing enzymes.

Shota Atsumi of the University of California, Davis, who developed a deletion approach to produce aliphatic aldehydes (Metab. Eng. 2014, DOI: 10.1016/j.ymben.2014.07.012), is happy to see that such a strategy also works with aromatic ones.

Laura Jarboe of Iowa State University says this approach opens up a lot of possibilities for microbial production of previously problematic products; she wonders if this strain can produce other aldehydes in addition to aromatic ones.