Science Focus (issue 25)

9 oxidases identified in wax worm saliva. In addition, the gut microbiome of wax worms also appears to involve in the digestion of PE, with the genus Acinetobacter suggested to be the major contributor to the effect [3]. Degradation of Polyethylene Terephthalate (PET) by Bacteria Around the time of Bertocchini’s discovery, a group of scientists in Japan also discovered the ability of bacteria to degrade a different type of plastic [4]. Named Ideonella sakaiensis, the bacterium was able to degrade polyethylene terephthalate (PET), the main component of plastic bottles. This bacterium produced two digestive enzymes known as PETase (or PET hydrolase) and MHETase to dismantle the polymer (Figure 2). The former acts on the ester bonds in PET, breaking the polymer down into its monomers, mono(2-hydroxyethyl) terephthalic acid (MHET); the latter further breaks down MHET into terephthalic acid (TPA) and ethylene glycol (EG). Further metabolisms enable the utilization of these compounds as energy and carbon sources by the bacterium. To have any real impact on the degradation of plastic waste, the stability and efficiency of individual enzymes need to be tremendously enhanced – this is precisely what scientist Hal Alper has been working on [5]. Using artificial intelligence, his team ran through a database of enzymes to devise an optimal combination of mutations that would speed up the degradation of PET. When five mutations were introduced to the wild-type PETase, the resulting enzyme FAST-PETase could nearly completely degrade untreated, postconsumer-PET in one week, and work between 30 °C and 50 °C and various pH levels. Furthermore, scientists have also successfully combined PETase and MHETase by physically connecting the two enzymes with a linker peptide to create a “superenzyme” capable of degrading PET at a rate six times faster than using PETase alone [6, 7]. These approaches hold immense potential for accelerating the decomposition of PET, taking us one step closer to solving the real-world problem on plastic waste management. More interestingly, there was another “delicious” breakthrough made by a team of scientists at Edinburgh [8]. They found an enzymatic pathway to convert post-consumer PET waste into vanillin – the main component in vanilla flavoring! Once the PET plastic was broken down into TPA and EG, genetically engineered Escherichia coli bacteria expressing five different enzymes were added to the degradation products, which results in a step-by-step synthesis of vanillin from TPA at a conversion rate of up to 79%. This biosynthetic pathway offers us with a way to upcycle plastic waste, creating a product with a higher value. The race to further harness enzymes for plastic degradation is underway, and could open up the possibility for a cleaner future. With the aid of the power of nature, we are closer to turning the tables on plastic pollution. Figure 1 Chemical structure of PE. Figure 2 Degradation pathway of PET 處理塑膠廢料一直都是全球面對的挑戰,亟需各方關 注和創新的解決方案。然而試想想,如果大自然早已給予 我們解決方案呢?在與塑膠污染的搏鬥中,人類已經發 現蠕蟲和細菌驚人的塑膠分解能力 [1]。讓我們看看這 些生物如何分解塑膠,過程背後的原理,以及未來發展 的方向。