3 years ago

A novel steroid-coenzyme A ligase from Novosphingobium sp. strain Chol11 is essential for an alternative degradation pathway for bile salts.

Johannes Holert, Sven Thierbach, Bodo Philipp, Kevin Christopher Ludwig, Onur Yücel
Bile salts such as cholate are steroid compounds with a C5 carboxylic side chain and occur ubiquitously in vertebrates. Upon their excretion into soils and waters bile salts can serve as growth substrates for diverse bacteria. Novosphingobium sp. strain Chol11 degrades 7-hydroxy bile salts via 3-keto-7-deoxy-Δ(4,6) metabolites by dehydration of the 7-hydroxyl group catalyzed by the 7α-hydroxysteroid dehydratase Hsh2. This reaction has not been observed in the well-studied 9-10-seco degradation pathway used by other steroid-degrading bacteria indicating that strain Chol11 uses an alternative pathway. A reciprocal BLASTp analysis showed that known side chain degradation genes from other cholate-degrading bacteria (Pseudomonas stutzeri Chol1, Comamonas testosteroni CNB-2 and Rhodococcus jostii RHA1) were not found in the genome of strain Chol11. Characterization of a transposon mutant of strain Chol11 showing altered growth with cholate identified a novel steroid-24-oyl-coenzyme A ligase named Scl1. Unmarked deletion of scl1 resulted in a strong growth rate decrease with cholate, while growth with steroids with a C3-side chain or without a side chain was not affected. Intermediates with a 7-deoxy-3-keto-Δ(4,6) structure such as 3,12-dioxo-4,6-choldienoic acid (DOCDA) were shown to be likely physiological substrates of Scl1. Furthermore, a novel CoA-dependent DOCDA degradation metabolite with an additional double bond in the side chain was identified. These results support that Novosphingobium sp. strain Chol11 harbors an alternative pathway for cholate degradation, in which side chain degradation is initiated by the CoA-ligase Scl1 and proceeds via reaction steps catalyzed by so-far unknown enzymes different from other steroid degrading bacteria.IMPORTANCE This study provides further evidence of the diversity of metabolic pathways for the degradation of steroid compounds in environmental bacteria. The knowledge about these pathways contributes to the understanding of the CO2-releasing part of the global C-cycle. Furthermore, it is useful for investigating the fate of pharmaceutical steroids in the environment, some of which may act as endocrine disruptors.

Publisher URL: http://doi.org/10.1128/AEM.01492-17

DOI: 10.1128/AEM.01492-17

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