Kurth, Julia: The TsdA family of thiosulfate dehydrogenases/tetrathionate reductases. - Bonn, 2017. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5n-46962
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5n-46962,
author = {{Julia Kurth}},
title = {The TsdA family of thiosulfate dehydrogenases/tetrathionate reductases},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2017,
month = apr,

note = {TsdA enzymes are a phylogenetically widespread family of periplasmic c-type diheme cytochromes which catalyse both thiosulfate oxidation and tetrathionate reduction. The reaction directionality varies between enzymes isolated from different bacteria and is in line with their different physiological functions: TsdA enables the purple sulfur bacteria Allochromatium vinosum (Av) and Marichromatium purpuratum (Mp) to use thiosulfate as an electron donor for respiration or photosynthesis whereas the enzyme from the human gut pathogen Campylobacter jejuni (Cj) allows the organism to use tetrathionate as a terminal electron acceptor.
The bifunctionality of the TsdA enzyme allowed experimental determination of the reduction potential of the tetrathionate/thiosulfate couple ETT/TS. This was very important as calculations from the relevant, constantly reevaluated thermodynamic data had yielded conflicting results. An ETT/TS value of +198 mV was obtained by experimental means which is much more positive than the value of +24 mV widely cited in the field of microbial bioenergetics. As a consequence more free energy is available to be harnessed during the respiratory reduction of tetrathionate to thiosulfate than was previously recognized.
The TsdA active site heme, Heme 1, shows an unusual His/Cys ligation which appears to be of special importance in sulfur-based energy metabolism. Whereas this heme-ligating cysteine is conserved in TsdAs from different organisms, the ligand constellation of the electron transfer heme, Heme 2, differs depending on the source organism. In vitro as well as in vivo experiments with CjTsdA revealed that structural differences in the immediate environment of Heme 2 contribute to defining the reaction directionality of the enzyme.
An essential part of the TsdA reaction cycle is the covalent linkage between thiosulfate and the heme-ligating cysteine in the active site involving electron transfer to or from the TsdA hemes, respectively. In addition, a thiol-disulfide exchange is an important step of the TsdA reaction mechanism. To get a closer insight into the TsdA reaction mechanism, sulfite which was shown to be a competitive inhibitor of TsdA can be used as substrate mimic. Hints were obtained that a positive shift in the Heme 1 reduction potential occurs during formation of this covalent bond facilitating electron transfer during the reaction cycle. The diheme cytochrome c TsdB acts as an effective electron acceptor of TsdA in vitro when TsdA and TsdB originate from the same source organism. When present, the diheme cytochrome c TsdB is the immediate electron acceptor of TsdA-type thiosulfate dehydrogenases. For AvTsdA and the MpTsdBA fusion protein the high potential iron-sulfur protein HiPIP was identified as a suitable electron acceptor.
In contrast to all other tsdA containing organisms, a membrane attachment of TsdA by the lipoprotein TsdC seems to be indispensable for tetrathionate reduction in W. succinogenes.},

url = {http://hdl.handle.net/20.500.11811/7172}

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