Berberine, palmatine, jatrorrhizine, and other protoberberine alkaloids (PBAs) from Huanglian exhibit potent antimicrobial, anti-inflammatory, and anticancer activities. Yet their structural complexity makes scalable production challenging. Plant extraction depends on crop cycles and climate, while chemical synthesis suffers from low regioselectivity and harsh conditions. Heterologous biosynthesis in microbes has been hindered by poorly expressed plant enzymes—particularly cytochrome P450s, berberine bridge enzyme (BBE), and (S)-tetrahydroprotoberberine oxidase (STOX).
To bypass these obstacles, the team designed a five-module cascade: a biocatalytic module for benzylisoquinoline alkaloid (BIA) formation, an N-methylation module, a BBE module, a 9-O-methylation module, and a final chemical module for methylenation and aromatization. This strategy replaces problematic plant enzymes (CYP719A and STOX) with a single chemical step using dibromomethane and heating.
The coclaurine N-methyltransferase (CjCNMT) from Coptis japonica was mutated at three residues (N92T/G98S/I234V), boosting activity 9.1-fold. The berberine bridge enzyme (EcBBE) from Eschscholzia californica gained a 6.2-fold improvement through a single I419F mutation, which shortened the distance between the substrate and the FAD cofactor.
Each module was expressed in separate E. coli strains to avoid metabolic burden, enabling independent optimization. In a 50 mL whole cell cascade, the key intermediate (S)-6a was produced at 1.93 g/L with 59% yield from substrate 1a. The chemical module then converted this intermediate into berberine (44% overall yield) via sequential methylenation and aromatization.
By simply swapping enzyme variants in the methylation module, the team accessed five additional PBAs: demethyleneberberine (48%), jatrorrhizine (39%), berberrubine (31%), columbamine (42%), and palmatine (44%). Each product was obtained with high purity without laborious chromatography.
The study establishes a paradigm for hybrid chemoenzymatic synthesis that could be extended to other plant derived bioactive molecules, accelerating the modernization of traditional medicine.