Electron accepting units of the diheme cytochrome c TsdA, a bifunctional thiosulfate dehydrogenase/tetrathionate reductase

Julia M. Kurth, José A. Brito, Jula Reuter, Alexander Flegler, Tobias Koch, Thomas Franke, Eva-Maria Klein, Sam F. Rowe, Julea N. Butt, Kevin Denkmann, Ines A. C. Pereira, Margarida Archer, Christiane Dahl

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The enzymes of the thiosulfate dehydrogenase (TsdA) family are wide-spread diheme c-type cytochromes. Here, redox carriers were studied mediating the flow of electrons arising from thiosulfate oxidation into respiratory or photosynthetic electron chains. In a number of organisms, including Thiomonas intermedia and Sideroxydans lithotrophicus the tsdA gene is immediately preceded by tsdB encoding for another diheme cytochrome. Spectrophotometric experiments in combination with enzymatic assays in solution showed that TsdB acts as an effective electron acceptor of TsdA in vitro when TsdA and TsdB originate from the same source organism. While TsdA covers a range from -300 mV to +150 mV, TsdB is redox active between -100 to +300 mV, thus enabling electron transfer between these hemoproteins. The three-dimensional structure of the TsdB-TsdA fusion protein from the purple sulfur bacterium Marichromatium purpuratum was solved by X-ray crystallography to 2.75 Å resolution providing insights into internal electron transfer. In the oxidized state, this tetraheme cytochrome c contains three hemes with axial His/Met ligation, while heme 3 exhibits the His/Cys coordination typical for TsdA active sites. Interestingly, thiosulfate is covalently bound to Cys330 on heme 3. In several bacteria including Allochromatium vinosum, TsdB is not present, precluding a general and essential role for electron flow. Both, AvTsdA and the MpTsdBA fusion react efficiently in vitro with high potential iron sulfur protein from A. vinosum (Em +350 mV). HiPIP not only acts as direct electron donor to the reaction center in anoxygenic phototrophs but can also be involved in aerobic respiratory chains.
Original languageEnglish
Pages (from-to)24804-24818
Number of pages15
JournalJournal of Biological Chemistry
Early online date30 Sep 2016
Publication statusPublished - 25 Nov 2016


  • crystal structure
  • cytochrome c
  • enzyme kinetics
  • heme
  • protein chemistry
  • respiratory chain
  • TsdA
  • electron acceptor
  • thiosulfate dehydrogenase

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