Engineering Micro-organisms for Green Chemical Synthesis

© The Wallace Lab, University of Edinburgh, UK

The Microbiology Society is undertaking a project entitled A Sustainable Future as part of our 75th Anniversary, which aims to highlight the Sustainable Development Goals (SDGs) to our members and empower them to use their research to evidence and impact the goals. Earlier this year, we put a call out to our members to submit case studies in the following three areas: antimicrobial resistance, soil health and the circular economy.

This case study is written by Dr Stephen Wallace, a UK Research and Innovation (UKRI) Future Leaders Fellow and Senior Lecturer in Biotechnology at the University of Edinburgh, UK. It focuses on the circular economy; an alternative to a traditional linear economy (make, use, dispose), in which we keep resources in use for as long as possible, extract the maximum value from them while in use, then recover and regenerate products and materials at the end of each service life.

Developed in the 1930s, the launch of nylon was transformative – it was one of the first synthetic materials to achieve widespread use. Strong and stretchy nylon’s versatility is due to it being a plastic that can be moulded into fabric form. It can be processed and mixed with other substances to make different shapes and textures. Today more than two million tonnes of the fabric – used to make clothing, furniture and even parachutes – is produced globally each year, with a market value of around £5 billion.

But nylon’s popularity and ubiquitous use comes at a high environmental cost. Like other plastics its production relies on fossil fuels. The intensive and complex chemical process consumes vast amounts of energy and releases nitric oxide – a greenhouse gas 300 times more potent than carbon dioxide. Its environmental impact is so high that eliminating nitric oxide emissions from nylon production would be the equivalent of replacing every car in the UK with an electric car.

Poor environmental credentials, combined with the dwindling supply of fossils fuels, threaten to create a perfect storm as demand for nylon and other synthetic materials continues to rise. To tackle this, researchers have sought out more sustainable methods to produce the raw chemicals that make up the material. One of the key components, adipic acid, has been the focus of our research group. Instead of using fossil-fuels to make this chemical we have explored other naturally occurring sources of carbon.

One of the largest untapped renewable resources of carbon on the planet can be found in plants. Millions of tons of lignin—the tough, indigestible structural part of a plant—piles up each year as waste from the paper industry and biorefineries that use plants to make ethanol-based biofuels. Lignin has long been of interest to the biotechnology industry as a potential source of renewable carbon that could be turned into biofuels and other useful materials. But only a small proportion is currently being recycled due to the difficulties of processing this large and complex polymeric molecule.

Finding a renewable source of carbon is just one of the challenges facing researchers. The carbon source needs to be processed to turn it into the desired chemical. To avoid the environmental cost of traditional chemical approaches much attention has been on reprogramming microbes. Their natural metabolic abilities have already been harnessed to ferment food and beer. Now, advances in genome sequencing and bioengineering technologies offer the opportunity to turn them into living factories churning out materials and chemicals.

One of the drawbacks of traditional approaches to developing bio-based renewables through, for example, using microbes to ferment even simple plant sugars, is that it generates low yields (less than 1%) of the desired product. The fermentation process taxes the microbes metabolism and the final product is often toxic at high concentrations. This restricts the microbes growth and reduces the quantity of the final product.

Our team developed an alternative approach to overcome both challenges. We built a biosynthetic pathway in Escherichia coli which operates independently of the bacteria’s central metabolism. Using a synthetic biology approach, we engineered instructions to make four enzymes, needed to make adipic acid, into a plasmid —a physically separate strand of DNA — and inserted this into the bacterial cell. The result is that engineered E. coli can convert a naturally occurring chemical called guaiacol —the main component of lignin — into adipic acid within 24 hours. Unlike the traditional fossil-fuel based process, this one-pot reaction takes place at low temperatures, requires no additives and produces no nitric oxide or other byproducts. The use of this separate biosynthetic pathway also allows conversion of up to 60%.

The Wallace Lab, University of Edinburgh
© The Wallace Lab, University of Edinburgh, UK

As well as nylon manufacture, adipic acid is an important chemical for the manufacture of many other synthetic materials. Every year 2.5 billion kilograms of the chemical is produced to feed the ever-growing demands of industries as diverse as automotive, textiles, electronic and global construction. Around 40% of its global production is used to make a diverse range of products from polyutherane foams to plasticisers in PVC. Feeding these growing industries with a sustainable source of adipic acid will be vital.

As well as lignin, our team are exploring the use other sustainable feedstocks, including waste from the textile and food industries. Beyond nylon our approach raises other exciting possibilities for creating a circular economy. If bacteria can be programmed to help make nylon from plant waste – we must ask ourselves what else they could do, and where the limits lie. From new materials to medicines, the possibilities of this approach to create a sustainable future are staggering.

References

Jack T. Suitor, Simon Varzandeh, Stephen Wallace, 2020. One-Pot Synthesis of Adipic Acid from Guaiacol in Escherichia coli. ACS Synth. Biol.

About the Author

Dr Stephen Wallace’s is a UKRI Future Leaders Fellow and Senior Lecturer in Biotechnology at the University of Edinburgh. His multidisciplinary research group are inspired by the chemical ingenuity of micro-organisms and how they can be genetically programmed to produce industrially-important chemicals via synthetic biology. More information about the Wallace Lab is available here and a video about Stephen’s work is available here.