Cloning and characterization of the NAD-dependent 7α-hydroxysteroid dehydrogenase from Bacteroides fragilis

The NAD-linked 7α-hydroxysteroid dehydrogenase (7-HSDH) from Bacteroides fragilis ATCC 25285 was characterized and its gene cloned. The enzyme displayed optimal activities at pH 8.5 (NAD reduction) and 6.5 (NADH oxidation). The lowest Km and highest Vmax values were observed with chenodeoxycholic acid and its conjugates. The protein had subunits of 27.4 kDa and a native size of 110 kDa, suggesting a homotetrameric composition. The enzyme was relatively thermostable, retaining 95% of initial activity after 1 h at 65°C. A DNA probe based on the N-terminal amino acid sequence hybridized to a 2373-bp HindIII fragment of B. fragilis DNA. This fragment was cloned into E. coli and sequenced, revealing a 780-bp open reading frame. The predicted amino acid sequence of the ORF showed strong sequence similarity to three other bacterial 7-HSDHs, all in the short-chain dehydrogenase family. The regulation of expression of this gene is currently under investigation.