Heme ligation and redox chemistry in two bacterial thiosulfate dehydrogenase (TsdA) enzymes

Abstract

Thiosulfate dehydrogenases (TsdAs) are bidirectional bacterial di-heme enzymes that catalyze the interconversion of tetrathionate and thiosulfate at measurable rates in both directions. In contrast to our knowledge of TsdA activities, information on the redox properties in the absence of substrates is rather scant. To address this deficit, we combined magnetic CD (MCD) spectroscopy and protein film electrochemistry (PFE) in a study to resolve heme ligation and redox chemistry in two representative TsdAs. We examined the TsdAs from Campylobacter jejuni, a microaerobic human pathogen, and from the purple sulfur bacterium Allochromatium vinosum. In these organisms, the enzyme functions as a tetrathionate reductase and a thiosulfate oxidase, respectively. The active site Heme 1 in both enzymes has His/Cys ligation in the ferric and ferrous states and the midpoint potentials (Em) of the corresponding redox transformations are similar, −185 mV versus standard hydrogen electrode (SHE). However, fundamental differences are observed in the properties of the second, electron transferring, Heme 2. In C. jejuni, TsdA Heme 2 has His/Met ligation and an Em of +172 mV. In A. vinosum TsdA, Heme 2 reduction triggers a switch from His/Lys ligation (Em, −129 mV) to His/Met (Em, +266 mV), but the rates of interconversion are such that His/Lys ligation would be retained during turnover. In summary, our findings have unambiguously assigned Em values to defined axial ligand sets in TsdAs, specified the rates of Heme 2 ligand exchange in the A. vinosum enzyme, and provided information relevant to describing their catalytic mechanism(s).

X-ray–induced photoreduction of heme metal centers rapidly induces active-site perturbations in a protein-independent manner

“…For heme proteins, there are essentially three reported cases: myoglobin (10,(19)(20)(21), cytochrome c peroxidase, and horseradish peroxidase (8,9). For a more general approach to study reduction kinetics, a set of six diverse heme proteins was selected; metmyoglobin from horse heart (hhMb), a diheme c protein, thiosulfate dehydrogenase from Allochromatium vinosum (AvTsdA) (22)(23)(24)(25), a B-class dye-decolorizing peroxidase (KpDyP) (26), chlorite dismutases from Nitrospira defluvii (NdCld) (27) and from Cyanothece sp. PCC7425 (CCld) (28), and a coproheme decarboxylase from Listeria monocytogenes (LmChdC) (29).…”

Heterotrophic Sulfur Oxidation of Halomonas titanicae SOB56 and Its Habitat Adaptation to the Hydrothermal Environment

“…In A. vinosum , the distal axial ligand of the second heme switch from Lys 235 to Met 236 is observed upon reduction of TsdA ( Brito et al, 2015 ). However, an asparagine residue at the Lys 235 equivalent position is highly conserved among proteobacterial thiosulfate dehydrogenases, for example, TsdA from the T. intermedia and S. lithotrophus belonging to Betaproteobacteria , as well as Campylobacter jejuni from Epsilonproteobacteria ( Denkmann et al, 2012 ; Jenner et al, 2019 ). In our results, all Halomonas species and referred M. purpuratum , from the same class as A. vinosum belonging to Gammaproteobacteria , possess TsdA homologues with an Asn residue instead of Lys.…”

“…In our results, all Halomonas species and referred M. purpuratum , from the same class as A. vinosum belonging to Gammaproteobacteria , possess TsdA homologues with an Asn residue instead of Lys. As reviewed TsdA containing conserved Asn 235 , the distal axial coordination of the second heme was Met 236 ( Denkmann et al, 2012 ; Brito et al, 2015 ; Jenner et al, 2019 ). Furthermore, it is reported that introducing Lys in place of Asn in the TsdA of C. jejuni , the second heme allows Lys to provide the distal ligand ( Jenner et al, 2019 ).…”

“…It was reported that thiosulfate dehydrogenases (TsdA) also catalyze the oxidation from thiosulfate to tetrathionate ( Mandal et al, 2020 ). In this reaction, thiosulfate is oxidized in the periplasm by thiosulfate dehydrogenase, which requires c -type cytochrome as an electron acceptor and/or a c-type heme molecule in the protein ( Denkmann et al, 2012 ; Kurth et al, 2016 ; Jenner et al, 2019 ). Several thiosulfate-oxidizing and tetrathionate-forming thiosulfate dehydrogenases have been identified and characterized in both chemolithotrophic and phototrophic sulfur bacteria.…”

Thiosulfate oxidation in sulfur‐reducing Shewanella oneidensis and its unexpected influences on the cytochrome c content

“…7), most of them may not catalyse thiosulfate/sulfur oxidation. The TsdA family proteins are bi‐functional, displaying activity of either thiosulfate dehydrogenase or tetrathionate reductases depending on bacteria species (Denkmann et al ., 2012; Liu et al ., 2013; Jenner et al ., 2019). When TsdA catalyse thiosulfate oxidation, they interact with TsdB.…”