Researchers from NUS have successfully synthesised L-tyrosine directly from carbon dioxide (CO₂) for the first time, using an approach that integrates electrochemical and biological systems.

L-tyrosine is a non-essential amino acid that plays a crucial role in protein synthesis, serving as a precursor for neurotransmitters and various bioactive compounds. It is widely used in the pharmaceutical, dietary supplement, as well as the food and feed industries. L-tyrosine, for example, is a precursor for the synthesis of a Parkinson’s disease drug 3,4-dihydroxy-l-phenylalanine (L-DOPA). It is also a precursor for melanin, the natural polymer responsible for eye, hair and skin colour.

Traditional production methods of L-tyrosine such as protein hydrolysis and chemical synthesis require energy-intensive and complex separation processes, leading to low yields and high costs. This has driven a shift towards alternatives methods including microbial fermentation and enzymatic synthesis, as they allow are more efficient, both in terms of their energy requirements and waste production.

Using engineered bacteria or yeast for microbial fermentation allows for the production of amino acids at a large scale, however even these more efficient methods have limitations. For example, they rely on feedstocks like glucose and this leads to a higher production cost, carbon loss and the generation of unwanted by-products. Enzymatic synthesis on the other hand can provide more targeted pathways for amino acid production, but enzyme instability, must first be overcome.

Given these challenges, researchers continue to seek more sustainable production methods of L-tyrosine that use inexpensive and renewable resources, yet produce high yields.

Best of both worlds

Motivated to address these challenges, a research team led by Assistant Professor Wang Lei from NUS Chemical and Biomolecular Engineering, has developed an abiotic/biotic cascade strategy involving CO₂ as the main feedstock to produce L-tyrosine. This approach was inspired by photosynthesis, the process whereby plants convert atmospheric CO₂ into amino acids.

Abiotic/biotic cascade systems are hybrid systems that combine chemical (abiotic) and biological (biotic) processes to synthesise compounds that might be challenging to produce with either process alone. Abiotic reactions typically involve chemical catalysts that create intermediate compounds for the biotic step, which then uses microorganisms or enzymes to further process the intermediates. However, one of the challenges in designing these systems lies in addressing the differences between the electrochemical and biological components. For example, the electrochemical CO₂ reduction process typically yields short-chain carbon molecules, which are not ideal substrates for producing larger molecules like L-tyrosine. In addition, biological systems using Escherichia coli (E. coli) have limited tolerance to the operating conditions of electrochemical reactors, such as high current densities.

To resolve these issues, Wang and team spatially decoupled the electrochemical and biological processes. They first reduced CO₂ in a solid-state electrochemical reactor in the presence of a CuAg catalyst to produce a mixture of acetic acid and ethanol. This mixture is then introduced into a separate biological reactor, where a genetically engineered E. coli strain converts it into L-tyrosine.

Their method, published in Science Advances, involved some modifications to the abiotic/biotic cascade system. First, the team refined the electrochemical reactor design. In a typical CO₂ reduction process, ethanol and acetate are produced alongside common salts like potassium hydroxide (KOH) and potassium carbonate (K₂CO₃), complicating the separation process. Asst Prof Wang’s team addressed this by using a solid electrolyte reactor, which prevents the formation of salt mixtures and allows for the production of pure ethanol and acetate, thus improving the overall efficiency of the cascade system.

In the second step of this approach, E. coli transforms acetic acid and ethanol into L-tyrosine through a series of engineered metabolic steps, using acetic acid and ethanol as carbon sources. The researchers genetically engineered E. coli strains to incorporate an additional ethanol utilisation pathway alongside the existing acetic acid pathway, allowing the bacteria to use both acetic acid and ethanol as feedstocks. They discovered that the ethanol pathway boosted the acetic acid pathway, increasing L-tyrosine production by nearly five-fold compared to strains that only utilised acetic acid.

Dual opportunity

According to Asst Prof Wang, this is the first time L-tyrosine has been synthesised using CO₂ as the feedstock, and a combination of electrochemical and biological systems.

Their approach presents a dual opportunity to address the demand for sustainable chemical production as well as environmental concerns related to greenhouse gas emissions. By combining electrochemical CO₂ reduction with microbial fermentation, this approach offers a viable pathway for converting CO₂ into valuable products like L-tyrosine. Since waste CO₂ from industrial applications is a major contributor to greenhouse gas emissions, the use of CO₂ as the feedstock aligns with global efforts to reduce emissions and promote sustainability.

“Our work turns CO₂ molecules into valuable products, offering promising pathways toward a cleaner, carbon-neutral future. While this study focused on L-tyrosine, the same principles could potentially be applied to the production of other valuable chemicals and amino acids, further expanding the scope of CO₂ utilisation technologies,” Asst Prof Wang added.