The efficient Δ1-dehydrogenation of a wide spectrum of 3-ketosteroids in a broad pH range by 3-ketosteroid dehydrogenase from Sterolibacterium denitrificans

Biotransformation of Δ1-Progesterone Using Selected Entomopathogenic Filamentous Fungi and Prediction of Its Products' Bioactivity

This research aimed at obtaining new derivatives of pregn-1,4-diene-3,20-dione (Δ1-progesterone) (2) through microbiological transformation. For the role of catalysts, we used six strains of entomopathogenic filamentous fungi (Beauveria bassiana KCh J1.5, Beauveria caledonica KCh J3.3, Isaria fumosorosea KCh J2, Isaria farinosa KCh KW1.1, Isaria tenuipes MU35, and Metarhizium robertsii MU4). The substrate (2) was obtained by carrying out an enzymatic 1,2-dehydrogenation on an increased scale (3.5 g/L) using a recombinant cholest-4-en-3-one Δ1-dehydrogenase (AcmB) from Sterolibacterium denitrificans. All selected strains were characterized by the high biotransformation capacity for the used substrate. As a result of the biotransformation, six steroid derivatives were obtained: 11α-hydroxypregn-1,4-diene-3,20-dione (3), 6β,11α-dihydroxypregn-1,4-diene-3,20-dione (4), 6β-hydroxypregn-1,4-diene-3,11,20-trione (5), 6β,17α-dihydroxypregn-1,4-diene-3,20-dione (6), 6β,17β-dihydroxyandrost-1,4-diene-3-one (7), and 12β,17α-dihydroxypregn-1,4-diene-3,20-dione (8). The results show evident variability of the biotransformation process between strains of the tested biocatalysts from different species described as entomopathogenic filamentous fungi. The obtained products were tested in silico using cheminformatics tools for their pharmacokinetic and pharmacodynamic properties, proving their potentially high biological activities. This study showed that the obtained compounds may have applications as effective inhibitors of testosterone 17β-dehydrogenase. Most of the obtained products should, also with a high probability, find potential uses as androgen antagonists, a prostate as well as menopausal disorders treatment. They should also demonstrate immunosuppressive, erythropoiesis-stimulating, and anti-inflammatory properties.

Reconstruction of the Steroid 1(2)-Dehydrogenation System from Nocardioides simplex VKM Ac-2033D in Mycolicibacterium Hosts

Microbial 1(2)-dehydrogenation of 3-ketosteroids is an important basis for the production of many steroid pharmaceuticals and synthons. When using the wild-type strains for whole cell catalysis, the undesirable reduction of the 20-carbonyl group, or 1(2)-hydrogenation, was observed. In this work, the recombinant strains of Mycolicibacterium neoaurum and Mycolicibacterium smegmatis were constructed with blocked endogenous activity of 3-ketosteroid-9α-hydroxylase, 3-ketosteroid-1(2)-dehydrogenase (3-KSD), and expressing 3-KSD encoded by the gene KR76_27125 (kstD2NS) from Nocardioides simplex VKM Ac-2033D. The in vivo activity of the obtained recombinant strains against phytosterol, 6α-methyl-hydrocortisone, and hydrocortisone was studied. When using M. smegmatis as the host strain, the 1(2)-dehydrogenation activity of the constructed recombinant cells towards hydrocortisone was noticeably higher compared to those on the platform of M. neoaurum. A comparison of the strengths of inducible acetamidase and constitutive hsp60 promoters in M. smegmatis provided comparable results. Hydrocortisone biotransformation by M. smegmatis BD/pMhsp_k expressing kstD2NS resulted in 95.4% prednisolone yield, and the selectivity preferred that for N. simplex. Mycolicibacteria showed increased hydrocortisone degradation at 35 °C compared to 30 °C. The presence of endogenous steroid catabolism in Mycolicibacterium hosts does not seem to confer an advantage for the functioning of KstD2NS. The results allow for the evaluation of the prospects for the development of simple technological methods for the selective 1(2)-dehydrogenation of 3-ketosteroids by growing bacterial cells.

Structure, Mutagenesis, and QM:MM Modeling of 3-Ketosteroid Δ1-Dehydrogenase from Sterolibacterium denitrificans─The Role of a New Putative Membrane-Associated Domain and Proton-Relay System in Catalysis

3-Ketosteroid Δ1-dehydrogenases (KstD) are important microbial flavin enzymes that initiate the metabolism of steroid ring A and find application in the synthesis of steroid drugs. We present a structure of the KstD from Sterolibacterium denitrificans (AcmB), which contains a previously uncharacterized putative membrane-associated domain and extended proton-relay system. The experimental and theoretical studies show that the steroid Δ1-dehydrogenation proceeds according to the Ping-Pong bi-bi kinetics and a two-step base-assisted elimination (E2cB) mechanism. The mechanism is validated by evaluating the experimental and theoretical kinetic isotope effect for deuterium-substituted substrates. The role of the active-site residues is quantitatively assessed by point mutations, experimental activity assays, and QM/MM MD modeling of the reductive half-reaction (RHR). The pre-steady-state kinetics also reveals that the low pH (6.5) optimum of AcmB is dictated by the oxidative half-reaction (OHR), while the RHR exhibits a slight optimum at the pH usual for the KstD family of 8.5. The modeling confirms the origin of the enantioselectivity of C2-H activation and substrate specificity for Δ4-3-ketosteroids. Finally, the cholest-4-en-3-one turns out to be the best substrate of AcmB in terms of ΔG of binding and predicted rate of dehydrogenation.

1,2-Hydrogenation and Transhydrogenation Catalyzed by 3-Ketosteroid Δ1-Dehydrogenase from Sterolibacterium denitrificans-Kinetics, Isotope Labelling and QM:MM Modelling Studies

Bacteria and fungi that are able to metabolize steroids express 3-ketosteroid-Δ1-dehydrogenases (KstDs). KstDs such as AcmB form Sterolibacterium denitrificans Chol-1 catalyze the enantioselective 1α,2β-dehydrogenation of steroids to their desaturated analogues, e.g., the formation of 1,4-androstadiene-3,17-dione (ADD) from 4-androsten-3,17-dione (AD). The reaction catalyzed by KstD can be reversed if the appropriate electron donor, such as benzyl viologen radical cation, is present. Furthermore, KstDs can also catalyze transhydrogenation, which is the transfer of H atoms between 3-ketosteroids and 1-dehydrosteroids. In this paper, we showed that AcmB exhibits lower pH optima for hydrogenation and dehydrogenation by 3.5-4 pH units than those observed for KstD from Nocardia corallina. We confirmed the enantiospecificity of 1α,2β-hydrogenation and 1α,2β-transhydrogenation catalyzed by AcmB and showed that, under acidic pH conditions, deuterons are introduced not only at 2β but also at the 1α position. We observed a higher degree of H/D exchange at Y363, which activates the C2-H bond, compared to that at FAD, which is responsible for redox at the C1 position. Furthermore, for the first time, we observed the introduction of the third deuteron into the steroid core. This effect was explained through a combination of LC-MS experiments and QM:MM modelling, and we attribute it to a decrease in the enantioselectivity of C2-H activation upon the deuteration of the 2β position. The increase in the activation barrier resulting from isotopic substitution increases the chance of the formation of d3-substituted 3-ketosteroids. Finally, we demonstrate a method for the synthesis of 3-ketosteroids chirally deuterated at 1α,2β positions, obtaining 1α,2β-d2-4-androsten-3,17-dione with a 51% yield (8.61 mg).

A New 3-Ketosteroid-Δ1–Dehydrogenase with High Activity and Broad Substrate Scope for Efficient Transformation of Hydrocortisone at High Substrate Concentration

3-Ketosteroid-Δ¹-dehydrogenases (KstDs [EC 1.3.99.4]) catalyze the Δ¹-dehydrogenation of steroids and are a class of important enzymes for steroid biotransformations. In this study, nine putative kstD genes from different origins were selected and overexpressed in Escherichia coli BL21(DE3). These recombinant enzymes catalyzed the Δ¹-desaturation of a variety of steroidal compounds. Among them, the KstD from Propionibacterium sp. (PrKstD) displayed the highest specific activity and broad substrate spectrum. The detailed catalytic characterization of PrKstD showed that it can convert a wide range of 3-ketosteroid compounds with diverse substituents, ranging from substituents at the C9, C10, C11 and C17 position through substrates without C4-C5 double bond, to previously inactive C6-substituted ones such as 11β,17-dihydroxy-6α-methyl-pregn-4-ene-3,20-dione. Reaction conditions were optimized for the biotransformation of hydrocortisone in terms of pH, temperature, co-solvent and electron acceptor. By using 50 g/L wet resting E. coli cells harboring PrKstD enzyme, the conversion of hydrocortisone was about 92.5% within 6 h at the substrate concentration of 80 g/L, much higher than the previously reported results, demonstrating the application potential of this new KstD.

Universal capability of 3-ketosteroid Δ1-dehydrogenases to catalyze Δ1-dehydrogenation of C17-substituted steroids

BACKGROUND

3-Ketosteroid Δ1-dehydrogenases (KSTDs) are the enzymes involved in microbial cholesterol degradation and modification of steroids. They catalyze dehydrogenation between C1 and C2 atoms in ring A of the polycyclic structure of 3-ketosteroids. KSTDs substrate spectrum is broad, even though most of them prefer steroids with small substituents at the C17 atom. The investigation of the KSTD's substrate specificity is hindered by the poor solubility of the hydrophobic steroids in aqueous solutions. In this paper, we used 2-hydroxpropyl-β-cyclodextrin (HBC) as a solubilizing agent in a study of the KSTDs steady-state kinetics and demonstrated that substrate bioavailability has a pivotal impact on enzyme specificity.

RESULTS

Molecular dynamics simulations on KSTD1 from Rhodococcus erythropolis indicated no difference in ΔGbind between the native substrate, androst-4-en-3,17-dione (AD; - 8.02 kcal/mol), and more complex steroids such as cholest-4-en-3-one (- 8.40 kcal/mol) or diosgenone (- 6.17 kcal/mol). No structural obstacle for binding of the extended substrates was also observed. Following this observation, our kinetic studies conducted in the presence of HBC confirmed KSTD1 activity towards both types of steroids. We have compared the substrate specificity of KSTD1 to the other enzyme known for its activity with cholest-4-en-3-one, KSTD from Sterolibacterium denitrificans (AcmB). The addition of solubilizing agent caused AcmB to exhibit a higher affinity to cholest-4-en-3-one (Ping-Pong bi bi KmA = 23.7 μM) than to AD (KmA = 529.2 μM), a supposedly native substrate of the enzyme. Moreover, we have isolated AcmB isoenzyme (AcmB2) and showed that conversion of AD and cholest-4-en-3-one proceeds at a similar rate. We demonstrated also that the apparent specificity constant of AcmB for cholest-4-en-3-one (kcat/KmA = 9.25∙106 M-1 s-1) is almost 20 times higher than measured for KSTD1 (kcat/KmA = 4.71∙105 M-1 s-1).

CONCLUSIONS

We confirmed the existence of AcmB preference for a substrate with an undegraded isooctyl chain. However, we showed that KSTD1 which was reported to be inactive with such substrates can catalyze the reaction if the solubility problem is addressed.

Application of microbial 3-ketosteroid Δ1-dehydrogenases in biotechnology

3-Ketosteroid Δ¹-dehydrogenase catalyzes the 1(2)-dehydrogenation of 3-ketosteroid substrates using flavin adenine dinucleotide as a cofactor. The enzyme plays a crucial role in microbial steroid degradation, both under aerobic and anaerobic conditions, by initiating the opening of the steroid nucleus. Indeed, many microorganisms are known to possess one or more 3-ketosteroid Δ¹-dehydrogenases. In the pharmaceutical industry, 3-ketosteroid Δ¹-dehydrogenase activity is exploited to produce Δ¹-3-ketosteroids, a class of steroids that display various biological activities. Many of them are used as active pharmaceutical ingredients in drug products, or as key precursors to produce pharmaceutically important steroids. Since 3-ketosteroid Δ¹-dehydrogenase activity requires electron acceptors, among other considerations, Δ¹-3-ketosteroid production has been industrially implemented using whole-cell fermentation with growing or metabolically active resting cells, in which the electron acceptors are available, rather than using the isolated enzyme. In this review we discuss biotechnological applications of microbial 3-ketosteroid Δ¹-dehydrogenases, covering commonly used steroid-1(2)-dehydrogenating microorganisms, the bioprocess for preparing Δ¹-3-ketosteroids, genetic engineering of 3-ketosteroid Δ¹-dehydrogenases and related genes for constructing new, productive industrial strains, and microbial fermentation strategies for enhancing the product yield. Furthermore, we also highlight the recent development in the use of isolated 3-ketosteroid Δ¹-dehydrogenases combined with a FAD cofactor regeneration system. Finally, in a somewhat different context, we summarize the role of 3-ketosteroid Δ¹-dehydrogenase in cholesterol degradation by Mycobacterium tuberculosis and other mycobacteria. Because the enzyme is essential for the pathogenicity of these organisms, it may be a potential target for drug development to combat mycobacterial infections.

Enhancing the sustainability of KsdD as a biocatalyst for steroid transformation by immobilization on epoxy support

The Δ1-dehydrogenation of 3-ketosteroid substrates is a crucial reaction in the production of steroids. Although 3-ketosteroid Δ1-dehydrogenase (KsdD) catalyzes this reaction with high efficiency and selectivity, the low stability and high cost of the purified enzyme catalyst have limited its industrial application. In this study, an epoxy support was used to evaluate the covalent immobilization of KsdD from Pimelobacter simplex, and the best androsta-1,4-diene-317-dione (ADD) production was achieved after optimized immobilization of KsdD enzyme in 1.5 M NaH₂PO₄- Na₂HPO₄ buffer (pH 6.5) for 12 h at 25 °C. The immobilized KsdD exhibited higher tolerance toward 20 % methanol. The dehydrogenation reaction reached a conversion efficiency of up to 90.0 % in 2 h when using 0.6 mg/mL of 4-androstene-317-dione (AD). The W299A and W299 G mutants of KsdD were also immobilized, and both showed the better catalytic performance with higher k_cat/K_M values compared with the wild type (WT). The immobilized W299A, W299 G and WT KsdD respectively maintained 70.5, 65.7 and 38.7 % of their initial activity at the end of 15 reaction cycles. Furthermore, the W299A retained 66.3 % of the initial activity after 30 days of incubation at 4 °C, and was more stable than free KsdD, Thus, the immobilized W299A is a promising biocatalyst for steroid dehydrogenation. In this study, we investigated the application of immobilized enzymes for the dehydrogenation of steroids, which will be of great importance for improving the development of green technology and sustainable use of biocatalysts in the steroid manufacturing industry.

Application of Immobilized Cholest-4-en-3-one Δ1-Dehydrogenase from Sterolibacterium Denitrificans for Dehydrogenation of Steroids

Cholest-4-en-3-one Δ1-dehydrogenase (AcmB) from Sterolibacterium denitrificans was successfully immobilized on 3-aminopropyltrimethoysilane functionalized mesoporous cellular foam (MCF) and Santa Barbara Amorphous (SBA-15) silica supports using adsorption or covalently with glutaraldehyde or divinyl sulfone linkers. The best catalyst, AcmB on MCF linked covalently with glutaraldehyde, retained the specific activity of the homogenous enzyme while exhibiting a substantial increase of the operational stability. The immobilized enzyme was used continuously in the fed-batch reactor for 27 days, catalyzing 1,2-dehydrogenation of androst-4-en-3-one to androst-1,4-dien-3-one with a final yield of 29.9 mM (8.56 g/L) and 99% conversion. The possibility of reuse of the immobilized catalyst was also demonstrated and resulted in a doubling of the product amount compared to that in the reference homogenous reactor. Finally, it was shown that molecular oxygen from the air can efficiently be used as an electron acceptor either reoxidizing directly the enzyme or the reduced 2,4-dichlorophenolindophenol (DCPIPH2).