Enzymatic mechanism of polystyrene biodegradation: key depolymerisation enzymes in the Stenotrophomonas maltophilia sp. ZSL2493

Annual global plastic waste exceeds 287 million tonnes, devastating ecosystems and health. As a promising solution, yellow mealworm larvae have shown the ability to degrade plastics. In this study, we explored the regional characteristics of the yellow mealworm and isolated a novel enteric bacterium, S . maltophilia ZSL2493, from the intestinal tract of yellow mealworm larvae, which achieved a biodegradation efficiency of 14.7% of polystyrene (PS) within 30 days. The two most promising candidate enzymes, kynurenine 3-monooxygenase (Kmo) and 4-hydroxyphenylpyruvate dioxygenase (Hpd), were identified by transcriptome mining and molecular docking simulations. Their depolymerisation ability was also confirmed by heterologous expression experiments. Recombinant Kmo and Hpd reduced the mass of PS by 6.14% and 3.96%, respectively, within 30 days. Quantitative analysis by liquid chromatography showed that the yield of styrene monomer was 1.37 μg/mL (Kmo-treated group) and 0.95 μg/mL (Hpd-treated group), respectively, after 30 days of incubation, which is a direct evidence of the enzyme's cleavage of polymer chains. This study elucidates the key enzymatic drivers in the biodegradation of PS and promotes the development of enzymatic recycling strategies for difficult-to-degrade plastics.