Redox and Chemical Activities of the Hemes in the Sulfur Oxidation Pathway Enzyme SoxAX

Thiosulfate Dehydrogenase (TsdA) from Allochromatium vinosum

“…Combination of two thiosulfate molecules at the active site, liberating two electrons, could then proceed with reduction of the two hemes of TsdA. This suggestion conforms in part to an alternative reaction mechanism discussed recently for SoxXA (57). Whereas Bradley et al (57) promoted the idea that SoxXA might mediate the direct coupling of thiosulfate to SoxY without dissociation of any heme ligands, we consider the possibility that Cys 96 temporarily exits the heme 1 iron coordination during the reaction cycle, leading to a change in the redox potential such that electron transfer into heme 1 is facilitated.…”

“…This suggestion conforms in part to an alternative reaction mechanism discussed recently for SoxXA (57). Whereas Bradley et al (57) promoted the idea that SoxXA might mediate the direct coupling of thiosulfate to SoxY without dissociation of any heme ligands, we consider the possibility that Cys 96 temporarily exits the heme 1 iron coordination during the reaction cycle, leading to a change in the redox potential such that electron transfer into heme 1 is facilitated. The observed switch at heme 2 from His/Lys to His/Met ligation upon reduction, leading to greater disparity in redox potential between heme 1 and heme 2 (discussed in more detail below), could be a further means to drive the reaction to tetrathionate production.…”

“…transfer of solely the thiosulfate sulfone sulfur instead of the complete thiosulfate moiety to SoxYZ) is the cause for persulfuration of the active site cysteine (6). It should however, be noted that even recombinant SoxA that had been produced in E. coli in the absence of thiosulfate or other Sox pathway proteins carried the modification, at least in part of the protein molecules (7,57). The A. vinosum TsdA and all its variants studied here were recombinantly expressed in E. coli, and we also observed partial persulfide modification in some structures.…”

“…Furthermore, we showed that when we replaced Cys 96 by the stronger field ligands methionine and histidine, the activity of the enzyme was completely abolished. For the R. sulfidophilus SoxXA, it has also been proposed that the thiolate ligand to the active site heme is protonated upon reduction (20,57). It is difficult to envision how a protonated cysteine could be a suitable acceptor for covalent attachment of thiosulfate.…”

“…However, it is still a matter of debate to what extent the heme-ligating cysteine is involved in the reaction and how it exerts its function. The His/Cys ligation corresponds to an extremely low redox potential of the corresponding heme in SoxA and DsrJ (23,(57)(58)(59). Values of Ϫ432 Ϯ 15 and Ϫ479 Ϯ 10 mV (versus NHE at pH 7.0) have been reported for the SoxA hemes in P. pantotropus and S. novella SoxA (12,14).…”