Expression of ksdD gene encoding 3-ketosteroid-?1-dehydrogenase from Arthrobacter simplex in Bacillus subtilis

Glycosylation of Quercetin by Selected Entomopathogenic Filamentous Fungi and Prediction of Its Products' Bioactivity

Quercetin is the most abundant flavonoid in food products, including berries, apples, cauliflower, tea, cabbage, nuts, onions, red wine and fruit juices. It exhibits various biological activities and is used for medical applications, such as treating allergic, inflammatory and metabolic disorders, ophthalmic and cardiovascular diseases, and arthritis. However, its low water solubility may limit quercetin's therapeutic potential. One method of increasing the solubility of active compounds is their coupling to polar molecules, such as sugars. The attachment of a glucose unit impacts the stability and solubility of flavonoids and often determines their bioavailability and bioactivity. Entomopathogenic fungi are biocatalysts well known for their ability to attach glucose and its 4-O-methyl derivative to bioactive compounds, including flavonoids. We investigated the ability of cultures of entomopathogenic fungi belonging to Beauveria, Isaria, Metapochonia, Lecanicillium and Metarhizium genera to biotransform quercetin. Three major glycosylation products were detected: (1), 7-O-β-D-(4″-O-methylglucopyranosyl)-quercetin, (2) 3-O-β-D-(4″-O-methylglucopyranosyl)-quercetin and (3) 3-O-β-D-(glucopyranosyl)-quercetin. The results show evident variability of the biotransformation process, both between strains of the tested biocatalysts from different species and between strains of the same species. Pharmacokinetic and pharmacodynamic properties of the obtained compounds were predicted with the use of cheminformatics tools. The study showed that the obtained compounds may have applications as effective modulators of intestinal flora and may be stronger hepato-, cardio- and vasoprotectants and free radical scavengers than quercetin.

Biotransformation of Phytosterols into Androstenedione—A Technological Prospecting Study

Androstenedione (AD) is a key intermediate in the body’s steroid metabolism, used as a precursor for several steroid substances, such as testosterone, estradiol, ethinyl estradiol, testolactone, progesterone, cortisone, cortisol, prednisone, and prednisolone. The world market for AD and ADD (androstadienedione) exceeds 1000 tons per year, which stimulates the pharmaceutical industry’s search for newer and cheaper raw materials to produce steroidal compounds. In light of this interest, we aimed to investigate the progress of AD biosynthesis from phytosterols by prospecting scientific articles (Scopus, Web of Science, and Google Scholar databases) and patents (USPTO database). A wide variety of articles and patents involving AD and phytosterol were found in the last few decades, resulting in 108 relevant articles (from January 2000 to December 2021) and 23 patents of interest (from January 1976 to December 2021). The separation of these documents into macro, meso, and micro categories revealed that most studies (articles) are performed in China (54.8%) and in universities (76%), while patents are mostly granted to United States companies. It also highlights the fact that AD production studies are focused on “process improvement” techniques and on possible modifications of the “microorganism” involved in biosynthesis (64 and 62 documents, respectively). The most-reported “process improvement” technique is “chemical addition” (40%), which means that the addition of solvents, surfactants, cofactors, inducers, ionic liquids, etc., can significantly increase AD production. Microbial genetic modifications stand out in the “microorganism” category because this strategy improves AD yield considerably. These documents also revealed the main aspects of AD and ADD biosynthesis: Mycolicibacterium sp. (basonym: Mycobacterium sp.) (40%) and Mycolicibacterium neoaurum (known previously as Mycobacterium neoaurum) (32%) are the most recurrent species studied. Microbial incubation temperatures can vary from 29 °C to 37 °C; incubation can last from 72 h to 14 days; the mixture is agitated at 140 to 220 rpm; vegetable oils, mainly soybean, can be used as the source of a mixture of phytosterols. In general, the results obtained in the present technological prospecting study are fundamental to mapping the possibilities of AD biosynthesis process optimization, as well as to identifying emerging technologies and methodologies in this scenario.

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.

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.

Identification, Biological Characteristics, and Active Site Residues of 3-Ketosteroid Δ1-Dehydrogenase Homologues from Arthrobacter simplex

3-Ketosteroid Δ¹-dehydrogenase (KsdD) is the key enzyme responsible for Δ¹-dehydrogenation, which is one of the most valuable reactions for steroid catabolism. Arthrobacter simplex has been widely used in the industry due to its superior bioconversion efficiency, but KsdD information is not yet fully clear. Here, five KsdD homologues were identified in A. simplex CGMCC 14539. Bioinformatic analysis indicated their distinct properties and structures. Each KsdD was functionally confirmed by transcriptional response, overexpression, and heterologous expression. The substantial difference in substrate profiles might be related to the enzyme loop structure. Two promising enzymes (KsdD3 and KsdD5) were purified and characterized, exhibiting strong organic solvent tolerance and clear preference for 4-ene-3-oxosteroids. KsdD5 seemed to be more versatile due to good activity on substrates with or without a substituent at C11 and high optimal temperature and also possessed unique residues. It is the first time that KsdDs have been comprehensively disclosed in the A. simplex industrial strain.

Biotransformation of Phytosterols to Androst-1,4-Diene-3,17-Dione by Mycobacterium sp. ZFZ Expressing 3-Ketosteroid-Δ1-Dehydrogenase

As an important hormone drug intermediate, androst-1,4-diene-3,17-dione can be bio-converted from phytosterols. However, separation and purification in the downstream process are very difficult due to the similarity in structure and physiological characteristics between ADD and androstenedione (AD). This phenomenon was correlated to the insufficient enzyme activity of 3-ketosteroid-Δ1-dehydrogenase (KSDD), which specifically catalyzes the C1,2 dehydrogenation of AD. In order to obtain a highly purified ADD from phytosterols, the dehydrogenation effect of different kinds of KSDDs and the transcription effect of four promoter sequences on ksdd were analyzed in Mycobacterium sp. ZFZ (ZFZ), the cell host that transform phytosterols to AD in the oil-aqueous system. A tandem KSDD expression cassette containing strain ZFZ-2111 yielded 2.06 ± 0.09 g L−1 ADD, with a molar ratio of ADD/AD at 41.47:1.00 in 120 h. In waste cooking oil-aqueous media, the proportion of ADD in the fermentation by ZFZ-2111 was 92%. The present study provides a reliable theoretical basis for the step-by-step transformation of phytosterols to ADD.

Facilitating Protein Expression with Portable 5'-UTR Secondary Structures in Bacillus licheniformis

The 5'-untranslated region (5'-UTR) of prokaryotic mRNAs plays an essential role in post-transcriptional regulation. Bacillus species, such as Bacillus subtilis and Bacillus licheniformis, have gained considerable attention as microbial cell factories for the production of various valuable chemicals and industrial proteins. In this work, we developed a portable 5'-UTR sequence for enhanced protein output in the industrial strain B. licheniformis DW2. This sequence contains only ∼30 nt and forms a hairpin structure located right before the open reading frame. The optimized Shine-Dalgarno (SD) sequence was presented as a single strand on the loop of the hairpin for better ribosome recognition and recruitment. By optimizing the free energy of folding, this 5'-element could effectively enhance the expression of eGFP by ∼50-fold and showed good adaptability for other target proteins, including RFP, nattokinase, and keratinase. This 5'-UTR could promote the accessibility of both the SD sequence and start codon, leading to improved efficiency of translation initiation. Furthermore, the hairpin structure protected mRNA against 5'-exonucleases, resulting in enhanced mRNA stability. It is well-known that the stable structure at a ribosome binding site (RBS) impedes initiation in Escherichia coli. In this study, we presented a unique structure at a RBS that can effectively enhance protein production, which is an exception of this prevailing concept. By adjusting a single thermodynamic parameter and holding the other factors affecting protein output constant, a series of 5'-UTR elements with different expression strengths could be rationally designed for wide use in Bacillus sp.

A progesterone biosensor derived from microbial screening

Bacteria are an enormous and largely untapped reservoir of biosensing proteins. We describe an approach to identify and isolate bacterial allosteric transcription factors (aTFs) that recognize a target analyte and to develop these TFs into biosensor devices. Our approach utilizes a combination of genomic screens and functional assays to identify and isolate biosensing TFs, and a quantum-dot Förster Resonance Energy Transfer (FRET) strategy for transducing analyte recognition into real-time quantitative measurements. We use this approach to identify a progesterone-sensing bacterial aTF and to develop this TF into an optical sensor for progesterone. The sensor detects progesterone in artificial urine with sufficient sensitivity and specificity for clinical use, while being compatible with an inexpensive and portable electronic reader for point-of-care applications. Our results provide proof-of-concept for a paradigm of microbially-derived biosensors adaptable to inexpensive, real-time sensor devices.

The role and mechanism of microbial 3-ketosteroid Δ1-dehydrogenases in steroid breakdown

3-Ketosteroid Δ¹-dehydrogenases are FAD-dependent enzymes that catalyze the introduction of a double bond between the C1 and C2 atoms of the A-ring of 3-ketosteroid substrates. These enzymes are found in a large variety of microorganisms, especially in bacteria belonging to the phylum Actinobacteria. They play a critical role in the early steps of the degradation of the steroid core. 3-Ketosteroid Δ¹-dehydrogenases are of particular interest for the etiology of some infectious diseases, for the production of starting materials for the pharmaceutical industry, and for environmental bioremediation applications. Here we summarize and discuss the biochemical and enzymological properties of these enzymes, their microbial sources, and their natural diversity. The three-dimensional structure of a 3-ketosteroid Δ¹-dehydrogenase in connection with the enzyme mechanism is highlighted.

Identification of steroid C27 monooxygenase isoenzymes involved in sterol catabolism and stepwise pathway engineering of Mycobacterium neoaurum for improved androst-1,4-diene-3,17-dione production

Cholesterol oxidase, steroid C27 monooxygenase and 3-ketosteroid-Δ1-dehydrogenase are key enzymes involved in microbial catabolism of sterols. Here, three isoenzymes of steroid C27 monooxygenase were firstly characterized from Mycobacterium neoaurum as the key enzyme in sterol C27-hydroxylation. Among these three isoenzymes, steroid C27 monooxygenase 2 exhibits the strongest function in sterol catabolism. To improve androst-1,4-diene-3,17-dione production, cholesterol oxidase, steroid C27 monooxygenase 2 and 3-ketosteroid-Δ1-dehydrogenase were coexpressed to strengthen the metabolic flux to androst-1,4-diene-3,17-dione, and 3-ketosteroid 9α-hydroxylase, which catalyzes the androst-1,4-diene-3,17-dione catabolism, was disrupted to block the androst-1,4-diene-3,17-dione degradation pathway in M. neoaurum JC-12. Finally, the recombinant strain JC-12S2-choM-ksdd/ΔkshA produced 20.1 g/L androst-1,4-diene-3,17-dione, which is the highest reported production with sterols as substrate. Therefore, this work is hopes to pave the way for efficient androst-1,4-diene-3,17-dione production through metabolic engineering.

New Insights on Steroid Biotechnology

Nowadays steroid manufacturing occupies a prominent place in the pharmaceutical industry with an annual global market over $10 billion. The synthesis of steroidal active pharmaceutical ingredients (APIs) such as sex hormones (estrogens, androgens, and progestogens) and corticosteroids is currently performed by a combination of microbiological and chemical processes. Several mycobacterial strains capable of naturally metabolizing sterols (e.g., cholesterol, phytosterols) are used as biocatalysts to transform phytosterols into steroidal intermediates (synthons), which are subsequently used as key precursors to produce steroidal APIs in chemical processes. These synthons can also be modified by other microbial strains capable of introducing regio- and/or stereospecific modifications (functionalization) into steroidal molecules. Most of the industrial microbial strains currently available have been improved through traditional technologies based on physicochemical mutagenesis and selection processes. Surprisingly, Synthetic Biology and Systems Biology approaches have hardly been applied for this purpose. This review attempts to highlight the most relevant research on Steroid Biotechnology carried out in last decades, focusing specially on those works based on recombinant DNA technologies, as well as outlining trends and future perspectives. In addition, the need to construct new microbial cell factories (MCF) to design more robust and bio-sustainable bioprocesses with the ultimate aim of producing steroids à la carte is discussed.

Efficient androst-1,4-diene-3,17-dione production by co-expressing 3-ketosteroid-Δ1 -dehydrogenase and catalase in Bacillus subtilis

AIMS

3-ketosteroid-Δ¹ -dehydrogenase (KSDD), a flavin adenine dinucleotide (FAD)-dependent enzyme involved in sterol metabolism, specifically catalyses the conversion of androst-4-ene-3,17-dione (AD) to androst-1,4-diene-3,17-dione (ADD). However, the low KSDD activity and the toxic effects of hydrogen peroxide (H₂ O₂ ) generated during the biotransformation of AD to ADD with FAD regeneration hinder its application on AD conversion. The aim of this work was to improve KSDD activity and eliminate the toxic effects of the generated H₂ O₂ to enhance ADD production.

METHODS AND RESULTS

The ksdd gene obtained from Mycobacterium neoaurum JC-12 was codon-optimized to increase its expression level in Bacillus subtilis, and the KSDD activity reached 12·3 U mg^-1 , which was sevenfold of that of codon-unoptimized gene. To improve AD conversion, catalase was co-expressed with KSDD in B. subtilis 168/pMA5-ksdd^opt -katA to eliminate the toxic effects of H₂ O₂ generated during AD conversion. Finally, under optimized bioconversion conditions, fed-batch strategy was carried out and the ADD yield improved to 8·76 g l^-1 .

CONCLUSIONS

This work demonstrates the potential to improve enzyme activity by codon-optimization and eliminate the toxic effects of H₂ O₂ by co-expressing catalase.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study showed the highest ADD productivity ever reported and provides a promising strain for efficient ADD production in the pharmaceutical industry.

Bioconversion of 6-(N-methyl-N-phenyl)aminomethyl androstane steroids by Nocardioides simplex

The newly synthesized (α/β)-diastereomers of 6-(N-methyl-N-phenyl)aminomethylandrost-4-ene-3,17-dione (5) and 6-(N-methyl-N-phenyl)aminomethylandrost-4-en-17β-ol-3-one (6) were firstly investigated as substrates for the whole cells of Nocardioides simplex VKM Ac-2033D in comparison with their unsubstituted analogs, - androst-4-ene-3,17-dione (1) and androst-4-en-17β-ol-3-one (2). 1(2)-Dehydroderivatives were identified as the major bioconversion products from all the substrates tested. When using the mixtures of (α/β)-stereoisomers of 5 and 6 as the substrates, only β-stereoisomers of the corresponding 1,4-diene-steroids were formed. Along with 1(2)-dehydrogenation, N. simplex VKM Ac-2033D promoted oxidation of the hydroxyl group at C-17 position of 6: both 6(α) and 6(β) were transformed to the corresponding 17-keto derivatives. No steroid core destruction was observed during the conversion of the 6-substituted androstanes 5 and 6, while it was significant when 1 or 2 was used as the substrate. The results suggested high potentials of N. simplex VKM Ac-2033D for the generation of novel 1(2)-dehydroanalogs.

Biotransformation of dehydroepiandrosterone (DHEA) by environmental strains of filamentous fungi

Study on the ability of selected filamentous fungus species to transform dehydroepiandrosterone was performed (DHEA) and interesting DHEA derivatives were obtained with high yield.

Steroid Bioconversions

Steroid modifications by selected wild-type and engineered strains of microorganisms became an effective tool for the production of high-valued steroidal drugs and their precursors for the pharmaceutical industry. Some microorganisms are effective at the performance of sterol side-chain degradation, oxyfunctionalization of steroid core, and redox reactions at different positions of the steroid molecule. A number of bioprocesses using steroid-transforming microbial strains are well established on an industrial level. Although a range of biocatalytic methods has been developed, selection of suitable microorganisms, as well as creation of new engineered strains, is of great importance for generation of improved bioprocesses and production schemes for obtaining known and new metabolites with potent biological activity. The achievements in genetic and metabolic engineering of steroid-transforming strains in combination with novel approaches in the enzymatic and whole-cell biocatalysis provide a platform for highly effective and selective biotransformations.Here, we briefly review the current state and prospects in the field of microbial bioconversions with special attention to the application of molecular microbiology methods for the generation of new whole cell biocatalysts.

A mutant form of 3-ketosteroid-Δ1-dehydrogenase gives altered androst-1,4-diene-3, 17-dione/androst-4-ene-3,17-dione molar ratios in steroid biotransformations by Mycobacterium neoaurum ST-095

Mycobacterium neoaurum ST-095 and its mutant M. neoaurum JC-12, capable of transforming phytosterol to androst-1,4-diene-3,17-dione (ADD) and androst-4-ene-3,17-dione (AD), produce very different molar ratios of ADD/AD. The distinct differences were related to the enzyme activity of 3-ketosteroid-Δ1-dehydrogenase (KSDD), which catalyzes the C1,2 dehydrogenation of AD to ADD specifically. In this study, by analyzing the primary structure of KSDDI (from M. neoaurum ST-095) and KSDDII (from M. neoaurum JC-12), we found the only difference between KSDDI and KSDDII was the mutation of Val366 to Ser366. This mutation directly affected KSDD enzyme activity, and this result was confirmed by heterologous expression of these two enzymes in Bacillus subtilis. Assay of the purified recombinant enzymes showed that KSDDII has a higher C1,2 dehydrogenation activity than KSDDI. The functional difference between KSDDI and KSDDII in phytosterol biotransformation was revealed by gene disruption and complementation. Phytosterol transformation results demonstrated that ksdd  I and ksdd  II gene disrupted strains showed similar ADD/AD molar ratios, while the ADD/AD molar ratios of the ksdd  I and ksdd  II complemented strains were restored to their original levels. These results proved that the different ADD/AD molar ratios of these two M. neoaurum strains were due to the differences in KSDD. Finally, KSDD structure analysis revealed that the Val366Ser mutation could possibly play an important role in stabilizing the active center and enhancing the interaction of AD and KSDD. This study provides a reliable theoretical basis for understanding the structure and catalytic mechanism of the Mycobacteria KSDD enzyme.

Identification of a highly efficient stationary phase promoter in Bacillus subtilis

A promoter that enabled high-level expression of the target gene during the stationary phase in the absence of an inducer would facilitate the efficient production of heterogeneous proteins at a low cost. In this study, a genome-scale microarray-based approach was employed to identify promoters that induced high-level expression of the target genes in Bacillus subtilis from the late log phase to the stationary phase without an inducer. Eleven candidate promoters were selected based on B. subtilis microarray data and the quantitative PCR analysis. Among the selected promoters, Pylb exhibited the highest activity with the reporter bgaB during the stationary phase. Compared with P43 (a commonly used constitutive promoter), promoter Pylb could express two reporter genes (egfp and mApple), and the expression levels of EGFP and RFP were 7.8- and 11.3-fold higher than that of P43, respectively. This finding was verified by overexpression of the genes encoding pullulanase and organophosphorus hydrolase, the activities of which were 7.4- and 2.3-fold higher, respectively, when driven by Pylb compared with P43. Therefore, our results suggest that the Pylb promoter could be used to overexpress target genes without an inducer; this method could facilitate the identification and evaluation of attractive promoters in the genome.

Enhanced Production of Androst-1,4-Diene-3,17-Dione by Mycobacterium neoaurum JC-12 Using Three-Stage Fermentation Strategy

To improve the androst-1,4-diene-3,17-dione (ADD) production from phytosterol by Mycobacterium neoaurum JC-12, fructose was firstly found favorable as the initial carbon source to increase the biomass and eliminate the lag phase of M. neoaurum JC-12 in the phytosterol transformation process. Based on this phenomenon, two-stage fermentation by using fructose as the initial carbon source and feeding glucose to maintain strain metabolism was designed. By applying this strategy, the fermentation duration was decreased from 168 h to 120 h with the ADD productivity increased from 0.071 g/(L·h) to 0.108 g/(L·h). Further, three-stage fermentation by adding phytosterol to improve ADD production at the end of the two-stage fermentation was carried out and the final ADD production reached 18.6 g/L, which is the highest reported ADD production using phytosterol as substrate. Thus, this strategy provides a possible way in enhancing the ADD production in pharmaceutical industry.

Genetic differences in ksdD influence on the ADD/AD ratio of Mycobacterium neoaurum

Mycobacterium neoaurum TCCC 11028 (MNR) and M. neoaurum TCCC 11028 M3 (MNR M3) significantly differ in the ratio of androst-1,4-diene-3,17-dione (ADD) to androst-4-ene-3,17-dione (AD) produced. The large fluctuations are related to the dehydrogenation activity of 3-ketosteroid-Δ(1)-dehydrogenase (KsdD). Analysis of the primary structure of KsdD showed that the Ser-138 of KsdD-MNR changed to Leu-138 of KsdD-MNR M3 because of C413T in the ksdD gene. This phenomenon directly affected KsdD activity. The effect of the primary structure of KsdD on dehydrogenation activity was confirmed through exogenous expression. Whole-cell transformation initially revealed that KsdD-MNR showed a higher dehydrogenation activity than KsdD-MNR M3. Then, ksdD gene replacement strain was constructed by homologous recombination. The results of steroid transformation experiments showed that the ability of the MNR M3ΔksdD::ksdD-MNR strain to produce ADD was improved and it returned to the similar level of the MNR strain. This result indicated that the ADD/AD ratio of the two M. neoaurum strains was influenced by the difference in ksdD. The mechanism by which residue mutations alter enzyme activity may be connected with the crystal structure of KsdD from Rhodococcus erythropolis SQ1. As a key amino acid residue in the active center position, Ser-138 played an important role in maintaining the active center in the hydrophobic environment of KsdD. This study may serve as a basis for future studies on the structural analysis and catalytic mechanism of dehydrogenase.