The catabolism of 3,3′-thiodipropionic acid in Variovorax paradoxus strain TBEA6: A proteomic analysis

Variovorax paradoxus strain TBEA6 is one of the few organisms known to utilize 3,3'-thiodipropionate (TDP) as the only source of carbon and energy. It cleaves TDP to 3-mercapto-propionate (3MP), which is a direct precursor for polythioester synthesis. To establish this process in V. paradoxus TBEA6, it is crucial to unravel its TDP metabolism. Therefore, a proteomic approach with subsequent deletion of interesting genes in the bacterium was chosen. Cells were cultivated with D-gluconate, TDP or 3-sulfinopropionate as the only carbon sources. Proteins with high abundances in gels of cells cultivated with either of the organic sulfur compounds were analyzed further. Thereby, we did not only confirm parts of the already postulated TDP metabolism, but also eight new protein candidates for TDP degradation were detected. Deletions of the corresponding genes (two enoyl-CoA hydratases (Ech-20 and Ech-30), an FK506-binding protein, a putative acetolactate synthase, a carnitinyl-CoA dehydratase, and a putative crotonase family protein) were obtained. Only the deletions of both Ech-20 and Ech-30 led to a TDP negative phenotype. The deletion mutant of VPARA_05510, which encodes the putative crotonase family protein showed reduced growth with TDP. The three genes are located in one cluster with genes proven to be involved in TDP metabolism. Thermal shift assays showed an increased stability of Ech-20 with TDP-CoA but not with TDP. These results indicate that Ech-20 uses TDP-CoA as a substrate instead of TDP. Hence, we postulate a new putative pathway for TDP metabolism. Ech-30 interacts with neither TDP-CoA nor TDP but might interact with other CoA-activated intermediates of the proposed pathway. Further enzyme characterization is necessary to unravel the complete pathway from TDP to 3MP.