When we think of sweet smells we do not immediately think of bacteria, but scientists from the University of California, Davis are engineering bacteria to make esters to be used in scents and fragrances, as well as in flavourings, paints and fuels.
Shota Atsumi's laboratory in the UC Davis Department of Chemistry is the hub of the research, and his team’s latest work, published recently in the journal Nature Chemical Biology, is looking for a more sustainable way of developing industrial chemicals which are normally derived from oil or gas.
"Our motivation is to make chemicals from renewable sources instead," says the assistant professor of chemistry; and adds that scents and flavorings make up a $20 billion industry worldwide.
Nature’s ‘great system’
Esters are molecules in which two chains of carbon atoms are linked through an oxygen atom. They are made chemically by reacting an alcohol with an organic acid. But the thermodynamics of this reaction mean that it tends to run the other way, meaning it is easier to break up an ester than to make it.
Living cells can also make esters. For example, yeasts produce small amounts of esters that give flavours to wine and beer, without requiring high temperatures or special conditions.
"The reaction is chemically difficult but biologically easy," explains Atsumi. "Nature gives you a great system to work with."
Tweaking the pathway
Atsumi explains that nature uses a class of enzymes called alcohol O-acetyltransferases to make esters from acyl-Coenzyme A (acyl-CoA) molecules. Changing the acyl- part of acyl-CoA that goes into the reaction changes the type of ester that is produced.
The assistant professor and his team’s work was partly supported by a Hellman Fellowship, and sees him, graduate student Gabriel Rodriquez, and postdoctoral researcher Yohei Tashiro take genes for biochemical pathways from yeast and introduced them into E. coli bacteria, a reliable test system for genetic engineering.
By tweaking the acyl-CoA pathway, they could manipulate one half of the ester: by adjusting the pathway that produces alcohols in the cell, and by shutting down other potential pathways, they could adjust the other half.
Because of this, the team were able to pick the final ester made by the bacteria.
The technique, which has been patented, opens up possibilities for producing many different esters in biological systems, according to Atsumi.
The source material for the bacteria is based on sugars, which can come from renewable biomass.
Ultimately, Atsumi says he hopes to engineer these chemical pathways into cyanobacteria (blue-green algae), single-celled organisms that can draw energy directly from sunlight and carbon from the atmosphere.