Characterization and engineering control of the effects of reactive oxygen species on the conversion of sterols to steroid synthons in Mycobacterium neoaurum

The conversion of sterols to steroid synthons by engineered mycobacteria comprises one of the basic ways for the production of steroid medications in the pharmaceutical industry. Here, we revealed that high amounts of reactive oxygen species (ROS) generate during the conversion process of sterols, which impairs the cell viability of mycobacterial cells and thus hinders the conversion of sterols to steroid synthons. Accordingly, the endogenous antioxidants for detoxifying ROS in mycobacteria, ROS scavenging enzymes and low molecular weight thiols, were examined. The results revealed that three antioxidants, catalase (CAT), mycothiol (MSH), and ergothioneine (EGT), demonstrated efficacy toward neutralizing the excessive ROS produced during sterol metabolism. CAT overexpression or MSH or EGT augmentation enhanced the conversion of phytosterols to 22-hydroxy-23,24-bisnorchol-4-ene-3-one (4-HBC) by 18.9%, 23.8%, and 32.1%, respectively, and also enhanced the cell viability, indicating the benefits of these antioxidants in reducing ROS-induced stress. Further combinatorial augmentation of CAT, MSH, and EGT demonstrated enhanced effects toward intracellular ROS scavenging, resulting in 54.2% greater cell viability and 47.5% enhancement in 4-HBC production. These findings indicated that the excessive ROS induces cell stress, in turn limiting the conversion of sterols, whereas neutralization of the excessive ROS by combined control of CAT, MSH, and EGT serves as an effective strategy to boost the conversion productivity of sterols to steroid synthons.