Researchers have decoded a bacterial enzyme essential for the breakdown of styrene, an element used in the large-volume production of polystyrene, which traditionally lacks biotechnology recycling methods.
Studying the role of a particular bacterial enzyme paved the way for the biotechnological degradation of styrene.
Polystyrene, composed of styrene units, is the most commonly used plastic by volume, often found in packaging materials. Unlike PET, which can be both produced and recycled through biotechnological methods, polystyrene manufacturing remains strictly chemical. Additionally, this type of plastic cannot be broken down by biotechnological means.
Researchers are looking for ways to address this problem: an international team led by Dr Xiaodan Li from the Paul Scherrer Institute, Switzerland, in collaboration with Professor Dirk Tischler, head of the Microbial Biotechnology Research Group at the University from the Ruhr in Bochum, Germany, has decoded a bacterial enzyme that plays a key role in the degradation of styrene. This paves the way for biotechnology applications. The researchers published their results in the journal Natural chemistry in an article published on May 14, 2024.
Styrene in the environment
“Several million tonnes of styrene are produced and transported each year,” explains Dirk Tischler. “In doing so, some is also unintentionally released into the environment. » This is not, however, the only Source of styrene in the environment: it is present naturally in coal tar and lignite tar, can be present in trace amounts in the essential oils of certain plants and is form during the decomposition of plant matter. “It is therefore not surprising that microorganisms have learned to manipulate it, or even metabolize it,” explains the researcher.
Dirk Tischler was part of an international research team. Credit: RUB, Marquard
Fast but complex: microbial degradation of styrene
Bacteria and fungi, as well as the human body, activate styrene with the help of oxygen and form styrene oxide. While styrene itself is toxic, styrene oxide is even more harmful. Rapid metabolization is therefore crucial. “In some microorganisms as well as the human body, the epoxide formed by this process typically undergoes conjugation with glutathione, making it both more soluble in water and easier to break down and excrete” , explains Dirk Tischler. “This process is very fast, but also very costly for the cells. One molecule of glutathione must be sacrificed for each molecule of styrene oxide.
The formation of the glutathione conjugate and the question of whether, or rather how, the glutathione can be recovered are part of ongoing research at the MiCon Graduate School of the Ruhr University in Bochum, funded by the German Research Foundation (DFG). Some microorganisms have developed a more effective variant. They use a small membrane protein, namely styrene oxide isomerase, to break down the epoxy.
Styrene oxide isomerases are more effective
“Even after the first enrichment of styrene oxide isomerase from the soil bacterium Rhodococcus, we observed its reddish color and showed that this enzyme is bound in the membrane,” explains Dirk Tischler. Over the years, he and his team have studied various enzymes in the family and used them primarily in biocatalysis. All of these styrene oxide isomerases have high catalytic efficiency, are very fast and do not require any additional substances (co-substrates). They therefore enable rapid detoxification of toxic styrene oxide in the body and also a powerful biotechnological application in the field of fine chemical synthesis.
“To optimize these, we need to understand their function,” emphasizes Dirk Tischler. “We have made considerable progress in this area thanks to our international collaboration between researchers from Switzerland, Singapore, the Netherlands and Germany.” The team showed that the enzyme exists in nature in the form of a trimer comprising three identical units. Structural analyzes revealed that there is a heme cofactor between each subunit and that it is charged with an iron ion. Heme forms an essential part of the so-called active pocket and plays an important role in substrate binding and processing. The iron ion of the heme cofactor activates the substrate by coordinating oxygen atom styrene oxide. “This means that a new biological function of heme in proteins has been comprehensively described,” concludes Dirk Tischler.
