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Deep-sea enzyme breaks down PET

Researchers from the universities of Kiel, Hamburg and Düsseldorf have, with the help of complex analysis methods, identified in a database with water specimens from all over the world an enzyme that stems from bacteria from prehistoric times – so-called archaea – in the deep waters off Venezuela. The newly discovered enzyme PET46 is said to be able, unlike the previously known biocatalysts with the ability to break down plastics, to withstand warm temperatures and consequently to break down the polyethylene terephthalate (PET) plastic more quickly. According to the research team, this special characteristic and other biochemical properties make PET46 a highly interesting candidate to combat plastic pollution, both in the sea and on the land.
 

The study entitled "An archaeal lid-containing feruloyl esterase degrades polyethylene terephthalate" was published in September 2023 in the trade journal Communications Chemistry. According to the information supplied, scientists from the working group headed by the Director of Studies, Professor Ruth Schmitz-Streit from the Christian Albrechts University of Kiel (CAU), participated in the study along with researchers from the University of Hamburg and the Heinrich Heine University of Düsseldorf (HHU). In the study, it was possible to demonstrate for the first time with the help of microorganisms from the deep sea that polymers such as PET are continuously broken down by an enzyme. According to the research team, the results broaden the knowledge not only about PET-degrading enzymes, but also about the basic mechanism. They also increase the understanding of how many PET-degrading enzymes there could be in the world's seas.
 
According to the study, the research team has succeeded for the first time in identifying the PET-degrading enzyme PET46 and describing it biochemically. For this, the gene was identified from a deep-sea specimen with the aid of similarities to known sequences, the correspondingly coded gene was synthesised, the protein in the bacterium Escherichia coli was produced, and then subsequently investigated biochemically and structurally. The enzyme PET46 stemming from prehistoric bacteria (archaea) has, according to the research team, many unusual properties, differing significantly from previously discovered enzymes and extending the known inventory of PET-degrading enzymes with a fundamentally new structure. In fact, the breakdown process with PET46 takes place through a different mechanism of substrate-binding than with the previously known PET-degrading enzymes: PET46 has a "lid" of 45 amino acids above the active enzyme centre, which exerts a positive effect on the PET-degrading activity of the enzyme. "Because of these special characteristics, PET46 is very robust, can also withstand elevated temperatures and can break down both very long-chain PET molecules – so-called polymers – and shorter-chain PET molecules – so-called oligomers, as a result of which the degradation of PET can take place continuously," explains the head of studies, Professor Ruth Schmitz-Streit. According to the study, the analysis of PET46 has also resulted at molecular level in significant similarities with another enzyme, namely ferulic acid esterase. This is an enzyme that splits and breaks down the natural polymer lignin in plant cell walls. Because of the structural similarities between lignin and PET, PET-degrading enzymes such as PET46 could be important for the composting of wood on the forest floor, says Schmitz-Streit. In the next step, the properties of PET46 are to be further improved in order to make the breaking down of PET more efficient. In future, the enzyme is to be produced industrially and PET is to be broken down in special facilities. PET46 is, however, only an initial approach because the research team is also studying plastic litter from the sea for other possible enzymes. "The aim of our research is to find new enzymes that can, at the end of it all, recycle PET on an industrial scale," says Dr. Pablo Perez-Garcia, a microbiologist from the University of Hamburg who is the initial author of the study.
 
The research results came about in a project called PLASTISEA, which is coordinated by the GEOMAR Helmholtz Centre for Ocean Research, Kiel, and funded by the Federal Ministry of Education and Research (BMBF) as part of "New biotechnological processes on the basis of marine resources – BioProMare".
 
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