Catalytic Mechanism and Engineering of Hydroxysteroid Dehydrogenases in the Biosynthesis of Ursodeoxycholic Acid

Hydroxysteroid dehydrogenases (HSDHs) are crucial NAD(P)H-dependent enzymes that catalyze the redox reactions of steroid substrates, playing an essential role in bile acid metabolism and the biosynthesis of ursodeoxycholic acid (UDCA). This review systematically summarizes the classification and catalytic mechanisms of HSDHs. It highlights advances made in the past 10 years in the discovery of novel HSDHs and in the application of protein engineering strategies to optimize their catalytic efficiency, substrate specificity, and cofactor dependency. Key approaches include rational design and directed evolution of HSDHs, cofactor regeneration systems, enzyme immobilization, and process optimization for industrial applications. The integration of these strategies has significantly enhanced the sustainable biosynthesis of UDCA, overcoming challenges such as low enzyme stability, cofactor imbalance, and process scalability. Last, the challenges in advancing HSDH engineering are highlighted, providing theoretical insights to support industrial-scale UDCA production.