How to make ubiquitous plastics biodegradable, understanding the function of specific bacterial enzymes paves the way for biotech degradation of styrene.
Polystyrene (PS) is made from styrene monomer and is the most used plastic, e.g. for packaging. Unlike PET, which can now be produced and recycled using biotechnological methods, the production of polystyrene has so far been a purely chemical process. This plastic also cannot be broken down by biotechnological means.
Researchers are looking for ways to rectify this situation: an international team led by Dr. Xiaodan Li from the Paul Scherrer Institute in Switzerland, in collaboration with Prof. Dirk Tischler, head of the Microbial Biotechnology Research Group at the Ruhr University in Bochum, Germany, decoded a bacterial enzyme that plays a key role in styrene degradation. This paves the way for biotechnology applications.On May 14, 2024, the researchers published their findings in the journal Nature Chemistry.
Dirk Tischler is the leader of the international research team.
Styrene in the environment
Dirk Tischler says, “Millions of tons of styrene are produced and transported every year, and in the process some of it is also inadvertently released into the environment.” However, this is not the only source of styrene in the environment; it occurs naturally in coal tar and lignite tar, as well as in the essential oils of some plants, and is formed during the decomposition of plant material. “Not surprisingly, therefore, microbes have learned to process and even metabolize it.”
Rapid but complex: microbial degradation of styrene
Bacteria and fungi, as well as the human body, activate styrene with the help of oxygen to form styrene oxide. While styrene itself is toxic, oxidized styrene is much more harmful. Therefore rapid metabolism is vital. “In some microorganisms and in humans, the epoxide formed by this process usually undergoes glutathione conjugation, which makes it more water-soluble and easier to break down and excrete.” Dirk Tischler explains, “The process is very fast, but also very expensive for the cell. One glutathione molecule is sacrificed for every molecule of oxidized styrene."
The MiCon Graduate School of the Ruhr University Bochum, funded by the German Research Foundation (DFG), is currently investigating the formation of glutathione conjugates and whether or how glutathione is recycled. Some microbes have developed a more efficient variant. They use a small membrane protein, styrene oxide isomerase, to break down the epoxide.
Oxidized styrene isomerase is more effective
Dirk Tischler explains, “Even after the first enrichment of the soil bacterium Rhodococcus erythropolis for oxidized styrene isomerase, we observed its red color and showed that this enzyme is bound in the membrane.”
Over the years, he and his team have studied various enzymes of this family and used them mainly for biocatalysis. All of these styrene oxide isomerases have high catalytic efficiency, are very fast and do not require any additional substances (co-substrates). Thus, they allow rapid detoxification of toxic styrene oxide in living organisms and also have potential biotechnological applications in the field of fine chemical synthesis.
Dirk Tischler noted, “In order to optimize the latter, we really need to understand their Through international collaboration between researchers in Switzerland, Singapore, the Netherlands and Germany, we have made considerable progress in this area.”
The team showed that this enzyme exists in nature as a trimer of three identical units. Structural analysis showed that each subunit has a heme cofactor between them and contains iron ions. Heme forms an important part of the so-called active pocket, which is associated with the immobilization and conversion of substrates. The iron ions of the heme cofactor activate the substrate by coordination with the oxygen atom of the oxidized styrene.
“This means that a new biological function of heme in proteins has been fully characterized,” concludes Dirk Tischler.
