Polyurethane-degrading enzymes: Structural analysis and development

Fully synthetic polymers, more commonly known as plastics, have become ubiquitous materials with many applications. Plastics are typically derived from petrochemical monomers, polymerised together to produce materials that have desirable properties, such as, plasticity, low cost, hydrophobicity, and resistance to chemical and microbial degradation. These properties have led to 79 per cent of all plastic waste accumulating in the natural environment and in landfill sites, 12 per cent being incinerated and just 7per cent being recycled. Polyurethanes (PURs) are a common group of these fully synthetic and resistant plastics and represent 5 to 6 per cent of annual plastic production. 18 million tonnes (Mt) were produced in 2016, expected to rise to 22Mt by 2020. PURs are used to produce foams, sealants, durable elastomers, waterproofing coatings, and synthetic fibres. PUR waste represents a more significant environmental and health challenge than other plastics. Combustion of PUR leads to the release of carbon monoxide and hydrogen cyanide. Recycling and sorting of PUR can be difficult due to the mixture of both thermoset and thermoplastic polymers. Some PUR can be mechanically recycled into lower value products such as carpet underlay but are often mixed with virgin polymers to produce a usable product. It is therefore key to deal with these PURs more sustainably at the end of their product life, both to remove waste and produce virgin plastic. Although chemical recycling routes have been explored with regards to PUR waste, the glycolysis methods proposed are high energy (190 degrees C), whilst biological degradation utilising enzymes may offer a greener solution.

Biologically derived novel recycling methods utilising enzymes, are exciting prospects that have quickly become high-potential solutions to the plastic waste problem. One example is the development of PETase for use in an industrially viable, circular, recycling process for PET. PUR-degrading enzymes are currently not efficient enough to develop a PUR depolymerisation recycling process. One approach to improving their activity is protein engineering, which has been successfully demonstrated for engineering PETase enzymes. Targeted engineering would utilise structural data on the enzymes obtained by either homology modelling or x-ray crystallography. Targeted amino-acid modifications include modifications to the active sites to eliminate rate limiting steps, increase thermal stability, and adding polymer binding domains can be used to enhance plastic degrading activity.

The aim of this project is to identify and engineer a reliable enzyme for the depolymerisation recycling of specific PUR plastics, releasing monomers to input into the resynthesis of virgin PUR. The initial aim of this research is to identify and express potential PUR-degrading enzymes and characterise their activity. This will initially target PUR-degrading enzyme candidates that have been previously identified in literature, such as esterases with activity on solid insoluble PUR substrates, and enzymes with suspected urethane bond cleaving activity. The second aim of this research will be to structurally analyse these enzymes using homology modelling or X ray crystallography where necessary to understand their urethane bond cleaving activity. Candidates with confirmed urethane bond activity can be used to identify further candidate PUR degrading enzymes across species. The final aim of this research is targeted engineering of the candidates to improve potential industrial viability. This stage will involve computational simulation to identify potential sites for modification, and site directed mutagenesis of these enzymes to alter the active site, thermal stability, or binding domains, to improve their activity. Overall, this identification, characterisation and engineering of PUR degrading enzymes will produce tools for the industrial recycling of PUR waste into virgin PUR.

This CDT will deliver impact aligned to the following agendas:

People
A2P will provide over 60 PhD graduates with the skill sets required to deliver innovative sustainable products and processes into the UK chemicals manufacturing industry. A2P will inspire and develop leaders who will:
- understand the needs of industrial end-users;
- embed sustainability across a range of sectors; and
- catalyse the transition to a more productive and resilient UK economy.

Economy
A2P will promote a step change towards a circular economy that embraces resilience and efficiency in terms of atoms and energy. The benefits of adopting more sustainable design principles and smarter production are clear. For example, the global production of active pharmaceutical ingredients (APIs) has been estimated at 65,000-100,000 tonnes per annum. The scale of associated waste is > 10 million tonnes per annum with a disposal cost of more than £15 billion. Consequently, even a modest efficiency increase by applying new, more sustainable chemical processes would deliver substantial economic savings and environmental wins. A2P will seek and deliver systematic gains across all sectors of the chemicals manufacturing industry. Our goals of providing cross-scale training in chemical sciences with economic and life- cycle awareness will drive uptake of sustainable best practice in UK industry, leading to improved economic competitiveness.

Knowledge
This CDT will deliver significant new knowledge in the development of more sustainable processes and products. It will integrate the philosophy of sustainability with catalysis, synthetic methodology, process engineering, and scale-up. Critical concepts such as energy/resource efficiency, life cycle analysis, recycling, and sustainability metrics will become seamlessly joined to what is considered a 'normal' approach to new molecular products. This knowledge and experience will be shared through publications, conferences and other engagement activities. A2P partners will provide efficient routes to market ensuring the efficient translation and transferal of new technologies is realised, ensuring impact is achieved.

Society
The chemistry-using industries manufacture a rich portfolio of products that are critical in maintaining a high quality of life in the UK. A2P will provide highly trained people and new knowledge to develop smarter, better products, whilst increasing the efficiency and sustainability of chemicals manufacture.
To amplify the impacts of our CDT, effective public engagement and technology transfer will become crucially important. As a general comment, 'sustainability' styled research is often regarded in a positive light by society, however, the science that underpins its effective implementation is often poorly appreciated. The University of Nottingham has developed an effective communication portfolio (with dedicated outreach staff) to tackle this issue. In addition to more traditional routes of scientific communication and dissemination, A2P will develop a portfolio of engagement and outreach activities including blogs, webpages, public outreach events, and contribution of material to our award-winning YouTube channel, www.periodicvideos.com.

A2P will build on our successful Sustainable Chemicals and Processes Industry Forum (SCIF), which will provide entry to networks with a wide range of chemical science end-users (spanning multinationals through to speciality SMEs), policy makers and regulators. We will share new scientific developments and best practice with leaders in these areas, to help realise the full impact of our CDT. Annual showcase events will provide a forum where knowledge may be disseminated to partners, we will broaden these events to include participants from thematically linked CDTs from across the UK, we will build on our track record of delivering hi-impact inter-CDT events with complementary centres hosted by the Universities of Bath and Bristol.