A role for yeast

In the world of sustainable biomanufacturing, Saccharomyces cerevisiae – commonly known as baker’s yeast – has evolved from its traditional role in fermentation to a cutting-edge platform for producing biofuels, specialty chemicals, and high-value compounds. In fact, it is now at the heart of innovative research aimed at engineering complex metabolic pathways for the production of fatty acids, alcohols, and their derivatives. These advancements are pushing the boundaries of biochemical manufacturing and offering exciting new possibilities for more sustainable, economically viable bio-based production systems.

Fatty acids and their derivatives, such as fatty alcohols and aldehydes, are crucial components in the production of various products, including cosmetics, pharmaceuticals, and biofuels. Their widespread use can be attributed to unique chemical properties like chain length, saturation, and branching.

Traditionally, fatty acids and their derivatives are extracted from plant and animal oils or synthesised chemically. While scalable and efficient, these methods rely on agriculture, cause deforestation, and generate chemical waste. By engineering S. cerevisiae to efficiently produce fatty acid derivatives, researchers are laying the foundation for greener, more efficient production systems that can support a circular bioeconomy.

Distilling odd-chain fatty alcohols

Producing odd-chain fatty alcohols has long been a challenge due to the natural predominance of even-chain fatty acids in most organisms. However, recent breakthroughs in metabolic engineering have enabled S. cerevisiae to produce odd-chain fatty compounds directly. This has been achieved in our lab at NUS SynCTI (Synthetic Biology for Clinical Technological Innovation) with impressive efficiency.

By introducing enzymes such as thioesterase, α-dioxygenase, and the yeast’s endogenous aldehyde reductases, we produced approximately 20 mg/L of fatty aldehydes and 20 mg/L of fatty alcohols by fermentation of the engineered yeast, all without the need for extensive fine-tuning of metabolic flux in the system. Building upon this pathway, there is great potential for scaling the system to produce odd-chain fatty alcohols sustainably.

Branching out from the conventional

S. cerevisiae has also been engineered to produce short-branched-chain fatty acids (SBCFAs). Due to their branched structure, SBCFAs often exhibit distinct chemical properties that make them suitable intermediates for a variety of industrial applications. Branching also alters physical properties like the melting point and volatility, meaning SBCFAs can be tailored to specific applications such as biofuels or specialty chemicals.

To facilitate the efficient production of SBCFAs, our team has developed a whole-cell biosensor using the weak organic acid transporter Pdr12p. By optimising the expression of the PDR12 gene, we can now fine-tune the biosensor’s sensitivity and operational range, enabling real-time monitoring of SBCFA production in S. cerevisiae. This system will allow for more efficient screening of SBCFA-producing strains to improve biosynthesis efforts.

Boosting aldehyde production

Fatty aldehydes and hydrocarbons have also been synthesised by genetically engineered S. cerevisiae. Naturally, an alcohol-forming fatty acyl-CoA reductase (FAR) enzyme consumes aldehyde intermediates derived from fatty acyl-CoAs without accumulating them. To overcome this, we have engineered the FAR to catalyse the direct production of fatty aldehydes in S. cerevisiae. After modifying the enzyme’s catalytic domains, we achieved a breakthrough production of 2,005 μg/L of fatty aldehyde – the highest reported concentration from fatty acyl-CoA in S. cerevisiae to date.

Engineered yeast strains have the potential for scalable production of a wide range of aldehyde-derived chemicals. Already, biodiesel candidates, alkanes and alkenes, have been produced using this system.

Overcoming challenges in yeast-based biomanufacturing

As these technologies continue to mature, engineered S. cerevisiae is poised to become a cornerstone of sustainable biomanufacturing. However, several scientific challenges remain to be addressed.

Strain optimisation is essential to enhance chemical production from yeast for economically viable large-scale fermentation. To achieve this, extensive genetic modifications are needed. These modifications would increase the efficiency of the biosynthesis pathway, minimise metabolic burden and enhance cellular tolerance against the toxic chemicals produced, thereby ensuring healthy cell growth and maintaining fermentation efficiency.

With our advancements in novel strain engineering approaches assisted by synthetic yeast and whole-cell biosensors, we aim to accelerate the development of S. cerevisiae into microbial cell factories for efficient chemical production, paving the way for sustainable and commercially viable yeast-based biomanufacturing processes.