Regioselective hydroxylation of cholecalciferol, cholesterol and other sterol derivatives by steroid C25 dehydrogenase

Synthesis of 25-Hydroxy-provitamin D3 by Direct Hydroxylation of Protected 7-Dehydrocholesterol

A new synthetic route to 25-hydroxy-provitamin D3 was elaborated. The synthesis consists of direct hydroxylation at C-25 of 7-dehydrocholesterol hetero Diels-Alder adducts. The adducts were prepared by [4 + 2] cycloaddition of azadienophiles to the steroidal diene. The hydroxylation reactions of adducts were carried out with different dioxiranes or with chromyl trifluoroacetate. The byproducts of these reactions were isolated and identified. The strengths and weaknesses of hydroxylation methods with different oxidizing agents were discussed.

Bioconversion of vitamin D3 into calcitriol by Actinomyces hyovaginalis isolate CCASU- A11-2

Vitamin D₃ is a fat-soluble prohormone that is activated inside the liver to produce 25-hydroxyvitamin D₃ (calcidiol), and in the kidney to produce the fully active 1α, 25-dihydroxy vitamin D₃ (calcitriol). A previous work piloted in our laboratory, resulted in a successful recovery of a local soil-promising Actinomyces hyovaginalis isolate CCASU-A11-2 capable of converting vitamin D₃ into calcitriol. Despite the rising amount of research on vitamin D₃ bioconversion into calcitriol, further deliberate studies on this topic can significantly contribute to the improvement of such a bioconversion process. Therefore, this work aimed to improve the bioconversion process, using the study isolate, in a 14 L laboratory fermenter (4 L fermentation medium composed of fructose (15 g/L), defatted soybean (15 g/L), NaCl (5 g/L), CaCO₃ 2 g/L); K₂HPO₄, (1 g/L) NaF (0.5 g/L) and initial of pH 7.8) where different experiments were undertaken to investigate the effect of different culture conditions on the bioconversion process. Using the 14 L laboratory fermenter, the calcitriol production was increased by about 2.5-fold (32.8 µg/100 mL) to that obtained in the shake flask (12.4 µg/100 mL). The optimal bioconversion conditions were inoculum size of 2% v/v, agitation rate of 200 rpm, aeration rate of 1 vvm, initial pH of 7.8 (uncontrolled); addition of vitamin D₃ (substrate) 48 h after the start of the main culture. In conclusion, the bioconversion of vitamin D₃ into calcitriol in a laboratory fermenter showed a 2.5-fold increase as compared to the shake flask level where, the important factors influencing the bioconversion process were the aeration rate, inoculum size, the timing of substrate addition, and the fixed pH of the fermentation medium. So, those factors should be critically considered for the scaling-up of the biotransformation process.

Bioconversion of vitamin D3 to bioactive calcifediol and calcitriol as high-value compounds

Biological catalysis is an important approach for the production of high-value-added compounds, especially for products with complex structures. Limited by the complex steps of chemical synthesis and low yields, the bioconversion of vitamin D₃ (VD₃) to calcifediol and calcitriol, which are natural steroid products with high added value and significantly higher biological activity compared to VD₃, is probably the most promising strategy for calcifediol and calcitriol production, and can be used as an alternative method for chemical synthesis. The conversion efficiency of VD₃ to calcifediol and calcitriol has continued to rise in the past few decades with the help of several different VD₃ hydroxylases, mostly cytochrome P450s (CYPs), and newly isolated strains. The production of calcifediol and calcitriol can be systematically increased in different ways. Specific CYPs and steroid C25 dehydrogenase (S25DH), as VD₃ hydroxylases, are capable of converting VD₃ to calcifediol and calcitriol. Some isolated actinomycetes have also been exploited for fermentative production of calcifediol and calcitriol, although the VD₃ hydroxylases of these strains have not been elucidated. With the rapid development of synthetic biology and enzyme engineering, quite a lot of advances in bioproduction of calcifediol and calcitriol has been achieved in recent years. Therefore, here we review the successful strategies of promoting VD₃ hydroxylation and provide some perspective on how to further improve the bioconversion of VD₃ to calcifediol and calcitriol.

Emerging Chemical Diversity and Potential Applications of Enzymes in the DMSO Reductase Superfamily

Molybdenum- and tungsten-dependent proteins catalyze essential processes in living organisms and biogeochemical cycles. Among these enzymes, members of the dimethyl sulfoxide (DMSO) reductase superfamily are considered the most diverse, facilitating a wide range of chemical transformations that can be categorized as oxygen atom installation, removal, and transfer. Importantly, DMSO reductase enzymes provide high efficiency and excellent selectivity while operating under mild conditions without conventional oxidants such as oxygen or peroxides. Despite the potential utility of these enzymes as biocatalysts, such applications have not been fully explored. In addition, the vast majority of DMSO reductase enzymes still remain uncharacterized. In this review, we describe the reactivities, proposed mechanisms, and potential synthetic applications of selected enzymes in the DMSO reductase superfamily. We also highlight emerging opportunities to discover new chemical activity and current challenges in studying and engineering proteins in the DMSO reductase superfamily. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Characterisation of the redox centers of ethylbenzene dehydrogenase

Ethylbenzene dehydrogenase (EbDH), the initial enzyme of anaerobic ethylbenzene degradation from the beta-proteobacterium Aromatoleum aromaticum, is a soluble periplasmic molybdenum enzyme consisting of three subunits. It contains a Mo-bis-molybdopterin guanine dinucleotide (Mo-bis-MGD) cofactor and an 4Fe-4S cluster (FS0) in the α-subunit, three 4Fe-4S clusters (FS1 to FS3) and a 3Fe-4S cluster (FS4) in the β-subunit and a heme b cofactor in the γ-subunit. Ethylbenzene is hydroxylated by a water molecule in an oxygen-independent manner at the Mo-bis-MGD cofactor, which is reduced from the Mo^VI to the Mo^IV state in two subsequent one-electron steps. The electrons are then transferred via the Fe-S clusters to the heme b cofactor. In this report, we determine the midpoint redox potentials of the Mo-bis-MGD cofactor and FS1-FS4 by EPR spectroscopy, and that of the heme b cofactor by electrochemically induced redox difference spectroscopy. We obtained relatively high values of > 250 mV both for the Mo^VI-Mo^V redox couple and the heme b cofactor, whereas FS2 is only reduced at a very low redox potential, causing magnetic coupling with the neighboring FS1 and FS3. We compare the results with the data on related enzymes and interpret their significance for the function of EbDH.

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).

The efficient Δ1-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 Δ¹-dehydrogenase (AcmB) from Sterolibacterium denitrificans, a key enzyme of the central degradation pathway of cholesterol, is a protein catalyzing Δ¹-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 - K₃[Fe(CN)₆]) used in the biocatalytic process yielding the best kinetic properties for the reaction with a DCPIP/PMS mixture (k_cat/K_m = 1.4·10⁵ s^-1·M^-1 at pH 9.0) followed by DCPIP (k_cat/K_m = 1.0·10⁵ s^-1·M^-1 at pH = 6.5) and K₃[Fe(CN)₆] (k_cat/K_m = 0.5·10² s^-1·M^-1 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.

Efficient biotransformation of vitamin D3 to 25-hydroxyvitamin D3 by a newly isolated Bacillus cereus strain

25-hydroxyvitamin D₃ has attracted considerable attention due to its great medical value and huge market demand in animal husbandry. Microbial production of 25-hydroxyvitamin D₃ has been recognized as an alternative superior to traditional chemical synthesis. In this study, a Gram-positive bacteria zju 4-2 (CCTCC M 2019385) was isolated from the soil using vitamin D₃ as the sole carbon source and was identified as Bacillus cereus according to its physiological characteristics and 16S rRNA analysis, which also showed a relatively high capacity for 25-hydroxyvitamin D₃ production. Through systematic optimization of different catalytic conditions, the optimal solvent system of vitamin D₃, vitamin D₃ addition time and concentration, temperature, and pH were shown to be propylene glycol/ethanol (v/v = 9:1), early stationary phase, 2 g/L, 37 °C, and pH 7.2, respectively. With these optimal conditions, 796 mg/L of 25-hydroxyvitamin D₃ was achieved after 48 h bioconversion with zju 4-2 at the shake flask level. Finally, up to 830 mg/L 25-hydroxyvitamin D₃ with a yield of 41.5% was obtained in a 5 L fermentation tank. Our developed biotransformation process with this newly isolated strain provides a platform to produce 25-hydroxyvitamin D₃ efficiently at industrialization scale.

Microbial transformation of cholesterol: reactions and practical aspects-an update

Cholesterol is a C27-sterol employed as starting material for the synthesis of valuable pharmaceutical steroids and precursors. The microbial transformations of cholesterol have been widely studied, since they are performed with high regio- and stereoselectivity and allow the production of steroidal compounds which are difficult to synthesize by classical chemical methods. In recent years, ongoing research is being conducted to discover novel biocatalysts and to develop biotechnological processes to improve existing biocatalysts and biotransformation reactions. The main objective of this review is to present the most remarkable advances in fungal and bacterial transformation of cholesterol, focusing on the different types of microbial reactions and biocatalysts, biotransformation products, and practical aspects related to sterol dispersion improvement, covering literature since 2000. It reviews the conversion of cholesterol by whole-cell biocatalysts and by purified enzymes that lead to various structural modifications, including side chain cleavage, hydroxylation, dehydrogenation/reduction, isomerization and esterification. Finally, approaches used to improve the poor solubility of cholesterol in aqueous media, such as the use of different sterol-solubilizing agents or two-phase conversion system, are also discussed.

Anaerobic C-H Oxyfunctionalization: Coupling of Nitrate Reduction and Quinoline Hydroxylation in Recombinant Pseudomonas putida

Whole-cell biocatalysis for C-H oxyfunctionalization depends on and is often limited by O₂ mass transfer. In contrast to oxygenases, molybdenum hydroxylases use water instead of O₂ as an oxygen donor and thus have the potential to relieve O₂ mass transfer limitations. Molybdenum hydroxylases may even allow anaerobic oxyfunctionalization when coupled to anaerobic respiration. To evaluate this option, the coupling of quinoline hydroxylation to denitrification is tested under anaerobic conditions employing Pseudomonas putida (P. putida) 86, capable of aerobic growth on quinoline. P. putida 86 reduces both nitrate and nitrite, but at low rates, which does not enable significant growth and quinoline hydroxylation. Introduction of the nitrate reductase from Pseudomonas aeruginosa enables considerable specific quinoline hydroxylation activity (6.9 U g_CDW ^-1 ) under anaerobic conditions with nitrate as an electron acceptor and 2-hydroxyquinoline as the sole product (further metabolization depends on O₂ ). Hydroxylation-derived electrons are efficiently directed to nitrate, accounting for 38% of the respiratory activity. This study shows that molybdenum hydroxylase-based whole-cell biocatalysts enable completely anaerobic carbon oxyfunctionalization when coupled to alternative respiration schemes such as nitrate respiration.

Bacterial steroid hydroxylases: enzyme classes, their functions and comparison of their catalytic mechanisms

The steroid superfamily includes a wide range of compounds that are essential for living organisms of the animal and plant kingdoms. Structural modifications of steroids highly affect their biological activity. In this review, we focus on hydroxylation of steroids by bacterial hydroxylases, which take part in steroid catabolic pathways and play an important role in steroid degradation. We compare three distinct classes of metalloenzymes responsible for aerobic or anaerobic hydroxylation of steroids, namely: cytochrome P450, Rieske-type monooxygenase 3-ketosteroid 9α-hydroxylase, and molybdenum-containing steroid C25 dehydrogenases. We analyze the available literature data on reactivity, regioselectivity, and potential application of these enzymes in organic synthesis of hydroxysteroids. Moreover, we describe mechanistic hypotheses proposed for all three classes of enzymes along with experimental and theoretical evidences, which have provided grounds for their formulation. In case of the 3-ketosteroid 9α-hydroxylase, such a mechanistic hypothesis is formulated for the first time in the literature based on studies conducted for other Rieske monooxygenases. Finally, we provide comparative analysis of similarities and differences in the reaction mechanisms utilized by bacterial steroid hydroxylases.

Four Molybdenum-Dependent Steroid C-25 Hydroxylases: Heterologous Overproduction, Role in Steroid Degradation, and Application for 25-Hydroxyvitamin D3 Synthesis

Side chain-containing steroids are ubiquitous constituents of biological membranes that are persistent to biodegradation. Aerobic, steroid-degrading bacteria employ oxygenases for isoprenoid side chain and tetracyclic steran ring cleavage. In contrast, a Mo-containing steroid C-25 dehydrogenase (S25DH) of the dimethyl sulfoxide (DMSO) reductase family catalyzes the oxygen-independent hydroxylation of tertiary C-25 in the anaerobic, cholesterol-degrading bacterium Sterolibacterium denitrificans Its genome contains eight paralogous genes encoding active site α-subunits of putative S25DH-like proteins. The difficult enrichment of labile, oxygen-sensitive S25DH from the wild-type bacteria and the inability of its active heterologous production have largely hampered the study of S25DH-like gene products. Here we established a heterologous expression platform for the three structural genes of S25DH subunits together with an essential chaperone in the denitrifying betaproteobacterium Thauera aromatica K172. Using this system, S25DH₁ and three isoenzymes (S25DH₂, S25DH₃, and S25DH₄) were overproduced in a soluble, active form allowing a straightforward purification of nontagged αβγ complexes. All S25DHs contained molybdenum, four [4Fe-4S] clusters, one [3Fe-4S] cluster, and heme B and catalyzed the specific, water-dependent C-25 hydroxylations of various 4-en-3-one forms of phytosterols and zoosterols. Crude extracts from T. aromatica expressing genes encoding S25DH₁ catalyzed the hydroxylation of vitamin D₃ (VD₃) to the clinically relevant 25-OH-VD₃ with >95% yield at a rate 6.5-fold higher than that of wild-type bacterial extracts; the specific activity of recombinant S25DH₁ was twofold higher than that of wild-type enzyme. These results demonstrate the potential application of the established expression platform for 25-OH-VD₃ synthesis and pave the way for the characterization of previously genetically inaccessible S25DH-like Mo enzymes of the DMSO reductase family.IMPORTANCE Steroids are ubiquitous bioactive compounds, some of which are considered an emerging class of micropollutants. Their degradation by microorganisms is the major process of steroid elimination from the environment. While oxygenase-dependent steroid degradation in aerobes has been studied for more than 40 years, initial insights into the anoxic steroid degradation have only recently been obtained. Molybdenum-dependent steroid C₂₅ dehydrogenases (S25DHs) have been proposed to catalyze oxygen-independent side chain hydroxylations of globally abundant zoo-, phyto-, and mycosterols; however, so far, their lability has allowed only the initial characterization of a single S25DH. Here we report on a heterologous gene expression platform that allowed for easy isolation and characterization of four highly active S25DH isoenzymes. The results obtained demonstrate the key role of S25DHs during anoxic degradation of various steroids. Moreover, the platform is valuable for the efficient enzymatic hydroxylation of vitamin D₃ to its clinically relevant C-25-OH form.

Electrocatalytic Hydroxylation of Sterols by Steroid C25 Dehydrogenase from Sterolibacterium denitrificans

The electrochemically driven catalysis of the complex molybdoenzyme steroid C25 dehydrogenase (S25DH) from the β-Proteobacterium Sterolibacterium denitrificans is reported. S25DH catalyses the oxygen-independent regioselective hydroxylation of the tertiary C25 atom of sterols and also their derivatives. Cholest-4-en-3-one is a native substrate for S25DH, which produces 25-hydroxycholest-4-en-3-one as a product of catalytic turnover. Cholecalciferol (vitD₃ ) is also a substrate. S25DH was immobilised on a modified gold working electrode with the co-adsorbent chitosan. The complexes ferricyanide ([Fe(CN)₆ ]^3- ) and ferrocenium methanol (FM⁺ ) are effective artificial electron acceptors from S25DH and act as mediators of electron transfer between the electrode and the enzyme. 2-Hydroxypropyl-β-cyclodextrin (HPCD) was employed as a sterol solubiliser, in addition to 2-methoxyethanol. The catalytic activity varied, depending upon the concentration of solubiliser in the reaction mixture. Parallel studies with [Fe(CN)₆ ]^3- as a chemical (as opposed to electrochemical) oxidant coupled to HPLC analysis show that S25DH is capable of oxidising both vitD₃ and its less stable isomer, pre-vitD₃ , and that the former substrate is stabilised by HPCD.

Reaction mechanism of sterol hydroxylation by steroid C25 dehydrogenase - Homology model, reactivity and isoenzymatic diversity

Steroid C25 dehydrogenase (S25DH) is a molybdenum-containing oxidoreductase isolated from the anaerobic Sterolibacterium denitrificans Chol-1S. S25DH is classified as 'EBDH-like' enzyme (EBDH, ethylbenzene dehydrogenase) and catalyzes the introduction of an OH group to the C25 atom of a sterol aliphatic side-chain. Due to its regioselectivity, S25DH is proposed as a catalyst in production of pharmaceuticals: calcifediol or 25-hydroxycholesterol. The aim of presented research was to obtain structural model of catalytic subunit α and investigate the reaction mechanism of the O₂-independent tertiary carbon atom activation. Based on homology modeling and theoretical calculations, a S25DH α subunit model was for the first time characterized and compared to other S25DH-like isoforms. The molecular dynamics simulations of the enzyme-substrate complexes revealed two stable binding modes of a substrate, which are stabilized predominantly by van der Waals forces in the hydrophobic substrate channel. However, H-bond interactions involving polar residues with C3=O/C3-OH in the steroid ring appear to be responsible for positioning the substrate. These results may explain the experimental kinetic results which showed that 3-ketosterols are hydroxylated 5-10-fold faster than 3-hydroxysterols. The reaction mechanism was studied using QM:MM and QM-only cluster models. The postulated mechanism involves homolytic CH cleavage by the MoO ligand, giving rise to a radical intermediate with product obtained in an OH rebound process. The hypothesis was supported by kinetic isotopic effect (KIE) experiments involving 25,26,26,26-[²H]-cholesterol (4.5) and the theoretically predicted intrinsic KIE (7.0-7.2). Finally, we have demonstrated that the recombinant S25DH-like isoform catalyzes the same reaction as S25DH.

Optimization of overexpression of a chaperone protein of steroid C25 dehydrogenase for biochemical and biophysical characterization

Molybdenum is an essential nutrient for metabolism in plant, bacteria, and animals. Molybdoenzymes are involved in nitrogen assimilation and oxidoreductive detoxification, and bioconversion reactions of environmental, industrial, and pharmaceutical interest. Molybdoenzymes contain a molybdenum cofactor (Moco), which is a pyranopterin heterocyclic compound that binds a molybdenum atom via a dithiolene group. Because Moco is a large and complex compound deeply buried within the protein, molybdoenzymes are accompanied by private chaperone proteins responsible for the cofactor's insertion into the enzyme and the enzyme's maturation. An efficient recombinant expression and purification of both Moco-free and Moco-containing molybdoenzymes and their chaperones is of paramount importance for fundamental and applied research related to molybdoenzymes. In this work, we focused on a D1 protein annotated as a chaperone of steroid C25 dehydrogenase (S25DH) from Sterolibacterium denitrificans Chol-1S. The D1 protein is presumably involved in the maturation of S25DH engaged in oxygen-independent oxidation of sterols. As this chaperone is thought to be a crucial element that ensures the insertion of Moco into the enzyme and consequently, proper folding of S25DH optimization of the chaperon's expression is the first step toward the development of recombinant expression and purification methods for S25DH. We have identified common E. coli strains and conditions for both expression and purification that allow us to selectively produce Moco-containing and Moco-free chaperones. We have also characterized the Moco-containing chaperone by EXAFS and HPLC analysis and identified conditions that stabilize both forms of the protein. The protocols presented here are efficient and result in protein quantities sufficient for biochemical studies.