Degradation of Bile Acids by Soil and Water Bacteria

Bacterial stigmasterol degradation involving radical flavin delta-24 desaturase and molybdenum-dependent C26 hydroxylase

Sterols are ubiquitous membrane constituents that persist to a large extent in the environment due to their water insolubility and chemical inertness. Recently, an oxygenase-independent sterol degradation pathway was discovered in a cholesterol-grown denitrifying bacterium Sterolibacterium (S.) denitrificans. It achieves hydroxylation of the unactivated primary C26 of the isoprenoid side chain to an allylic alcohol via a phosphorylated intermediate in a four-step ATP-dependent enzyme cascade. However, this pathway is incompatible with the degradation of widely distributed steroids containing a double bond at C22 in the isoprenoid side chain such as the plant sterol stigmasterol. Here, we have enriched a prototypical delta-24 desaturase from S. denitrificans, which catalyzes the electron acceptor-dependent oxidation of the intermediate stigmast-1,4-diene-3-one to a conjugated (22,24)-diene. We suggest an α4β4 architecture of the 440 kDa enzyme, with each subunit covalently binding an flavin mononucleotide cofactor to a histidyl residue. As isolated, both flavins are present as red semiquinone radicals, which can be reduced by stigmast-1,4-diene-3-one but cannot be oxidized even with strong oxidizing agents. We propose a mechanism involving an allylic radical intermediate in which two flavin semiquinones each abstract one hydrogen atom from the substrate. The conjugated delta-22,24 moiety formed allows for the subsequent hydroxylation of the terminal C26 with water by a heterologously produced molybdenum-dependent steroid C26 dehydrogenase 2. In conclusion, the pathway elucidated for delta-22 steroids achieves oxygen-independent hydroxylation of the isoprenoid side chain by bypassing the ATP-dependent formation of a phosphorylated intermediate.

Comprehensive summary of steroid metabolism in Comamonas testosteroni TA441: entire degradation process of basic four rings and removal of C12 hydroxyl group

Comamonas testosteroni is one of the representative aerobic steroid-degrading bacteria. We previously revealed the mechanism of steroidal A,B,C,D-ring degradation by C. testosteroni TA441. The corresponding genes are located in two clusters at both ends of a mega-cluster of steroid degradation genes. ORF7 and ORF6 are the only two genes in these clusters, whose function has not been determined. Here, we characterized ORF7 as encoding the dehydrase responsible for converting the C12β hydroxyl group to the C10(12) double bond on the C-ring (SteC), and ORF6 as encoding the hydrogenase responsible for converting the C10(12) double bond to a single bond (SteD). SteA and SteB, encoded just upstream of SteC and SteD, are in charge of oxidizing the C12α hydroxyl group to a ketone group and of reducing the latter to the C12β hydroxyl group, respectively. Therefore, the C12α hydroxyl group in steroids is removed with SteABCD via the C12 ketone and C12β hydroxyl groups. Given the functional characterization of ORF6 and ORF7, we disclose the entire pathway of steroidal A,B,C,D-ring breakdown by C. testosteroni TA441.IMPORTANCEStudies on bacterial steroid degradation were initiated more than 50 years ago, primarily to obtain materials for steroid drugs. Now, their implications for the environment and humans, especially in relation to the infection and the brain-gut-microbiota axis, are attracting increasing attention. Comamonas testosteroni TA441 is the leading model of bacterial aerobic steroid degradation with the ability to break down cholic acid, the main component of bile acids. Bile acids are known for their variety of physiological activities according to their substituent group(s). In this study, we identified and functionally characterized the genes for the removal of C12 hydroxyl groups and provided a comprehensive summary of the entire A,B,C,D-ring degradation pathway by C. testosteroni TA441 as the representable bacterial aerobic degradation process of the steroid core structure.

Identification of "missing links" in C- and D-ring cleavage of steroids by Comamonas testosteroni TA441

Comamonas testosteroni TA441 is capable of aerobically degrading steroids through the aromatization and cleavage of the A- and B-rings, followed by D- and C-ring cleavage via β-oxidation. While most of the degradation steps have been previously characterized, a few intermediate compounds remained unidentified. In this study, we proposed that the cleavage of the D-ring at C13-17 required the ScdY hydratase, followed by C-ring cleavage via the ScdL1L2 transferase. The anticipated reaction was expected to yield 6-methyl-3,7-dioxo-decane-1,10-dioic acid-coenzyme A (CoA) ester. To confirm this hypothesis, we constructed a plasmid enabling the induction of targeted genes in TA441 mutant strains. Induction experiments of ScdL1L2 revealed that the major product was 3-hydroxy-6-methyl-7-oxo-decane-1,10-dioic acid-CoA ester. Similarly, induction experiments of ScdY demonstrated that the substrate of ScdY was a geminal diol, 17-dihydroxy-9-oxo-1,2,3,4,5,6,10,19-octanorandrost-8(14)-en-7-oic acid-CoA ester. These findings suggest that ScdY catalyzes the addition of a water molecule at C14 of 17-dihydroxy-9-oxo-1,2,3,4,5,6,10,19-octanorandrost-8(14)-en-7-oic acid-CoA ester, leading to D-ring cleavage at C13-17. Subsequently, the C9 ketone of the D-ring cleavage product is converted to a hydroxyl group, followed by C-ring cleavage, resulting in the production of 3-hydroxy-6-methyl-7-oxo-decane-1,10-dioic acid-CoA ester.IMPORTANCEStudies on bacterial steroid degradation were initiated more than 50 years ago primarily to obtain substrates for steroid drugs. In recent years, the role of steroid-degrading bacteria in relation to human health has gained significant attention, as emerging evidence suggests that the intestinal microflora plays a crucial role in human health. Furthermore, cholic acid, a major component of bile acid secreted in the intestines, is closely associated with the gut microbiota. While Comamonas testosteroni TA441 is recognized as the leading bacterial model for aerobic steroid degradation, the involvement of aerobic steroid degradation in the intestinal microflora remains largely unexplored. Nonetheless, the presence of C. testosteroni in the cecum suggests the potential influence of aerobic steroid degradation on gut microbiota. To establish essential information about the role of these bacteria, here, we identified the missing compounds and propose more details of C-, and D-ring cleavage, which have remained unclear until now.

Detection of Bile Acids in Complex Matrices Using a Transcription Factor-Based Biosensor

Bile acids play an important role in digestion and human health, are found throughout the gastrointestinal tract, and are excreted in feces. Therefore, bile acids are promising biomarkers for monitoring health and detecting fecal contamination in water sources. Here, we engineered a bile acid sensor by expressing the transcription factor BreR, a TetR-like repressor from Vibrio cholorae, in Escherichia coli. The sensor was further optimized by screening a promoter library. To further characterize the BreR sensor and increase its utility, we moved expression to a cell-free expression (CFE) system, resulting in an approximately 3 orders of magnitude increase in deoxycholic acid sensitivity. We next optimized this sensor to detect bile acids in fecal water, wastewater, and serum and transferred the CFE sensor to a paper-based assay to enhance fieldability.

The Hylemon-Björkhem pathway of bile acid 7-dehydroxylation: history, biochemistry, and microbiology

Bile acids are detergents derived from cholesterol that function to solubilize dietary lipids, remove cholesterol from the body, and act as nutrient signaling molecules in numerous tissues with functions in the liver and gut being the best understood. Studies in the early 20th century established the structures of bile acids, and by mid-century, the application of gnotobiology to bile acids allowed differentiation of host-derived "primary" bile acids from "secondary" bile acids generated by host-associated microbiota. In 1960, radiolabeling studies in rodent models led to determination of the stereochemistry of the bile acid 7-dehydration reaction. A two-step mechanism was proposed, which we have termed the Samuelsson-Bergström model, to explain the formation of deoxycholic acid. Subsequent studies with humans, rodents, and cell extracts of Clostridium scindens VPI 12708 led to the realization that bile acid 7-dehydroxylation is a result of a multi-step, bifurcating pathway that we have named the Hylemon-Björkhem pathway. Due to the importance of hydrophobic secondary bile acids and the increasing measurement of microbial bai genes encoding the enzymes that produce them in stool metagenome studies, it is important to understand their origin.

Current Trends and Perspectives in Microbial Bioconversions of Steroids

The microbiological transformation of sterols is currently the technological basis for the industrial production of valuable steroid precursors, the so-called synthons, from which a wide range of steroid and indane isoprenoids are obtained by combined chemical and enzymatic routes. These compounds include value-added corticoids, neurosteroids, sex hormones, bile acids, and other terpenoid lipids required by the medicine, pharmaceutical, food, veterinary, and agricultural industries.Progress in understanding the molecular mechanisms of microbial degradation of steroids, and the development and implementation of genetic technologies, opened a new era in steroid biotechnology. Metabolic engineering of microbial producers makes it possible not only to improve the biocatalytic properties of industrial strains by enhancing their target activity and/or suppressing undesirable activities in order to avoid the formation of by-products or degradation of the steroid core, but also to redirect metabolic fluxes in cells towards accumulation of new metabolites that may be useful for practical applications. Along with whole-cell catalysis, the interest of researchers is growing in enzymatic methods that make it possible to carry out selective structural modifications of steroids, such as the introduction of double bonds, the oxidation of steroidal alcohols, or the reduction of steroid carbonyl groups. A promising area of research is strain engineering based on the heterologous expression of foreign steroidogenesis systems (bacterial, fungal, or mammalian) that ensure selective formation of demanded hydroxylated steroids.Here, current trends and progress in microbial steroid biotechnology over the past few years are briefly reviewed, with a particular focus on the application of metabolic engineering and synthetic biology techniques to improve existing and create new whole-cell microbial biocatalysts.

Microbiota-derived metabolites in regulating the development and physiology of Caenorhabditis elegans

Microbiota consist of microorganisms that provide essential health benefits and contribute to the animal's physiological homeostasis. Microbiota-derived metabolites are crucial mediators in regulating host development, system homeostasis, and overall fitness. In this review, by focusing on the animal model Caenorhabditis elegans, we summarize key microbial metabolites and their molecular mechanisms that affect animal development. We also provide, from a bacterial perspective, an overview of host-microbiota interaction networks used for maintaining host physiological homeostasis. Moreover, we discuss applicable methodologies for profiling new bacterial metabolites that modulate host developmental signaling pathways. Microbiota-derived metabolites have the potential to be diagnostic biomarkers for diseases, as well as promising targets for engineering therapeutic interventions against animal developmental or health-related defects.

Isolation of Environmental Bacteria Able to Degrade Sterols and/or Bile Acids: Determination of Cholesterol Oxidase and Several Hydroxysteroid Dehydrogenase Activities in Rhodococcus, Gordonia, and Pseudomonas putida

Interest about the isolation and characterization of steroid-catabolizing bacteria has increased over time due to the massive release of these recalcitrant compounds and their deleterious effects or their biotransformation derivatives as endocrine disruptors for wildlife, as well as their potential use in biotechnological approaches for the synthesis of pharmacological compounds. Thus, in this chapter, an isolation protocol to select environmental bacteria able to degrade sterols, bile acids, and androgens is shown. Moreover, procedures for the determination of cholesterol oxidase or different hydroxysteroid dehydrogenase activities in Pseudomonas putida DOC21, Rhodococcus sp. HE24.12, Gordonia sp. HE24.4J and Gordonia sp. HE24.3 are also detailed.

Identification and Characterization of Some Genes, Enzymes, and Metabolic Intermediates Belonging to the Bile Acid Aerobic Catabolic Pathway from Pseudomonas

The study of the catabolic potential of microbial species isolated from different habitats has allowed the identification and characterization of bacteria able to assimilate bile acids and/or other steroids (e.g., testosterone and 4-androsten-3,17-dione) under aerobic conditions through the 9,10-seco pathway. From soil samples, we have isolated several strains belonging to genus Pseudomonas that grow efficiently in chemically defined media containing some cyclopentane-perhydrophenanthrene derivatives as carbon sources. Genetic and biochemical studies performed with one of these bacteria (P. putida DOC21) allowed the identification of the genes and enzymes belonging to the route involved in bile acids and androgens, the 9,10-seco pathway in this bacterium. In this manuscript, we describe the most relevant methods used in our lab for the identification of the chromosomal location and nucleotide sequence of the catabolic genes (or gene clusters) encoding the enzymes of this pathway, and the tools useful to establish the role of some of the enzymes that participate in this route.

Steroid Metabolism in Thermophilic Actinobacterium Saccharopolyspora hirsuta VKM Ac-666T

The application of thermophilic microorganisms opens new prospects in steroid biotechnology, but little is known to date on steroid catabolism by thermophilic strains. The thermophilic strain Saccharopolyspora hirsuta VKM Ac-666^T has been shown to convert various steroids and to fully degrade cholesterol. Cholest-4-en-3-one, cholesta-1,4-dien-3-one, 26-hydroxycholest-4-en-3-one, 3-oxo-cholest-4-en-26-oic acid, 3-oxo-cholesta-1,4-dien-26-oic acid, 26-hydroxycholesterol, 3β-hydroxy-cholest-5-en-26-oic acid were identified as intermediates in cholesterol oxidation. The structures were confirmed by ¹H and ¹³C-NMR analyses. Aliphatic side chain hydroxylation at C26 and the A-ring modification at C3, which are putatively catalyzed by cytochrome P450 monooxygenase CYP125 and cholesterol oxidase, respectively, occur simultaneously in the strain and are followed by cascade reactions of aliphatic sidechain degradation and steroid core destruction via the known 9(10)-seco-pathway. The genes putatively related to the sterol and bile acid degradation pathways form three major clusters in the S. hirsuta genome. The sets of the genes include the orthologs of those involved in steroid catabolism in Mycobacterium tuberculosis H37Rv and Rhodococcus jostii RHA1 and related actinobacteria. Bioinformatics analysis of 52 publicly available genomes of thermophilic bacteria revealed only seven candidate strains that possess the key genes related to the 9(10)-seco pathway of steroid degradation, thus demonstrating that the ability to degrade steroids is not widespread among thermophilic bacteria.

Investigations on the Degradation of the Bile Salt Cholate via the 9,10-Seco-Pathway Reveals the Formation of a Novel Recalcitrant Steroid Compound by a Side Reaction in Sphingobium sp. Strain Chol11

Bile salts such as cholate are steroid compounds from the digestive tracts of vertebrates, which enter the environment upon excretion, e.g., in manure. Environmental bacteria degrade bile salts aerobically via two pathway variants involving intermediates with Δ^1,4- or Δ^4,6-3-keto-structures of the steroid skeleton. Recent studies indicated that degradation of bile salts via Δ^4,6-3-keto intermediates in Sphingobium sp. strain Chol11 proceeds via 9,10-seco cleavage of the steroid skeleton. For further elucidation, the presumptive product of this cleavage, 3,12β-dihydroxy-9,10-seco-androsta-1,3,5(10),6-tetraene-9,17-dione (DHSATD), was provided to strain Chol11 in a co-culture approach with Pseudomonas stutzeri Chol1 and as purified substrate. Strain Chol11 converted DHSATD to the so far unknown compound 4-methyl-3-deoxy-1,9,12-trihydroxyestra-1,3,5(10)7-tetraene-6,17-dione (MDTETD), presumably in a side reaction involving an unusual ring closure. MDTETD was neither degraded by strains Chol1 and Chol11 nor in enrichment cultures. Functional transcriptome profiling of zebrafish embryos after exposure to MDTETD identified a significant overrepresentation of genes linked to hormone responses. In both pathway variants, steroid degradation intermediates transiently accumulate in supernatants of laboratory cultures. Soil slurry experiments indicated that bacteria using both pathway variants were active and also released their respective intermediates into the environment. This instance could enable the formation of recalcitrant steroid metabolites by interspecies cross-feeding in agricultural soils.