Biological upcycling of conditionally biodegradable plastics: enzymatic depolymerization to microbial valorization and consolidated bioprocessing frameworks

초록

Most biodegradable plastics, including polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS), poly-lactic acid (PLA), and poly(epsilon-caprolactone) (PCL), are conditionally biodegradable, as their decomposition occurs only under specific managed environments. This limitation highlights the need for advanced upcycling technologies to achieve a truly circular plastic economy. Among various approaches, biological upcycling offers a sustainable route by integrating enzymatic depolymerization with microbial valorization. The upcycling process involves the two following interconnected steps: (i) enzymatic depolymerization of polymers under mild and eco-friendly conditions and (ii) metabolic conversion of the released monomers into high-value products via native or engineered microbial pathways. This review summarizes the recent advances across both fronts-from the discovery and engineering of plastic-degrading enzymes to the design of microbial platforms for monomer upcycling. In particular, we focus on conditionally biodegradable plastics (PBAT, PBS, PLA, and PCL) and their depolymerized intermediates, such as terephthalic acid, adipic acid, succinic acid, lactic acid, 1,4-butanediol, and 6-hydroxyhexanoic acid. Building on these developments, we propose future directions for plastic upcycling frameworks inspired by lignocellulosic biomass conversion, including separate hydrolysis and fermentation, simultaneous saccharification and fermentation, and consolidated bioprocessing. By outlining design principles that inte-grate enzyme secretion, thermophilic-mesophilic task partitioning, transporter optimization, and redox-cofactor balancing, we envision a next-generation circular biomanufacturing system that transforms plastic waste into renewable resources, bridging the gap between biodegradation and true material circularity.

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