Why does sulfate inhibit selenate reduction: Molybdenum deprivation from Mo-dependent selenate reductase
Selenium pollution has become an increasingly serious global concern. Methane-fed selenate reduction has proven to be of great interest for the bioremediation of selenate-contaminated waters even with the coexistence of nitrate and dissolved oxygen. However, it is unclear if the common concurrent sulfate anion affects selenate removal. To address this question, we first introduced selenate (SeO42-) as the sole influent electron acceptor in a CH(4)-fed membrane biofilm reactor (CH(4)-MBfR); then we added different concentrations of sulfate (SO42-). The initial selenate removal efficiency (∼90%) was decreased by 50% in the presence of 15.6 μM of sulfate and completely inhibited after loading with 171.9 μM of sulfate. 16S rRNA gene sequencing showed that the selenate-reducing bacteria decreased after the addition of sulfate. Metagenomic sequencing showed that the abundance of genes encoding molybdenum (Mo)-dependent selenate reductase reduced by >50% when exposed to high concentrations of sulfate. Furthermore, the decrease in the total genes encoding all Mo-oxidoreductases was much greater than that of the genes encoding molybdate transporters, suggesting that the inhibition of selenate reduction by sulfate was most likely via the direct competition with molybdate for the transport system, leading to a lack of available Mo for Mo-dependent selenate reductases and thus reducing their activities. This result was confirmed by a batch test wherein the supplementation of molybdate mitigated the sulfate effect. Overall, this study shed light on the underlying mechanism of sulfate inhibition on selenate reduction and laid the foundation for applying the technology to practical wastewaters.
L. D. Shi, P. L. Lv, Z. F. Niu, C. Y. Lai, and H. P. Zhao,Why does sulfate inhibit selenate reduction: Molybdenum deprivation from Mo-dependent selenate reductase, Water research, 2020, 178, 115832.
Selenate and nitrate
Discovery of piperonal-converting oxidase involved in the metabolism of a botanical aromatic aldehyde
Piperonal-catabolizing microorganisms were isolated from soil, the one (strain CT39-3) exhibiting the highest activity being identified as Burkholderia sp. The piperonal-converting enzyme involved in the initial step of piperonal metabolism was purified from strain CT39-3. Gene cloning of the enzyme and a homology search revealed that the enzyme belongs to the xanthine oxidase family, which comprises molybdoenzymes containing a molybdopterin cytosine dinucleotide cofactor. We found that the piperonal-converting enzyme acts on piperonal in the presence of O2, leading to formation of piperonylic acid and H2O2. The growth of strain CT39-3 was inhibited by higher concentrations of piperonal in the culture medium. Together with this finding, the broad substrate specificity of this enzyme for various aldehydes suggests that it would play an important role in the defense mechanism against antimicrobial compounds derived from plant species.
Doi, S., Hashimoto, Y., Tomita, C., Kumano, T., and Kobayashi, M.,Discovery of piperonal-converting oxidase involved in the metabolism of a botanical aromatic aldehyde, Scientific Reports, 2016, 6.
Biosynthesis of selenate reductase in Salmonella enterica: critical roles for the signal peptide and DmsD
Salmonella enterica serovar Typhimurium is a Gram-negative bacterium with a flexible respiratory capability. Under anaerobic conditions, S. enterica can utilize a range of terminal electron acceptors, including selenate, to sustain respiratory electron transport. The S. enterica selenate reductase is a membrane-bound enzyme encoded by the ynfEFGH-dmsD operon. The active enzyme is predicted to comprise at least three subunits where YnfE is a molybdenum-containing catalytic subunit. The YnfE protein is synthesized with an N-terminal twin-arginine signal peptide and biosynthesis of the enzyme is coordinated by a signal peptide binding chaperone called DmsD. In this work, the interaction between S. enterica DmsD and the YnfE signal peptide has been studied by chemical crosslinking. These experiments were complemented by genetic approaches, which identified the DmsD binding epitope within the YnfE signal peptide. YnfE signal peptide residues L24 and A28 were shown to be important for assembly of an active selenate reductase. Conversely, a random genetic screen identified the DmsD V16 residue as being important for signal peptide recognition and selenate reductase assembly.
Connelly, K. R. S., Stevenson, C., Kneuper, H., and Sargent, F.,Biosynthesis of selenate reductase in Salmonella enterica: critical roles for the signal peptide and DmsD, Microbiology-Sgm, 2016, 162, 2136-2146.
Some of the common themes and variations between the different classes of nitrate and selenate reductases are reviewed.
Watts, C.A., Ridley, H., Dridge, E. J., Leaver, J. T., Reilly, A. J., Richardson, D. J., and Butler, C. S., Microbial reduction of selenate and nitrate: common themes and variations, Biochemical Society Transactions, 2005, 33, 173-175.