A ‘minted’ method for the sustainable, bio-based production of menthol enantiomers

We interact intimately with the chemical menthol every day: it is found in our food, drinks, toothpastes, soap, cleaning sprays, gum, etc. It imparts a cooling sensation and a minty flavour: it is in mint plants (Mentha) where we find the compound naturally. The chemical in our products is sourced either from mint oil extract or from chemical synthesis. There are complications with both processes. The former takes up land that could be used for food, and the plants produce mixtures containing unwanted compounds. The latter chemical process is not environmentally friendly as it uses solvents, high temperatures/pressures and generates waste. Another aspect to menthol is that it can come in eight different forms (stereoisomers). Whilst only one form, (–)-menthol, has the famous cooling properties, the other types of menthol can be used as flavours/fragrances or for other chemical processes or products. However, these other forms of menthol are challenging to access using the mint oil or chemical processes, so they are either not used or very expensive. In this project we aim to address a crucial basic biology question: how does mint make menthol? Then, we aim to use this knowledge to develop new ways to make menthol that are better for our planet than existing processes. Plants typically make molecules through a series of enzyme-catalysed chemical steps called biosynthetic pathways. Remarkably, the biosynthetic pathway to menthol is still not known, there are enzymes yet to be discovered. We aim to discover these missing steps in Mentha and closely related plants in the mint family. We will analyse the properties of these enzymes and attempt to understand how they differ to enzymes in related plants that do not make menthol. Once we have found and characterised these key missing enzymes, we will start to plan ways to make menthol stereoisomers using enzymes. We will take enzymes from plants and other sources and test them to determine what combinations we can use to reach our desired products. Eventually we will be able to mix and match different combinations of enzymes together to make all the different stereoisomers. We will start this process first just with purified enzymes but later we can transfer these pathways into a plant or microorganism like yeast. We will produce menthol isomers in these organisms, developing a new third way to make menthol, complementing the plant and chemical routes with a bio-based route. This project combines plant biology, chemistry and biocatalysis with an overriding spirit of engineering biology. The biological basis for this research is the solution of a core question in plant natural product biosynthesis: how mint makes menthol. The discovery could influence plant breeding efforts or contribute to gene edited biological products. Eventually we aim to generate bio-based systems for the controlled production of valuable chemicals. This work will contribute to the UK bioeconomy, aiding the development of a net-zero, sustainable, circular economy. Importantly, menthol is a global product that a huge proportion of the world’s population interact daily. Therefore, effecting only a fraction of the market could still make a significant impact.