Phytosterol conversion into C9 non-hydroxylated derivatives through gene regulation in Mycobacterium fortuitum

Androst-4-ene-3,17-dione (AD) and 22-hydroxy-23,24-bisnorchol-4-ene-3-one (4-HBC) are important drug intermediates that can be biosynthesized from phytosterols. However, the C9 hydroxylation of steroids via 3-ketosteroid 9α-hydroxylase (KSH) limits AD and 4-HBC accumulation. Five active KshAs, the oxidation component of KSH, were identified in Mycobacterium fortuitum ATCC 35855 for the first time. The deletion of kshAs indicated that the five KshA genes were jointly responsible for C9 hydroxylation during phytosterol biotransformation. MFKDΔkshA, the five KshAs deficient strain, blocked C9 hydroxylation and produced 5.37 g/L AD and 0.55 g/L 4-HBC. The dual function reductase Opccr knockout and 17β-hydroxysteroid dehydrogenase Hsd4A enhancement reduced 4-HBC content from 8.75 to 1.72% and increased AD content from 84.13 to 91.34%, with 8.24 g/L AD being accumulated from 15 g/L phytosterol. In contrast, hsd4A and thioesterase fadA5 knockout resulted in the accumulation of 5.36 g/L 4-HBC from 10 g/L phytosterol. We constructed efficient AD (MFKDΔkshAΔopccr_hsd4A) and 4-HBC (MFKDΔkshAΔhsd4AΔfadA5) producers and provided insights for further metabolic engineering of the M. fortuitum ATCC 35855 strain for steroid productions. KEY POINTS: • Five active KshAs were first identified in M. fortuitum ATCC 35855. • Deactivation of all five KshAs blocks the steroid C9 hydroxylation reaction. • AD or 4-HBC production was improved by Hsd4A, FadA5, and Opccr modification.

Engineered 3-Ketosteroid 9α-Hydroxylases in Mycobacterium neoaurum: an Efficient Platform for Production of Steroid Drugs

3-Ketosteroid 9α-hydroxylase (Ksh) consists of a terminal oxygenase (KshA) and a ferredoxin reductase and is indispensable in the cleavage of steroid nucleus in microorganisms. The activities of Kshs are crucial factors in determining the yield and distribution of products in the biotechnological transformation of sterols in industrial applications. In this study, two KshA homologues, KshA1_N and KshA2_N, were characterized and further engineered in a sterol-digesting strain, Mycobacterium neoaurum ATCC 25795, to construct androstenone-producing strains. kshA1 _N is a member of the gene cluster encoding sterol catabolism enzymes, and its transcription exhibited a 4.7-fold increase under cholesterol induction. Furthermore, null mutation of kshA1 _N led to the stable accumulation of androst-4-ene-3,17-dione (AD) and androst-1,4-diene-3,17-dione (ADD). We determined kshA2 _N to be a redundant form of kshA1 _N Through a combined modification of kshA1 _N, kshA2 _N, and other key genes involved in the metabolism of sterols, we constructed a high-yield ADD-producing strain that could produce 9.36 g liter^-1 ADD from the transformation of 20 g liter^-1 phytosterols in 168 h. Moreover, we improved a previously established 9α-hydroxy-AD-producing strain via the overexpression of a mutant KshA1_N that had enhanced Ksh activity. Genetic engineering allowed the new strain to produce 11.7 g liter^-1 9α-hydroxy-4-androstene-3,17-dione (9-OHAD) from the transformation of 20.0 g liter^-1 phytosterol in 120 h.IMPORTANCE Steroidal drugs are widely used for anti-inflammation, anti-tumor action, endocrine regulation, and fertility management, among other uses. The two main starting materials for the industrial synthesis of steroid drugs are phytosterol and diosgenin. The phytosterol processing is carried out by microbial transformation, which is thought to be superior to the diosgenin processing by chemical conversions, given its simple and environmentally friendly process. However, diosgenin has long been used as the primary starting material instead of phytosterol. This is in response to challenges in developing efficient microbial strains for industrial phytosterol transformation, which stem from complex metabolic processes that feature many currently unclear details. In this study, we identified two oxygenase homologues of 3-ketosteroid-9α-hydroxylase, KshA1_N and KshA2_N, in M. neoaurum and demonstrated their crucial role in determining the yield and variety of products from phytosterol transformation. This work has practical value in developing industrial strains for phytosterol biotransformation.

Influence of temperature on nucleus degradation of 4-androstene-3, 17-dione in phytosterol biotransformation by Mycobacterium sp

One of the steroid intermediates, 4-androstene-3, 17-dione (AD), in the biotransformation of phytosterols is valuable for the production of steroid medicaments. However, its degradation during the conversion process is one of the main obstacles to obtain high yields. In this study, the effect of temperature on nucleus degradation during microbial biotransformation of phytosterol was investigated. The results indicated that microbial degradation of phytosterol followed the AD-ADD-'9-OH-ADD' pathway, and that two important reactions involved in nucleus degradation, conversions of AD to ADD and ADD to 9-OH-ADD, were inhibited at 37°C. With a change in the culture temperature from 30 to 37°C, nucleus degradation was reduced from 39·9% to 17·6%, due to inhibition of the putative KstD and Ksh. These results suggested a simple way to decrease the nucleus degradation in phytosterol biotransformation and a new perspective on the possibilities of modifying the metabolism of strains used in industrial applications.

SIGNIFICANCE AND IMPACT OF THE STUDY

Nucleus degradation of products is one of the main problems encountered during phytosterol biotransformation. To solve this problem, the effect of temperature on nucleus degradation was investigated in the industrial production of steroid intermediates. The results are also helpful to the genetic modification of sterol-producing strains.

Comparative analysis of genes encoding key steroid core oxidation enzymes in fast-growing Mycobacterium spp. strains

A comparative genome analysis of Mycobacterium spp. VKM Ac-1815D, 1816D and 1817D strains used for efficient production of key steroid intermediates (androst-4-ene-3,17-dione, AD, androsta-1,4-diene-3,17-dione, ADD, 9α-hydroxy androst-4-ene-3,17-dione, 9-OH-AD) from phytosterol has been carried out by deep sequencing. The assembled contig sequences were analyzed for the presence putative genes of steroid catabolism pathways. Since 3-ketosteroid-9α-hydroxylases (KSH) and 3-ketosteroid-Δ(1)-dehydrogenase (Δ(1) KSTD) play key role in steroid core oxidation, special attention was paid to the genes encoding these enzymes. At least three genes of Δ(1) KSTD (kstD), five genes of KSH subunit A (kshA), and one gene of KSH subunit B of 3-ketosteroid-9α-hydroxylases (kshB) have been found in Mycobacterium sp. VKM Ac-1817D. Strains of Mycobacterium spp. VKM Ac-1815D and 1816D were found to possess at least one kstD, one kshB and two kshA genes. The assembled genome sequence of Mycobacterium sp. VKM Ac-1817D differs from those of 1815D and 1816D strains, whereas these last two are nearly identical, differing by 13 single nucleotide substitutions (SNPs). One of these SNPs is located in the coding region of a kstD gene and corresponds to an amino acid substitution Lys (135) in 1816D for Ser (135) in 1815D. The findings may be useful for targeted genetic engineering of the biocatalysts for biotechnological application.