Process development for the production of 15ß-hydroxycyproterone acetate using Bacillus megaterium expressing CYP106A2 as whole-cell biocatalyst.

BACKGROUND: CYP106A2 from Bacillus megaterium ATCC 13368 was first identified as a regio- and stereoselective 15ß-hydroxylase of 3-oxo-∆4-steroids. Recently, it was shown that besides 3-oxo-∆4-steroids, 3-hydroxy-∆5-steroids as well as di- and triterpenes can also serve as substrates for this biocatalyst. It is highly selective towards the 15ß position, but the 6ß, 7α/ß, 9α, 11α and 15α positions have also been described as targets for hydroxylation. Based on the broad substrate spectrum and hydroxylating capacity, it is an excellent candidate for the production of human drug metabolites and drug precursors. RESULTS: In this work, we demonstrate the conversion of a synthetic testosterone derivative, cyproterone acetate, by CYP106A2 under in vitro and in vivo conditions. Using a Bacillus megaterium whole-cell system overexpressing CYP106A2, sufficient amounts of product for structure elucidation by nuclear magnetic resonance spectroscopy were obtained. The product was characterized as 15ß-hydroxycyproterone acetate, the main human metabolite. Since the product is of pharmaceutical interest, our aim was to intensify the process by increasing the substrate concentration and to scale-up the reaction from shake flasks to bioreactors to demonstrate an efficient, yet green and cost-effective production. Using a bench-top bioreactor and the recombinant Bacillus megaterium system, both a fermentation and a transformation process were successfully implemented. To improve the yield and product titers for future industrial application, the main bottlenecks of the reaction were addressed. Using 2-hydroxypropyl-ß-cyclodextrin, an effective bioconversion of 98% was achieved using 1 mM substrate concentration, corresponding to a product formation of 0.43 g/L, at a 400 mL scale. CONCLUSIONS: Here we describe the successful scale-up of cyproterone acetate conversion from shake flasks to bioreactors, using the CYP106A2 enzyme in a whole-cell system. The substrate was converted to its main human metabolite, 15ß-hydroxycyproterone acetate, a highly interesting drug candidate, due to its retained antiandrogen activity but significantly lower progestogen properties than the mother compound. Optimization of the process led to an improvement from 55% to 98% overall conversion, with a product formation of 0.43 g/L, approaching industrial process requirements and a future large-scale application.

Structural basis of steroid binding and oxidation by the cytochrome P450 CYP109E1 from Bacillus megaterium.

Cytochrome P450 monooxygenases (P450s) are attractive enzymes for the pharmaceutical industry, in particular, for applications in steroidal drug synthesis. Here, we report a comprehensive functional and structural characterization of CYP109E1, a novel steroid-converting cytochrome P450 enzyme identified from the genome of Bacillus megaterium DSM319. In vitro and whole-cell in vivo turnover experiments, combined with binding assays, revealed that CYP109E1 is able to hydroxylate testosterone at position 16ß. Related steroids with bulky substituents at carbon C17, like corticosterone, bind to the enzyme without being converted. High-resolution X-ray structures were solved of a steroid-free form of CYP109E1 and of complexes with testosterone and corticosterone. The structural analysis revealed a highly dynamic active site at the distal side of the heme, which is wide open in the absence of steroids, can bind four ordered corticosterone molecules simultaneously, and undergoes substantial narrowing upon binding of single steroid molecules. In the crystal structures, the single bound steroids adopt unproductive binding modes coordinating the heme-iron with their C3-keto oxygen. Molecular dynamics (MD) simulations suggest that the steroids may also bind in ~180° reversed orientations with the C16 carbon and C17-substituents pointing toward the heme, leading to productive binding of testosterone explaining the observed regio- and stereoselectivity. The X-ray structures and MD simulations further identify several residues with important roles in steroid binding and conversion, which could be confirmed by site-directed mutagenesis. Taken together, our results provide unique insights into the CYP109E1 activity, substrate specificity, and regio/stereoselectivity. DATABASE: The atomic coordinates and structure factors have been deposited in the Protein Data Bank with accession codes 5L90 (steroid-free CYP109E1), 5L91 (CYP109E1-COR4), 5L94 (CYP109E1-TES), and 5L92 (CYP109E1-COR). ENZYMES: Cytochrome P450 monooxygenase CYP109E1, EC 1.14.14.1, UniProt ID: D5DKI8, Adrenodoxin reductase EC 1.18.1.6.