The efficient Δ-dehydrogenation of a wide spectrum of 3-ketosteroids in a broad pH range by 3-ketosteroid dehydrogenase from Sterolibacterium denitrificans.

Cholest-4-en-3-one Δ-dehydrogenase (AcmB) from Sterolibacterium denitrificans, a key enzyme of the central degradation pathway of cholesterol, is a protein catalyzing Δ-dehydrogenation of a wide range of 3-ketosteroids. In this study, we demonstrate the application of AcmB in the synthesis of 1-dehydro-3-ketosteroids and investigate the influence of reaction conditions on the catalytic performance of the enzyme. The recombinant AcmB expressed in E. coli BL21(DE3)Magic exhibits a broad pH optimum and pH stability in the range of 6.5 to 9.0. The activity-based pH optimum of AcmB reaction depends on the type of electron acceptor (2,6-dichloroindophenol - DCPIP, phenazine methosulfate - PMS or potassium hexacyanoferrate - K[Fe(CN)]) used in the biocatalytic process yielding the best kinetic properties for the reaction with a DCPIP/PMS mixture (k/K = 1.4·10 s·M at pH 9.0) followed by DCPIP (k/K = 1.0·10 s·M at pH = 6.5) and K[Fe(CN)] (k/K = 0.5·10 s·M at pH = 8.0). The unique feature of AcmB is its capability to convert both testosterone derivatives (C20-C22) as well as steroids substituted at C17 (C27-C30) such as cholest-4-en-3-one or (25R)-spirost-4-en-3-one (diosgenone). Apparent steady-state kinetic parameters were determined for both groups of AcmB substrates. In a batch reactor synthesis, the solubility of water-insoluble steroids was facilitated by the addition of a solubilizer, 2-hydroxypropyl-β-cyclodextrin, and organic co-solvent, 2-methoxyethanol. Catalytic properties characterization of AcmB was tested in fed-batch reactor set-ups, using 0.81 μM of isolated enzyme, PMS and aerobic atmosphere resulting in >99% conversion of the C17-C20 3-ketosteroids within 2 h. Finally, the whole cell E. coli system with recombinant enzyme was demonstrated as an efficient biocatalyst in the synthesis of 1-dehydro-3-ketosteroids.