Bacterial degradation of the steroid compound cholate
Microbial Ecology and Applied Microbiology
Final Report Abstract
Bile acids such as cholate are biological detergents that occur in the intestinal tract of all vertebrates, and their main function is to support the digestion of water-insoluble nutrients. Being steroid compounds that are synthesized from cholesterol in the liver, bile acids have a C5 carboxylic side chain attached to the steroid skeleton and can vary depending on the number and position of hydroxyl groups on the steroid rings. A considerable part of bile acids is released into the environment by vertebrate excretions (e.g. about 60 kg per cow and year), where they are subject to microbial degradation. The topic of this project was the elucidation of the metabolic pathways for bacterial bile acid degradation. This research has led to the discovery that there at least two pathway variants used by bacteria for the catabolism of bile acids. At the beginning of the project a novel bypass-reaction was discovered that is required for channeling different bile acids into a common pathway for the complete degradation of the steroid skeleton. The main part of the project was the elucidation of a so-far unexplored pathway for bile acid degradation in the family of the Sphingomonadaceae, which are frequent environmental bacteria with a high biodegradative potential. In Novosphingobium sp. strain Chol11 the stereospecific ketosteroid dehydratase Hsh2 was identified as a key enzyme in this pathway. From further genetic and biochemical analysis evidence of an unknown metabolic pathway for degradation of the carboxylic side chain in strain Chol11 was obtained, which is initiated by the novel acyl-CoA ligase Scl1. A proteomic analysis was performed and is currently being used to identify further enzymes involved in this unknown pathway for side chain degradation using strain Chol11 as a model organism. Interestingly, Hsh2 was also found to be a key enzyme for anaerobic degradation of bile acids in the denitrifying bacterium Azoarcus sp. strain Aa7. Irrespective of the metabolic pathway bacterial bile acid metabolism involves the transient released of steroid compounds as intermediates of the degradation. This release calls attention to transport processes across the cell envelope, and a gene for a putative efflux protein for androstadienediones (ADDs) as prominent degradation intermediates was found in this project. Bacterial bile acid degradation has impact on other scientific areas and on applied aspects, which were also addressed in this project. Generally, the knowledge about biological functions of bile acids is increasing. As bile acids have multiple signaling functions in humans, their biotechnological transformation is interesting for pharmaceutical science and has led to cooperation with industry. Bile acids can also act as pheromones for both vertebrate and invertebrate aquatic animals. Thus, bacterial metabolism may interfere with chemical communication of animal life. Additionally, intermediates of bile acid degradation are potential hormones, in particular the aforementioned ADDs. Regarding the high amount of bile acids brought into the environment by manure this calls attention to potential endocrine effects on biota in agricultural areas. In the laboratory, such endocrine effects on the nematode Caenorhabditis elegans could be shown. This proof-of-concept study led to a new cooperation with ecotoxicologists and gained also public interest (radio report Deutschlandfunk on 22nd of October 2019). In conclusion, bacterial bile acid degradation has multiple facets ranging from the biochemistry of steroidtransforming reactions and their industrial application to ecological aspects with relevance for agriculture. Several of these aspects are currently being further investigated in our laboratory.
Publications
- (2016). An unexplored pathway for degradation of cholate requires a 7α-hydroxysteroid dehydratase and contributes to a broad metabolic repertoire for the utilization of bile salts in Novosphingobium sp. strain Chol11. Environmental Microbiology 18:5187-5203
O. Yücel, S. Drees, N. Jagmann, T. Patschkowski and B. Philipp
(See online at https://doi.org/10.1111/1462-2920.13534) - (2016). Identification of bypass reactions leading to the formation of one central steroid degradation intermediate in metabolism of different bile salts in Pseudomonas sp. strain Chol1. Enivronmental Microbiology 18:3373-3389
J. Holert, O. Yücel, N. Jagmann, A. Prestel, H. M. Möller and B. Philipp
(See online at https://doi.org/10.1111/1462-2920.13192) - (2017). A novel steroid-coenzyme A ligase from Novosphingobium sp. strain Chol11 is essential for an alternative degradation pathway for bile salts. Applied and Environmental Microbiology 84:1-16
O. Yücel, J. Holert, K.C. Ludwig, S. Thierbach and B. Philipp
(See online at https://doi.org/10.1128/AEM.01492-17) - (2018). Functional characterization of three specific acyl-coenzyme A synthetases involved in anaerobic cholesterol degradation in Sterolibacterium denitrificans Chol1S. Applied and Environmental Microbiology 84:e02721-17
M. Warnke, T. Jung T, C. Jacoby, M Agne, FM Feller, B. Philipp, W. Seiche, B. Breit B and M. Boll
(See online at https://doi.org/10.1128/AEM.02721-17) - (2018). Genome sequence of the bile saltdegrading bacterium Novosphingobium sp. strain Chol11, a model organism for bacterial steroid catabolism. Genome Announcements 6:e01372-17
O. Yücel, D. Wibberg, B. Philipp and J. Kalinowski
(See online at https://doi.org/10.1128/genomeA.01372-17) - (2019). Steroids originating from bacterial bile acid degradation affect Caenorhabditis elegans and indicate potential risks for the fauna of manured soils. Scientific Reports 9:11120
M. N. Mendelski, R. Dölling, F. M. Feller, D. Hoffmann, L. Ramos Fangmeier, K. C. Ludwig, O. Yücel, A. Mährlein, R. J. Paul and B. Philipp
(See online at https://doi.org/10.1038/s41598-019-47476-y) - (2019). The 7α-hydroxysteroid dehydratase Hsh2 is essential for anaerobic degradation of the steroid skeleton of 7α-hydroxyl bile salts in the novel denitrifying bacterium Azoarcus sp. strain Aa7. Environmental Microbiology 21: 800-813
O. Yücel, S. R. Borgert, A. Poehlein, K. Niermann and B. Philipp
(See online at https://doi.org/10.1111/1462-2920.14508)
