NAD+-Dependent Enzymatic Route for the Epimerization of Hydroxysteroids

Epimerization of cholic and chenodeoxycholic acid (CA and CDCA, respectively) is a notable conversion for the production of ursodeoxycholic acid (UDCA). Two enantiocomplementary hydroxysteroid dehydrogenases (7 alpha- and 7 beta-HSDHs) can carry out this transformation fully selectively by specific oxidation of the 7 alpha-OH group of the substrate and subsequent reduction of the keto intermediate to the final product (7 beta-OH). With a view to developing robust and active biocatalysts, novel NADH-active 7 beta-HSDH species are necessary to enable a solely NAD(+)-dependent redox-neutral cascade for UDCA production. A wild-type NADH-dependent 7 beta-HSDH from Lactobacillus spicheri (Ls7 beta-HSDH) was identified, recombinantly expressed, purified, and biochemically characterized. Using this novel NAD(+)-dependent 7 beta-HSDH enzyme in combination with 7 alpha-HSDH from Stenotrophomonas maltophilia permitted the biotransformations of CA and CDCA in the presence of catalytic amounts of NAD(+), resulting in high yields (>90 %) of UCA and UDCA.