Understanding Enzyme Recycling Technology: Can Plastic-Eating Superenzymes Address the Plastic Crisis?

Each year, approximately 150 million tons of plastic are either landfilled or released into the environment, with over 8 million tons flowing into the oceans via rivers. Most of this plastic does not decompose fully but breaks down into microplastics. These microplastics are found not only in seawater but also ingested by marine life and even enter the human digestive system. Plastic pollution has become one of the most severe environmental issues facing humanity, and researchers are working tirelessly to find solutions to this pressing problem.

Overview of Enzyme Engineering

Entering the 21st century, humanity has advanced from the 20th century's foundational understanding of life, moving into the post-genomic era. This era focuses on exploring the core of life phenomena by examining the structure and function of genomes and their protein products, integrating all biological knowledge to establish a unified framework of general biology.

Enzyme engineering, a crucial component of biotechnology, involves using enzymes as catalysts within specific bioreactors to facilitate material transformation. Its applications span various fields, including industry, medicine, agriculture, chemical analysis, environmental protection, energy development, and theoretical life sciences.

In pollution control, enzyme technology has proven vital. Horseradish peroxidase catalyzes the oxidation of toxic aromatic compounds in the presence of hydrogen peroxide, forming insoluble precipitates. Lignin peroxidase degrades aromatic compounds and oxidizes polycyclic aromatic hydrocarbons. Laccase removes toxic phenols, and microbial lipases are used for bioremediation of oil spills and lipid-containing waste.

Meanwhile, the enzyme engineering industry has experienced rapid growth. In 1998, global industrial enzyme sales reached $1.6 billion, with projections indicating that by 2008, sales would rise to $3 billion.

The Dawn of Enzyme-Based PET Recycling: Dual Degradation by Soil Bacteria

Plastics consist of long-chain polymers, primarily categorized into thermoplastics and thermosetting plastics. Thermoplastics soften at high temperatures and harden upon cooling, making them recyclable, though their quality declines with recycling. Thermosetting plastics, once heated and cured, are nearly impossible to recycle.

Polyethylene terephthalate (PET) is a widely used thermoplastic made from petroleum-derived terephthalic acid (TPA) and ethylene glycol (EG). Global PET production increased from approximately 58 million tons in 2021 to 62 million tons in 2023, with projections reaching 72 million tons by 2025. PET's versatility allows it to be manufactured into transparent, opaque, or white materials based on its crystalline structure and particle size. It is commonly used for producing clothing fibers and containers, such as water bottles, and can also be molded into various packaging products like blister packs.

Developing efficient PET depolymerization technologies is a critical milestone for achieving true plastic recycling and advancing environmental protection. PET biodegradation has garnered significant attention due to the presence of various esterases in nature that can break down esters into acids and alcohols.

In 2012, researchers from Osaka University in Japan discovered a PET-degrading enzyme in compost, known as leaf compost cutinase (LLC). Although LLC can disrupt PET's chemical bonds, it is unstable at 65°C and degrades within days, limiting its industrial application. Effective PET depolymerization requires enzymes to remain stable at high temperatures.

In 2016, Yoshida and colleagues identified a soil bacterium, Ideonella sakaiensis 201-F6, from PET-contaminated sediment near a plastic recycling facility in Japan. I. sakaiensis is a Gram-negative, aerobic rod bacterium capable of utilizing PET as its primary carbon and energy source. It employs a dual-enzyme system to degrade PET: PETase hydrolyzes PET into BHET, MHET, and TPA, and subsequently, MHETase further converts MHET into TPA and EG, achieving complete PET degradation.

Recent Advances in PET Enzyme Degradation

Enhanced Degradation with PETase Mutants

Recent research has demonstrated that PETase mutants exhibit significantly improved PET degradation capabilities. PETase, structurally similar to cutinase that breaks down cutin, has been shown through crystal structure analysis and biochemical testing to possess an open active site and follows the catalytic mechanism of serine hydrolases. Scientists have engineered a double mutant of PETase, which shows markedly enhanced PET degradation activity at the catalytic center. Unlike the wild-type PETase, which requires hundreds of years to degrade PET in natural environments, this mutant can decompose the plastic within days.

Improving Efficiency with Dual-Enzyme Systems

Studies indicate that adding MHETase to the reaction significantly boosts PET degradation rates, with enzyme mixtures degrading PET twice as fast as PETase alone. Experiments have shown that increasing the concentrations of PETase and MHETase significantly enhances PET degradation rates, suggesting that the reaction is limited by enzyme concentration rather than substrate. Synergistic effect analyses further reveal that the presence of MHETase markedly improves overall degradation rates even at lower concentrations of PETase. The optimal ratio of PETase to MHETase is still undetermined.

Super-Enzyme Innovation Triples Degradation Speed

In recent experiments, researchers developed a "super-enzyme" by fusing MHETase and PETase into a single long-chain chimeric protein. This super-enzyme outperforms both individual PETase and MHETase in PET degradation and can effectively degrade polyethylene furanoate (PEF), a bio-plastic used for beer bottles. The chimeric protein's superior performance has tripled the degradation rate of PET and PEF, converting them into monomers within days. This innovation promises unlimited recycling and reusability of plastics, reducing dependence on fossil resources.

Additionally, a breakthrough in 2020 led to the discovery of a new enzyme capable of effectively degrading PET in just 10 hours. Researchers screened various bacteria and enzymes, including the leaf branch compost cutinase (LLC) discovered in 2012, resulting in hundreds of PET hydrolase mutants. One selected mutant demonstrated a PET degradation efficiency 10,000 times higher than natural LLC and remained stable at 72°C, near PET's melting temperature. This advance lays the groundwork for infinite PET recycling and is currently in the pilot production stage.

Commercialization Prospects of Enzyme-Based Recycling

In nature, microorganisms efficiently degrade natural polymers like cellulose and chitin through synergistic enzyme systems that have evolved over time to optimize degradation. Some soil bacteria, such as Ideonella sakaiensis, exhibit similar evolutionary traits and can process polyester substrates using a dual-enzyme system. However, commercializing this super-enzyme technology still faces several challenges.

Wankai New Materials Co., Ltd., a leading company in the bottle-grade PET industry, stands at the forefront of global production. The company is not only committed to green manufacturing but also actively expanding its PET recycling business. Its parent company, Zhink Group, has entered into a long-term strategic partnership with the French  CARBIOS to leverage its advanced enzyme degradation technology. This collaboration aims to establish China’s first industrial-scale PET biorecycling facility with a capacity of 50,000 tons per year. This initiative is expected to significantly enhance Wankai's sustainability competitiveness and drive the commercialization of enzyme-based recycling technologies.

Conclusion

Microorganisms and their enzymes offer new possibilities for plastic recycling, but our understanding of these processes is still in its early stages. With plastics primarily derived from fossil fuels and the worsening issue of environmental pollution, effective solutions are critical. The key to solving this problem lies in scaling up the reduction of plastics to their monomer components. Fortunately, the evolutionary wisdom of nature and scientific innovation provide new hope, potentially serving as breakthroughs in addressing plastic pollution.