PETase is an enzyme that breaks down polyethylene terephthalate (PET), a widely used plastic. It is an esterase enzyme that catalyzes PET breakdown through hydrolysis. PET is prevalent in products like single-use bottles, packaging, and clothing fibers. The discovery of PETase has generated interest due to its potential in addressing global plastic pollution.
The Discovery of a Plastic-Eating Enzyme
PETase was discovered in 2016 by Japanese scientists. They isolated the bacterium Ideonella sakaiensis from sediment at a plastic bottle recycling facility in Sakai, Japan. The bacterium exhibited an ability to use PET plastic as its primary source of carbon and energy.
This finding was significant, revealing a naturally evolved organism capable of consuming a material previously thought highly resistant to biodegradation. The bacterium achieved this degradation through the production of PETase. Scientists observed Ideonella sakaiensis could degrade a thin PET film within about six weeks, highlighting its adaptation to a plastic-rich environment.
How PETase Breaks Down Plastic
PET plastic consists of long chains of repeating molecular units, making it a polymer. PETase acts as a molecular catalyst, targeting the ester bonds that link these repeating units. Its active site initiates hydrolysis, using water molecules to cleave these bonds. This process disassembles the large PET polymer into smaller, simpler molecules.
The primary product of PETase activity is mono-2-hydroxyethyl terephthalate (MHET). For complete degradation, another enzyme, MHETase, often works with PETase. MHETase further breaks down MHET into its foundational monomers: terephthalic acid (TPA) and ethylene glycol (EG). These resulting monomers are the original chemical building blocks from which PET plastic is synthesized.
The enzyme’s structure features a catalytic triad of amino acids, which facilitates the nucleophilic attack on the ester bond. This enzymatic action allows for the deconstruction of PET into its constituent parts, which can then be utilized or recovered. The breakdown of PET by PETase can occur in days, a contrast to the hundreds of years it takes for natural environmental degradation.
Engineering Enhanced PETase Variants
While the initial PETase discovery was groundbreaking, the natural enzyme had limitations, including a slow reaction rate and narrow optimal temperature range. To overcome these, scientists use protein engineering to modify the enzyme’s structure, introducing mutations to enhance performance. Engineered variants, such as FAST-PETase, show improved activity.
These engineered enzymes often exhibit greater thermal stability, functioning efficiently at higher temperatures, which is beneficial for industrial applications. Scientists have also explored combining PETase with other enzymes to create more efficient systems. A notable example is the synergistic use of PETase with MHETase. This enzymatic cocktail can break down PET more completely and rapidly, as MHETase further processes the intermediate products generated by PETase into the pure monomers. Such dual-enzyme systems have shown capabilities to improve PET hydrolysis compared to single enzymes.
Potential for Industrial Biorecycling
The development of PETase and its enhanced variants holds promise for industrial biorecycling, offering a novel approach to plastic waste management. This technology enables the “upcycling” of waste PET plastic by breaking it down into its original, pure chemical components: terephthalic acid (TPA) and ethylene glycol (EG). These recovered monomers can then be used to synthesize new, high-quality PET plastic, effectively creating a closed-loop or circular economy for plastics. This contrasts with traditional mechanical recycling methods, which often lead to a reduction in material quality with each recycling cycle.
Enzymatic recycling provides an environmentally compatible alternative to conventional methods, operating under milder conditions with less energy and chemical input. Companies are exploring the industrial-scale application of PETase and related enzymes to process post-consumer PET waste, including bottles and textiles. The goal is to establish facilities where these enzymes can efficiently depolymerize large volumes of plastic into their constituent monomers. This approach could reduce the accumulation of plastic waste in landfills and the environment, while also lessening the reliance on fossil fuels for virgin plastic production.