Microbial Enzyme Could Make Plastics Biodegradable

Microbial Enzyme Could Make Plastics Biodegradable
by Robert Schreiber
Berlin, Germany (SPX) May 16, 2024

"Several million tons of styrene are produced and transported every year," says Dirk Tischler. "In the process, some of it also gets released unintentionally into the environment." This is not the only source of styrene in the environment, however: It occurs naturally in coal tar and lignite tar, can occur in traces in essential oils from some plants and is formed during the decomposition of plant material. "It is therefore not surprising that microorganisms have learned to handle or even to metabolize it," says the researcher.

Bacteria, fungi, and the human body activate styrene with oxygen, forming styrene oxide. While styrene is toxic, styrene oxide is more harmful. Rapid metabolization is crucial. "In some microorganisms as well as in the human body, the epoxide formed by this process usually undergoes glutathione conjugation, which makes it both more water-soluble and easier to break down and excrete," explains Dirk Tischler. "This process is very fast, but also very expensive for the cells. A glutathione molecule has to be sacrificed for every molecule of styrene oxide."

Research at the MiCon Graduate School at Ruhr University Bochum, funded by the German Research Foundation (DFG), is focused on the formation of the glutathione conjugate and the potential recovery of glutathione. Some microorganisms have developed a more efficient variant, using a small membrane protein, styrene oxide isomerase, to break down the epoxide.

"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 of the family and used them primarily in biocatalysis.

These styrene oxide isomerases have high catalytic efficiency, are very fast, and don't require additional substances. They allow rapid detoxification of styrene oxide in the organism and have potent biotechnological applications in fine chemical synthesis.

"In order to optimize the latter, we do need to understand their function," points out Dirk Tischler. "We made considerable progress in this area in our international collaboration between researchers from Switzerland, Singapore, the Netherlands, and Germany."

The team showed that the enzyme exists in nature as a trimer with three identical units. Structural analyses revealed a heme cofactor between each subunit, loaded with an iron ion. The heme forms an essential part of the active pocket and is relevant for the fixation and conversion of the substrate. The iron ion of the heme cofactor activates the substrate by coordinating the oxygen atom of the styrene oxide. "This means that a new biological function of heme in proteins has been comprehensively described," concludes Dirk Tischler.

Research Report:Structural Basis of the Meinwald Rearrangement Catalyzed by Styrene Oxide Isomerase