High-efficiency bioconversion of phytosterol to bisnoralcohol by metabolically engineered Mycobacterium neoaurum in a micro-emulsion system

21-Hydroxy-20-methylpregn-4-en-3-one (4-HBC, bisnoralcohol) is a crucial intermediate for the synthesis of steroidal drugs. Significant challenges including by-products formation and poor substrate solubility were still confronted in its main synthetic route by microbial conversion from phytosterol. Construction of a direct bioconversion pathway to 4-HBC and an efficient substrate emulsion system is therefore urgently required. In this study, three novel isoenzymes of 3-ketosteroid-Δ¹-dehydrogenase (KstD) and 3-ketosteroid 9α-hydroxylase (KsH) in Mycobacterium neoaurum were excavated and identified as KstD4, KstD5, and KsHA3. A strain capable of fully directing the synthesis of 4-HBC was metabolically engineered via serial genetic deletion combined with enhanced expression of cholesterol oxidase (ChOx2) and enoyl-CoA hydratase (EchA19). Moreover, a micro-emulsion system combined with soybean oil and hydroxypropyl-β-cyclodextrin improved substrate solubility and bioavailability. In batch fermentation, molar yield of 96.7% with 39.5 g L⁻¹ 4-HBC was obtained from 50 g L⁻¹ phytosterol. Our findings demonstrate the potential for industrial-scale biosynthesis of 4-HBC.