In recent years, rising living standards have made food safety a top consumer concern. The food industry mainly uses potassium sorbate, sodium benzoate, and other synthetic preservatives to control food safety issues, especially those from foodborne pathogens. However, excessive or improper use of these preservatives can disrupt metabolic balance, causing various diseases. (Pepper et al., 2020). Consequently, the latent health hazards associated with chemical preservatives, coupled with the regulatory constraints on their use, have precipitated an urgent societal demand for a novel class of antimicrobial agents that are both safe and efficacious. Honey and pickles have a longer shelf life than other foods. Researchers have found that these foods all contain a natural organic acid-phenyllactic acid (PLA, 2-hydroxy-3-phenylpropionic acid) (Xu et al., 2020). Empirical studies have substantiated that PLA diminishes the structural integrity of cell walls, attenuates the biosynthesis of extracellular polysaccharides, and suppresses the transcriptional activity of genes implicated in polysaccharide synthesis. Moreover, PLA penetrates the DNA within microbial genomes, thereby perturbing the transcriptional machinery, which leads to the disintegration of the cell wall and biofilm architecture, and subsequently, the perturbation of cellular homeostasis (Wu et al., 2023a). PLA demonstrates a comprehensive antimicrobial profile, effectively suppressing the proliferation of a diverse array of microorganisms, including various Gram-positive and Gram-negative bacteria, yeasts, molds, and other microbial entities (Xu et al., 2020). The favorable water solubility, thermal stability, extensive pH tolerance, and inherent safety of PLA collectively underpin its potential for extensive applications across the food industry (Cortes-Zavaleta et al., 2014; Sun et al., 2015; Zhou et al., 2018). PLA can serve as a novel preservative, effectively preventing food spoilage while retaining nutritional value and flavor (Mu et al., 2012). It can be added to animal feed as an alternative to antibiotics, to prevent and cure diseases, improve meat quality, and increase egg production (Wang et al., 2009). Additionally, PLA can be employed as an intermediate in the synthesis of various pharmaceuticals, such as Englitazone and Daiclzein (Danshensu) (Lu et al., 2018, Urban and Moore, 1992). The broad application prospect of PLA has aroused great concern in the industry and academia for its synthesis.

Within the metabolic pathway of lactic acid bacteria, L-phenylalanine (L-PHE) acts as a direct precursor to PPA. The synthesis is mediated by a sequential enzymatic cascade that converts L-PHE into PLA through intermediate PPA (Huccetogullari et al., 2019). Due to its affordability and stability, L-PHE is an excellent alternative substrate for synthesizing D-phenyllactic acid (D-PLA). The multi-enzyme cascade involves at least two sequential processes, outperforming traditional methods. Enzymes are in close proximity, enhancing mass transfer and reaction efficiency. Immediate product transfer from upstream to downstream reactions eliminates extensive separation and purification, minimizing waste and costs, especially with unstable intermediates (Liang and Liang, 2020, Wu et al., 2023b, Yang et al., 2023). The high efficiency of these cascades, combined with easy enzyme expression in E. coli, makes whole-cell catalysis widely used in compound synthesis. Zhang et al. (Zhang and Li, 2018) co-expressing L-phenylalanine oxidase and lactate dehydrogenase (LDH) in E. coli and achieved the catalytic synthesis of L-PLA from L-PHE using this recombinant bacterium, resulting in a production yield of 1.47 g l−1. Hou et al. (2019) optimized the expression levels of L-amino acid deaminase (L-AAD), LDH, and FDH in recombinant E. coli by regulating ribosome binding sites, duplicating genes, and optimizing induction conditions. This approach enabled the production of 54.0 g l−1 of PLA using whole-cell catalysis. The L-PHE-PPA-PLA cascade biocatalysts, mediated by whole-cell catalysts, appear to be a potent method for PLA production. However, biocatalysts typically exhibit drawbacks such as limited operational stability and a lack of reusability, which constrains their broader application.

Immobilized enzymes or cells are capable of localizing biocatalysts within defined spatial domains, thereby preserving their catalytic activity and enhancing both their stability and reusability (Li et al., 2020, Patil et al., 2025a). Metal-organic frameworks (MOFs) are a subclass of coordination polymers that consist of metal ions or clusters connected by organic ligands to form one-, two-, or three-dimensional porous networks. Their structures may be either crystalline or amorphous, depending on the synthesis protocol (Batten et al., 2013). These materials are characterized by their expansive specific surface area, structural porosity, and the ease with which they can be chemically modified (Andarzbakhsh et al., 2024, Patil et al., 2025b). As a class of porous materials, MOFs exhibit ordered pore architectures and tunable channel dimensions, which can reduce diffusion resistance and improve mass transfer kinetics for molecules under external driving forces, making them effective immobilization matrices for biocatalysts (Nadar et al., 2020, Patil et al., 2025c). The MOFs matrix protects encapsulated enzymes from denaturation under elevated temperatures, non-physiological pH, and organic solvents, enabling catalytic activity retention under industrially relevant harsh conditions (Liang et al., 2021). Enzyme encapsulation in ZIFs was pioneered by the Ge group and Shieh group, who developed biomimetic mineralization strategies (Li et al., 2014, Shieh et al., 2015). The Falcaro group further advanced MAF-based encapsulation for precise enzyme orientation control (Falcaro et al., 2017). For instance, Cai et al. demonstrated MOF-enabled enzyme stabilization in biosensor platforms under high ionic strength conditions (Cai et al., 2021). MOFs have emerged as a robust platform for enzyme immobilization owing to their unique structural and stability advantages: chemical robustness under harsh environments, exceptional thermal tolerance, long-term operational stability, and precision-engineered pore architectures(Wang et al., 2022). Immobilization of biocatalysts within MOFs can significantly enhance their operational stability, storage stability, and reusability, demonstrating good applicability across numerous reactions (Li et al., 2023, Li et al., 2024a, Yu et al., 2022).

In the present investigation, we engineered a recombinant E. coli strain capable of co-expressing L-AAD, D-LDH, and GDH. This engineered strain facilitated a sequential enzymatic cascade for the production of D-polylactic acid (D-PLA), coupled with the in-situ regeneration of the requisite coenzyme. To enhance the operational stability and reusability of the biocatalyst, we encapsulated the co-expressing E. coli within a ZIF-90 matrix, thereby creating immobilized cell systems. The immobilized E. coli@ZIF-90 demonstrated superior operational stability and reusability compared to their free-cell counterparts, thereby enabling the efficient and stable synthesis of D-PLA.