Microbial steroid transformations: current state and prospects

Studies of steroid modifications catalyzed by microbial whole cells represent a well-established research area in white biotechnology. Still, advances over the last decade in genetic and metabolic engineering, whole-cell biocatalysis in non-conventional media, and process monitoring raised research in this field to a new level. This review summarizes the data on microbial steroid conversion obtained since 2003. The key reactions of structural steroid functionalization by microorganisms are highlighted including sterol side-chain degradation, hydroxylation at various positions of the steroid core, and redox reactions. We also describe methods for enhancement of bioprocess productivity, selectivity of target reactions, and application of microbial transformations for production of valuable pharmaceutical ingredients and precursors. Challenges and prospects of whole-cell biocatalysis applications in steroid industry are discussed.

Cholesterol metabolism in Mycobacterium smegmatis

The metabolism of cholesterol in Mycobacterium smegmatis mc(2) 155 has been investigated by using a microarray approach. The transcriptome of M. smegmatis growing in cholesterol was compared with that of cells growing in glycerol as the sole carbon and energy sources during the middle exponential phase. Microarray analyses revealed that only 89 genes were upregulated at least threefold during growth on cholesterol compared with growth on glycerol. The upregulated genes are scattered throughout the 7 Mb M. smegmatis genome and likely reflect a general physiological adaptation of the bacterium to grow on this highly hydrophobic polycyclic compound. Nevertheless, 39 of the catabolic genes are organized in three specific clusters. These results not only supported the role of KstR and KstR2 as auto-regulated repressors of cholesterol catabolism, and revealed some metabolic similarities and differences on actinobacteria, but more important, they have facilitated the identification of new catabolic genes, opening a research scenario that might provide important clues on the role of cholesterol in tuberculosis infection.

Isolation of cholesterol- and deoxycholate-degrading bacteria from soil samples: evidence of a common pathway

Nineteen different steroid-degrading bacteria were isolated from soil samples by using selective media containing either cholesterol or deoxycholate as sole carbon source. Strains that assimilated cholesterol (17 COL strains) were gram-positive, belonging to the genera Gordonia, Tsukamurella, and Rhodococcus, and grew on media containing other steroids but were unable to use deoxycholate as sole carbon source. Surprisingly, some of the COL strains unable to grow using deoxycholate as sole carbon source were able to catabolize other bile salts (e.g., cholate). Conversely, strains able to grow using deoxycholate as the sole carbon source (two DOC isolates) were gram-negative, belonging to the genus Pseudomonas, and were unable to catabolize cholesterol and other sterols. COL and DOC were included into the corresponding taxonomic groups based on their morphology (cells and colonies), metabolic properties (kind of substrates that support bacterial growth), and genetic sequences (16S rDNA and rpoB). Additionally, different DOC21 Tn5 insertion mutants have been obtained. These mutants have been classified into two different groups: (1) those affected in the catabolism of bile salts but that, as wild type, can grow in other steroids and (2) those unable to grow in media containing any of the steroids tested. The identification of the insertion point of Tn5 in one of the mutants belonging to the second group (DOC21 Mut1) revealed that the gene knocked-out encodes an A-ring meta-cleavage dioxygenase needed for steroid catabolism.

Cholesterol oxidase ChoD is not a critical enzyme accounting for oxidation of sterols to 3-keto-4-ene steroids in fast-growing Mycobacterium sp. VKM Ac-1815D

Fast-growing strain of Mycobacterium sp. VKM Ac-1815D is capable of effective oxidizing of sterols (phytosterol, cholesterol, ergosterol) to androstenedione and other valuable 3-oxo-steroids. To elucidate the role of cholesterol oxidase in sterol catabolism by the strain, the choD gene has been cloned and sequenced. The deduced gene product (M(r) 63.5kDa) showed homologies over its entire length to a large number of proteins belonging to the InterPro-family EPR006076, which includes various FAD dependent oxidoreductases. The expression of choD in Escherichia coli was shown to result in the synthesis of membrane associated cholesterol oxidase. In addition to cholesterol, the enzyme oxidized β-sitosterol, dehydroepiandrosterone, ergosterol, pregnenolone, and lithocholic acid. Knock-out of choD in Mycobacterium sp. VKM Ac-1815D strain was obtained by the gene replacement technique. The mutant strain transformed sitosterol forming exclusively 3-keto-4-ene steroids with androstenedione as a major product, thus evidencing that choD knock out did not abrogate sterol A-ring oxidation. The results indicated that ChoD is not a critical enzyme responsible for modification of 3β-hydroxy-5-ene- to 3-keto-4-ene steroids in Mycobacterium sp. VKM Ac-1815D. Article from a special issue on steroids and microorganisms.

Catabolism and biotechnological applications of cholesterol degrading bacteria

Cholesterol is a steroid commonly found in nature with a great relevance in biology, medicine and chemistry, playing an essential role as a structural component of animal cell membranes. The ubiquity of cholesterol in the environment has made it a reference biomarker for environmental pollution analysis and a common carbon source for different microorganisms, some of them being important pathogens such as Mycobacterium tuberculosis. This work revises the accumulated biochemical and genetic knowledge on the bacterial pathways that degrade or transform this molecule, given that the characterization of cholesterol metabolism would contribute not only to understand its role in tuberculosis but also to develop new biotechnological processes that use this and other related molecules as starting or target materials.

Cholesterol catabolism as a therapeutic target in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is an intracellular pathogen that infects 10 million people worldwide and kills 2 million people every year. The uptake and utilization of nutrients by Mtb within the host cell is still poorly understood, although lipids play an important role in Mtb persistence. The recent identification of a large regulon of cholesterol catabolic genes suggests that Mtb can use host sterol for infection and persistence. In this review, we report on recent progress in elucidation of the Mtb cholesterol catabolic reactions and their potential utility as targets for tuberculosis therapeutic agents.

Characterization of the KstR-dependent promoter of the gene for the first step of the cholesterol degradative pathway in Mycobacterium smegmatis

The KstR-dependent promoter of the MSMEG_5228 gene of Mycobacterium smegmatis, which encodes the 3-β-hydroxysteroid dehydrogenase (3-β HSDMS) responsible for the first step in the cholesterol degradative pathway, has been characterized. Primer extension analysis of the P5228 promoter showed that the transcription starts at the ATG codon, thus generating a leaderless mRNA lacking a 5′ untranslated region (5′UTR). Footprint analyses demonstrated experimentally that KstR specifically binds to an operator region of 31 nt containing the quasi-palindromic sequence AACTGGAACGTGTTTCAGTT, located between the −5 and −35 positions with respect to the transcription start site. This region overlaps with the −10 and −35 boxes of the P5228 promoter, suggesting that KstR represses MSMEG_5228 transcription by preventing the binding of RNA polymerase. Using a P5228 –β-galactosidase fusion we have demonstrated that KstR is able to work as a repressor in a heterologous system like Escherichia coli. A 3D model of the KstR protein revealed folding typical of TetR-type regulators, with two domains, i.e. a DNA-binding N-terminal domain and a regulator-binding C-terminal domain composed of six helices with a long tunnel-shaped hydrophobic pocket that might interact with a putative highly hydrophobic inducer. The finding that similar P5228 promoter regions have been found in all mycobacterial strains examined, with the sole exception of Mycobacterium tuberculosis, provides new clues about the role of cholesterol in the pathogenicity of this micro-organism.