The second of two enzymes found to digest PET has been decoded by a group of researchers in Germany.
Over 50m tonnes of PET is produced each year to produce plastic, a versatile and durable material used in everyday life. Of the plastic produced, only 9% is recycled, meaning that our planet is now littered with plastic waste that takes hundreds of years to degrade.
In the search for a solution to this growing problem, Japanese researchers discovered in 2016 that bacterium that grows on PET contains two enzymes that are able to digest PET plastic polymers: PETase and MHETase. The simpler structure of the two – PETase – was decoded in 2018, and now scientists have done the same with MHETase.
‘MHETase is considerably larger than PETase and even more complex. A single MHETase molecule consists of 600 amino acids, or about 4,000 atoms,’ explains Dr Gert Weber from the joint Protein Crystallography group at the Helmholtz-Zentrum Berlin and Freie Universität Berlin, Germany.
‘MHETase has a surface that is about twice as large as the surface of PETase and has therefore considerably more potential to optimise it for decomposition of PET.’
In order to find the structure of the enzyme, the group chemically decomposed a PET plastic bottle and synthesised a chemical fragment from it that can bind to MHETase but cannot be cleaved by it. From this, crystals formed that could be used for structural investigations.
‘The structural investigations enabled us to watch MHETase virtually ‘at work’ and develop strategies for how to optimise this enzyme,’ said Weber, and ‘we can now exactly localise where the MHET molecule docks to MHETase and how MHET is then split into its two building blocks, terephthalic acid and ethylene glycol.’
‘Thanks to the clarification of the structure of this very important enzyme, we have now also been able to plan, produce and biochemically characterise variants that show significantly higher activity than natural MHETase and are even active against another intermediate product of PET degradation, BHET,’ adds Uwe Bornscheuer, also from the team.
Together, PETase and MHETase can sustainable synthesise new PET without the addition of crude oil – a known fossil fuel – in a closed biotechnological cycle.
The group are now looking to take their discovery and apply it for use in PET recycling.