‘Pac-Man’ enzyme breaks down bioplastics and penicillin in laboratory tests

In the ocean, plastic waste collects to form large garbage patches that endanger marine life. This is partly because the synthetic plastic polymers used today can only be broken down biologically—that is, by microorganisms—at a very slow rate. Instead, the physical fragmentation of these materials leads to the formation of microplastics and nanoplastics. However, microorganisms do colonize plastic waste in the environment, forming biofilms and creating a unique microbial habitat that researchers refer to as the “plastisphere.”

A research team at the University of Konstanz has identified a new enzyme that can not only degrade certain polyesters and bioplastics but also might provide bacteria with resistance to antibiotics. The discovery raises hopes that microorganisms can adapt to plastic degradation more rapidly than previously thought. Due to its structure featuring a wide-open active site, the researchers have named this “plastic-eating” enzyme the “Pac-Man enzyme.” Accelerating the environmental breakdown of plastic would require the use of biodegradable plastics wherever possible, as these materials can be decomposed by microorganisms.

The research project

In their research project, biologists Harry Lerner and David Schleheck from Konstanz investigated the complete microbial degradation of bioplastics. At the same time, they analyzed both the composition and the complete genetic makeup (the metagenome) of the microbial community involved in this degradation. For the study, the researchers used bioplastic materials—long-chain aliphatic polyesters (LCAP)—developed by chemist Stefan Mecking’s team. Their joint study has now been published in the journal The ISME Journal.

How did the research team conduct their study? “We buried small pieces of LCAP bioplastic film in the upper humus layer in the forest at the university’s botanical garden, about 10 centimeters (4 inches) deep,” explains Lerner. “This layer is where the breakdown of cellulose and other natural polymers, such as cutin—a plant-based polyester—takes place.”

In the laboratory, the team also mixed bioplastic powder into samples of the same forest soil. The forest samples were left untouched for a whole year, while in the laboratory, CO₂ production—and thus microbial respiration—was monitored in great detail over the course of one year to document the degradation of the materials.

“Cellulose, other types of bioplastics such as PHBV and PCL, as well as high-density plastic (HDPE) and untreated soil were used as controls in the laboratory. We found that all bioplastic materials were completely degraded within roughly 250 to 330 days. Cellulose broke down after about 80 days, whereas virtually no degradation occurred for HDPE,” says Lerner.

A surprising discovery

The analysis of the films retrieved from the forest soil held a surprise for the researchers: Electron microscopy revealed tiny holes in the material, each of which matched the size and shape of a single bacterial cell. “We hypothesized that bacteria are coated with plastic-depolymerases anchored to their cell surfaces. This would enable them to digest their way into the material and become embedded within the film, leaving behind microscopic holes of exactly this type,” explains Lerner.

In order to identify the microbes that accumulated in the soil samples during the degradation of the bioplastic materials, and the enzymes responsible for breaking them down, Lerner extracted the entire DNA from the soil’s microbial community, sequenced it and analyzed the vast amount of data with great patience and care.

The microbiologists identified a gene that was enriched exclusively in the forest soil containing LCAP. This gene encodes an esterase enzyme equipped with both a secretion signal, which directs the enzyme out of the cell, and a membrane-bound lipid anchor. This anchor ensures that the enzyme remains firmly attached to the bacterial cell surface. The “Pac-Man enzyme” had been discovered.

The enzyme held yet another surprise. “Its structure resembles that of esterases, but also that of beta-lactamases, which are bacterial enzymes that are capable of cleaving the beta-lactam ring of certain antibiotics, such as penicillin, thereby making bacteria resistant to antibiotics,” says Lerner. And indeed, the researchers were able to demonstrate the enzyme’s dual biochemical function—breaking down polyesters into monomers on the one hand and cleaving penicillin on the other—in the laboratory.

A turning point in the plastisphere?

“The plastisphere is a new habitat in our environment,” explains Schleheck. “Humans have only been introducing plastic into the environment in significant quantities for around 50 to 75 years. Since then, it has theoretically been available to microbial communities—such as bacteria, yeasts and fungi—as an additional source of carbon and energy for their growth. By ‘theoretically,’ I mean that they would certainly like to use the plastic as a growth substrate—but they cannot, because the materials are actually indigestible to microbial metabolism and are therefore hardly degraded.”

At least, so far. The “Pac-Man enzyme” could signal a turning point, indicating that certain microorganisms may be able to specialize in breaking down plastic. “I find this encouraging, because it seems that bacteria can adapt to breaking down polyester plastics more quickly than we expected. To tackle the environmental problem of plastic pollution, we humans need to work with the capabilities of microbes. Ideally, this would involve using only polymers with biochemical breaking points, such as the hydrolyzable ester bonds in polyesters like LCAP or other types of bioplastics,” concludes Schleheck.

Who’s behind this story?

Gaby Clark

MA in English, copy editor since 2021 with experience in higher education and health content. Dedicated to trustworthy science news.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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