Boosting the Biosynthesis of Ursodeoxycholic Acid via a Novel 7β-Hydroxysteroid Dehydrogenase from Olsenella sp. and Biotransformation Systematic Optimization

7β-Hydroxysteroid dehydrogenase (7β-HSDH), a key enzyme for the enzymatic synthesis of ursodeoxycholic acid, reduces the ketocarbonyl group at the C-7 site to a β-oriented hydroxyl group. In this study, a 7β-HSDH_Osp from Olsenella sp. An188 with perfected enzymatic properties (e.g., high conversion rate) was heterologously expressed in Escherichia coli in an efficient manner. In combination with 7α-hydroxysteroid dehydrogenase, the conversion rate of chenodeoxycholic acid (CDCA) to ursodeoxycholic acid (UDCA) reached 68.3% under the optimal conditions of the cascade "one-pot" catalytic system using cofactor self-recycling with individual expression of 7β-HSDH_Osp and 7α-HSDH_Cd. For the cascade "one-pot" catalytic system using cofactor self-recycling with coexpression of 7β-HSDH_Osp and 7α-HSDH_Cd, the conversion rates from CDCA to UDCA via whole-cell catalysis were higher than those via the free enzyme. The cofactor NADH regeneration system was more favorable for the production of intermediate 7K-LCA from CDCA using 7β-HSDH_Osp than the cofactor NADPH regeneration system. The "two-step" reaction in "one-pot" using cofactor regeneration with coexpression of 7α-HSDH_Es and LDH and 7β-HSDH_Osp and GDH had a high conversion rate (81%) from CDCA to UDCA. The conversion rate (90.25%) from CDCA to UDCA via the "two-step" reaction in "one-pot" using cofactor regeneration with individual expression of HSDHs (7β-HSDH_Osp and 7α-HSDH_Es) and coupled enzymes was higher compared to the "two-step" reaction in "one-pot" using cofactor regeneration with coexpression of HSDHs and coupled enzymes. The conversion rate of UDCA from 100 g·L–1 CDCA reached 84% using the optimized method.