A ()-3-Hydroxybutyrate Dehydrogenase Belonging to the 3-Hydroxyacyl-CoA Dehydrogenase Family Facilitates Hydroxyacid Degradation in Anaerobic Bacteria.

Ketone bodies, including acetoacetate, 3-hydroxybutyrate, and acetone, are produced in the liver of animals during glucose starvation. Enzymes for the metabolism of ()-3-hydroxybutyrate have been extensively studied, but little is known about the metabolism of its enantiomer ()-3-hydroxybutyrate. Here, we report the characterization of a novel pathway for the degradation of ()-3-hydroxybutyrate in anaerobic bacteria. We identify and characterize a stereospecific ()-3-hydroxylbutyrate dehydrogenase (3SHBDH) from Desulfotomaculum ruminis, which catalyzes the reversible NAD(P)H-dependent reduction of acetoacetate to form ()-3-hydroxybutyrate. 3SHBDH also catalyzes oxidation of d-threonine (2, 3) and l-allo-threonine (2, 3), consistent with its specificity for β-(3)-hydroxy acids. Isothermal calorimetry experiments support a sequential mechanism involving binding of NADH prior to ()-3-hydroxybutyrate. Homologs of 3SHBDH are present in anaerobic fermenting and sulfite-reducing bacteria, and experiments with Clostridium pasteurianum showed that 3SHBDH, acetate CoA-transferase (YdiF), and ()-3-hydroxybutyryl-CoA dehydrogenase (Hbd) are involved together in the degradation of ()-3-hydroxybutyrate as a carbon and energy source for growth. ()-3-hydroxybutyrate is a human metabolic marker and a chiral precursor for chemical synthesis, suggesting potential applications of 3SHBDH in diagnostics or the chemicals industry. ()-3-hydroxybutyrate is well studied as a component of ketone bodies produced by the liver and of bacterial polyesters. However, the biochemistry of its enantiomer ()-3-hydroxybutyrate is poorly understood. This study describes the identification and characterization of a stereospecific ()-3-hydroxylbutyrate dehydrogenase and its function in a metabolic pathway for the degradation of ()-3-hydroxybutyrate as a carbon and energy source in anaerobic bacteria. ()-3-hydroxybutyrate is a mammalian metabolic marker and a precursor for chemical synthesis and bioplastics, suggesting potential applications of these enzymes in diagnostics and biotechnology.