Applications of Synthetic Biology in Microbial and Enzymatic Systems for Microplastic Degradation: A Review

Microplastic pollution poses a persistent environmental challenge due to the chemical recalcitrance, low bioavailability, and environmental stability of synthetic polymers. Synthetic biology has emerged as a powerful, integrative framework for enhancing biological degradation of microplastics by systematically engineering enzymes, microbial chassis, and metabolic pathways. This narrative review examines recent advances in enzyme engineering, whole-cell engineering, and metabolic engineering that collectively enhance the efficiency, robustness, and scalability of microbial and enzymatic systems for plastic degradation. At the enzyme level, rational design, directed evolution, and computationally guided approaches have driven substantial improvements in the catalytic performance of plastic-degrading enzymes, particularly polyester hydrolases such as PETase, MHETase, cutinases, and LCC variants. Structure-guided mutagenesis and machine-learning–assisted workflows have yielded next-generation enzymes with enhanced activity, thermostability, and substrate affinity, enabling the depolymerization of semicrystalline and post-consumer plastics under increasingly mild, industrially relevant conditions. Domain fusion strategies further address mass-transfer limitations by improving enzyme–polymer interactions, especially for highly crystalline substrates. Beyond isolated enzymes, whole-cell engineering integrates enzyme production, localization, and activity within living systems. Surface display platforms, biofilm-based immobilization, secretion systems, and multi-enzyme cascades facilitate sustained enzyme–substrate contact, reduce diffusional losses, and enable sequential depolymerization. Engineered microbial chassis have demonstrated effective microplastic degradation in controlled environments, although catalytic efficiency, intermediate toxicity, and biosafety concerns currently limit deployment in open environments. Metabolic engineering complements depolymerization by enabling microbial assimilation and conversion of plastic-derived monomers into central metabolites or value-added products, supporting closed-loop recycling and upcycling concepts. However, pathway complexity, flux imbalance, and substrate toxicity remain significant constraints. Overall, the review highlights that the most effective synthetic biology strategies for microplastic degradation arise from integrating enzyme engineering with whole-cell and systems-level optimization. While technical and economic challenges persist, continued advances in computational design, process integration, and systems synthetic biology hold strong promise for developing scalable, environmentally safe solutions aligned with circular economy principles.