A new study by the ICM‑CSIC and the IQAC‑CSIC explains for the first time the molecular mechanisms preventing the enzymatic degradation of PET in its crystalline form—the most resistant fraction of plastics found in modern packaging and textiles.
Researchers from the Institute of Marine Sciences (ICM-CSIC) and the Institute of Advanced Chemistry of Catalonia (IQAC-CSIC) have identified the molecular mechanisms that block the enzymatic degradation of polyethylene terephthalate (PET), one of the world's most widely manufactured plastics, when it is in its crystalline state.
The study, published in The Journal of Physical Chemistry Letters, details why even the most advanced enzymes struggle to fully break down water bottles or polyester fibres. According to the study, the problem lies in the enormous amount of energy required for the enzyme to bind with the polymer chains when they are extremely ordered and compact, highlighting the need to design new biotechnological tools for circular and more sustainable recycling, thereby reducing dependence on fossil resources.
PET is one of the most produced plastics globally, present in millions of tonnes of waste that end up in landfills or the ocean. Although science has spent two decades perfecting enzymes called PETases to decompose this material into its original components, most only work efficiently on "amorphous" PET—the softer, more disordered portion that is more susceptible to enzymatic degradation. However, commercial products usually contain a high degree of crystallinity, with highly ordered molecules, which allows for the production of a more resistant plastic but poses a problem for biological degradation. Until now, it was not precisely understood what occurred at an atomic scale when an enzyme attempted to "bite" into these rigid structures.
Unsurmountable barriers
To carry out the work, the scientific team combined the analysis of experimental data on the shape of plastic chains with high-precision computational simulations. These simulations allowed them to observe how the enzyme binds to small fragments of plastic and to measure the energetic effort involved in this process.
"Our results demonstrate that, although the enzyme is theoretically capable of reaching the correct position to perform the chemical cut in both soft and crystalline plastic, the energetic cost to achieve this in the latter is prohibitive," explains ICM and IQAC researcher Francesco Colizzi, the lead author of the study.
The study details that, for crystalline PET, not only is significantly more energy needed for the chain to fit into the enzyme's active centre, but additional effort is also required to separate the plastic chains from one another. In the crystalline structure, the chains are so tightly packed that the enzyme simply lacks the mechanical strength required to isolate and process them. "It's like trying to untie a knot that is too tight; even if you know where the rope goes, you can't even begin to move it," Colizzi adds.
Redesigning for a sustainable future
This breakthrough does not merely describe a problem; it points toward a solution. By understanding that the limitation is structural and energetic, the scientific team can now focus on modifying the architecture of existing enzymes. In this regard, the research suggests that the PETases currently in use have design limitations that could be corrected through protein engineering, enabling them to tackle the rigidity of commercial PET without the need for costly and polluting chemical or thermal pre-treatments.
Ania Di Pede-Mattatelli, researcher at ICM-CSIC and first author of the study, highlights the social relevance of these findings:
"This knowledge is fundamental if we want enzymatic recycling to move from the laboratory to large-scale industry. If we manage to design enzymes that overcome these energy barriers, we will be much closer to a true circular economy where old bottles can be transformed into new bottles of the same quality, time and time again."
The scientific team also stresses that international collaboration is key to this next step. They are currently working with partners in other countries to apply these computational models to the development of more potent enzyme variants. The ultimate goal is to create a catalogue of biocatalysts optimised for different types of plastic waste, minimising the carbon footprint of the recycling process and offering a viable alternative to the production of virgin plastic derived from petroleum.
Ultimately, the study represents a paradigm shift, moving from observing "which enzymes work" to understanding "why they do not work on certain materials". With this map of molecular obstacles in hand, biotechnology now has a clear roadmap to tackle one of the greatest environmental challenges of the 21st century.