An Enzymatic Strategy for the Selective Methylation of High-Value-Added Tetrahydroprotoberberine Alkaloids

Tetrahydroprotoberberines (THPBs) are plant-specific alkaloids with significant medicinal value. They are present in trace amounts in plants and are difficult to chemically synthesize due to stereoselectivity and an unfavorable environment. In this study, a selective methylation strategy was developed for the biocatalysis of seven high-value-added THPB compounds using 4'-O-methyltransferase (Cj4'OMT), norcoclaurine 6-O-methyltransferase (Cj6OMT), and (S)-scoulerine 9-O-methyltransferase (SiSOMT and PsSOMT) in engineered E. coli. The methyltransferases Cj4'OMT, Cj6OMT, PsSOMT, and SiSOMT were expressed heterologously in E. coli. Compound 1 (10-methoxy-2,3,9-tetrahydroxyberbine) was synthesized using the recombinant E. coli strain Cj4'OMT and the substrate 2,3,9,10-tetrahydroxyberbine. Compound 2 (9-methoxy-2,3,10-tetrahydroxyberbine) was produced in the recombinant Escherichia coli (E. coli) strain PsSOMT, and compounds 2 and 3 (discretamine) were produced in the recombinant E. coli strain SiSOMT. Compounds 4 (9,10-methoxy-2,3-tetrahydroxyberbine) and 5 (corypalmine) were obtained by co-culturing the recombinant strains Cj4'OMT and SiSOMT with substrate. Compounds 6 (scoulerine) and 7 (isoscoulerine) were produced by co-culturing the substrate with the recombinant strains Cj4'OMT and Cj6OMT. To increase the yield of novel compound 2, the flask culture conditions of the engineered SiSOMT strain were optimized, resulting in the production of 165.74 mg/L of this compound. This study thus presents an enzymatic approach to the synthesis of high-value-added THPBs with minimum environmental wastage.

Conjugate Vaccines from Bacterial Antigens by Squaric Acid Chemistry: A Closer Look

By using O-SP-core (O-SPcNH2 ) polysaccharide, isolated from Vibrio cholera O1 lipopolysaccharide (LPS) and related synthetic substances, a detailed study of factors that affect conjugation of bacterial polysaccharides to protein carriers through squaric acid chemistry to form conjugate vaccines has been carried out. Several previously unrecognized processes that take place during the squarate labeling of the O-SPcNH2 and subsequent conjugation of the formed squarate (O-SPcNH-SqOMe) have been identified. The efficiency of conjugation at pH 8.5, 9.0, and 9.5 to bovine serum albumin (BSA) and to the recombinant tetanus toxin fragment C (rTT-Hc) has been determined. The study led to a protocol for more efficient labeling of O-SPcNH2 antigen with the methyl squarate group, to yield a higher-quality, more potent squarate conjugation reagent. Its use resulted in about twofold increases in conjugation efficiency (from 23-26 % on BSA to 51 % on BSA and 55 % on rTT-Hc). The spent conjugation reagent could be recovered and regenerated by treatment with MeI in the absence of additional base. The immunological properties of the experimental vaccine made from the regenerated conjugation reagent were comparable with those of the immunogen made from the parent O-SPcNH-SqOMe.