A novel biotransformation process of 4'-demethylepipodophyllotoxin to 4'-demethylepipodophyllic acid by Bacillus fusiformis CICC 20463, Part II: process optimization

Chemistry and Biological Activities of Naturally Occurring and Structurally Modified Podophyllotoxins

Plants containing podophyllotoxin and its analogues have been used as folk medicines for centuries. The characteristic chemical structures and strong biological activities of this class of compounds attracted attention worldwide. Currently, more than ninety natural podophyllotoxins were isolated, and structure modifications of these molecules were performed to afford a variety of derivatives, which offered optimized anti-tumor activity. This review summarized up to date reports on natural occurring podophyllotoxins and their sources, structural modification and biological activities. Special attention was paid to both structural modification and optimized antitumor activity. It was noteworthy that etoposide, a derivative of podophyllotoxin, could prevent cytokine storm caused by the recent SARS-CoV-2 viral infection.

Engineering the Entrance of a Flavonoid Glycosyltransferase Promotes the Glycosylation of Etoposide Aglycone

Enzyme entrances, which function as the first molecular filters, influence substrate selectivity and enzymatic activity. Because of low binding affinities, engineering enzyme entrances that recognize non-natural substrates is a major challenge for artificial biocatalyst design. Here, the entrance of flavonoid glycosyltransferase UGT78D2 was engineered to promote the recognition of the aglycone of etoposide, a chemotherapeutic agent. We found that Q258, S446, R444, and R450, the key residues surrounding the substrate entrance, specifically guide the flux of etoposide aglycone, which has a high steric hindrance, into the active site; this activity was inferred to be determined by the entrance size and hydrophobic and electrostatic interactions. Engineering the coordination of Q258 and S446 to increase the entrance size and hydrophobic interaction between UGT78D2 and etoposide aglycone increased the affinity by 10.10-fold and the conversion by 10%. The entrance-engineering strategy applied in this study can improve the design of artificial biocatalysts.

Novel tandem biotransformation process for the biosynthesis of a novel compound, 4-(2,3,5,6-tetramethylpyrazine-1)-4'-demethylepipodophyllotoxin

According to the structure of podophyllotoxin and its structure-function relationship, a novel tandem biotransformation process was developed for the directional modification of the podophyllotoxin structure to directionally synthesize a novel compound, 4-(2,3,5,6-tetramethylpyrazine-1)-4'-demethylepipodophyllotoxin (4-TMP-DMEP). In this novel tandem biotransformation process, the starting substrate of podophyllotoxin was biotransformed into 4'-demethylepipodophyllotoxin (product 1) with the demethylation of the methoxyl group at the 4' position by Gibberella fujikuroi SH-f13, which was screened out from Shennongjia prime forest humus soil (Hubei, China). 4'-Demethylepipodophyllotoxin (product 1) was then biotransformed into 4'-demethylpodophyllotoxone (product 2) with the oxidation of the hydroxyl group at the 4 position by Alternaria alternata S-f6, which was screened out from the gathered Dysosma versipellis plants in the Wuhan Botanical Garden, Chinese Academy of Sciences. Finally, 4'-demethylpodophyllotoxone (product 2) and ligustrazine were linked with a transamination reaction to synthesize the target product 4-TMP-DMEP (product 3) by Alternaria alternata S-f6. Compared with podophyllotoxin (i.e., a 50% effective concentration [EC(50)] of 529 μM), the EC(50) of 4-TMP-DMEP against the tumor cell line BGC-823 (i.e., 0.11 μM) was significantly reduced by 5,199 times. Simultaneously, the EC(50) of 4-TMP-DMEP against the normal human proximal tubular epithelial cell line HK-2 (i.e., 0.40 μM) was 66 times higher than that of podophyllotoxin (i.e., 0.006 μM). Furthermore, compared with podophyllotoxin (i.e., log P = 0.34), the water solubility of 4-TMP-DMEP (i.e., log P = 0.66) was significantly enhanced by 94%. For the first time, the novel compound 4-TMP-DMEP with superior antitumor activity was directionally synthesized from podophyllotoxin by the novel tandem biotransformation process developed in this work.