Petroleum-based plastics are durable, lightweight and cheap to produce, and have thus become ubiquitous in daily life. However, these advantages are also the major cause of a growing environmental disaster, as most plastics are used in short-lived products such as packaging that is disposed of within a year of manufacture, and globally less than 15% of this material is collected for recycling.
Polyethylene terepththalate (PET) is the most widely used synthetic polyester thermoplastic, used in textile fibres, packaging materials and beverage bottles, with around 60 million tons produced worldwide annually from its monomeric precursors, terephthalic acid (TPA) and ethylene glycol (EG), both of which are derived from fossil fuels. PET is a long-chain aromatic polyester that is chemically inert and is generally resistant to microbial degradation in the natural environment.
However, a number of enzymes have been identified as having PET hydrolase activity, most of which naturally target the plant polymer cutin, a complex polymeric lipid, typically composed of monomers including C16 and C18 ω-hydroxy fatty acids, linked together by ester bonds. Like PET, it is a long-chain hydrophobic polyester, albeit a largely aliphatic one.
An engineered, thermostable version of leaf-branch compost cutinase (LCC) has recently been shown to be the most efficient PET-degrading enzyme characterised to date, but one of the drawbacks of enzymatic degradation is the length of time taken to fully degrade the substrate. One approach to increase catalytic efficiency is to increase the enzyme’s localisation to its hydrophobic substrate by the addition of hydrophobins, which are protein modules that naturally aggregate at air-liquid boundaries through the orientation of a hydrophobic surface.
To test this hypothesis, we used a bacterial superglue system, where modules can be easily assembled in different combinations using an autocatalytic ester bond linkage to an immunoglobulin-like protein framework, to investigate the effect of hydrophobin addition to the activity of LCC on PET substrates.