Thiosulfate dehydrogenase: an unusual acidophilic c-type cytochrome

越来越多的证据表明，c型细胞色素具有不寻常的轴向组氨酸-半胱氨酸血红素铁配位，在硫基能量代谢中起关键作用。二血红素细胞色素c TsdA在体内根据宿主生物和生长条件发挥硫代硫酸盐脱氢酶或四硫代酸还原酶的作用，最近被确定为这一令人兴奋的蛋白质组的另一个成员。虽然TsdA不仅广泛存在于变形菌门，也广泛存在于其他细菌门中，但目前对其反应机制、生物物理结构特性以及确切的生理作用还没有详细的了解。该项目旨在通过关注两种酶的原型来填补这一空白：来自紫色硫细菌Allochromatium vinosum的[1]硫代硫酸盐脱氢酶，该酶不仅携带一个具有不寻常的轴向His/Cys配位的血红素，而且第二个血红素在还原时表现出前所未有的His/Lys到His/Met连接血红素2的切换。人类肠道病原体空肠弯曲杆菌的双功能四硫酸盐还原酶/硫代硫酸盐脱氢酶。这种酶似乎特别适合于催化四硫酸盐还原。结构比较表明，一个血红素基团的缺失/不同的轴向结扎是导致这两个TsdA原型不同性质的一个因素。与源生物和tsdA null以及互补突变体的分子遗传学研究，结合详细的生物物理，电化学和结构特征以及两种tsdA原型的比较，将回答以下问题[1]哪些分子细节是tsdA蛋白在硫代硫酸盐氧化或四硫代酸还原中优先发挥作用的基础？硫代硫酸盐脱氢酶/四硫代酸还原酶有哪些生理功能和优势？这些问题的答案将有助于彻底了解这种新颖而不寻常的细胞色素类型。此外，该项目的结果将阐明硫代硫酸盐不仅与致力于产生能量的硫代谢的细菌有关，而且与与人类健康有关的硫代硫酸盐/四硫代酸代谢有机异养生物有关。

Evidence is emerging that c-type cytochromes with an unusual axial histidine-cysteine coordination of the heme iron play a pivotal role in sulfur-based energy metabolism. The diheme cytochrome c TsdA that acts as thiosulfate dehydrogenase or tetrathionate reductase in vivo depending on the host organism and growth conditions, was recently identified as another member of this exciting group of proteins. Although wide spread not only in Proteobacteria but also in other bacterial phyla, detailed knowledge about the reaction mechanism, the biophysical and structural properties as well as the exact physiological role of TsdA is not available. This project is intended to fill this gap by focussing on two prototypes of the enzyme: [1] Thiosulfate dehydrogenase from the purple sulfur bacterium Allochromatium vinosum that does not only carry one heme with unusual axial His/Cys coordination but a second heme exhibiting an unprecedented switch of a His/Lys to His/Met ligation of heme 2 upon reduction. [2] The bifunctional tetrathionate reductase/thiosulfate dehydrogenase from the human gut pathogen Campylobacter jejuni. This enzyme appears to be especially adapted to catalyzing tetrathionate reduction. Structural comparisons indicate missing/different axial ligation of one heme group as one factor contributing to the different properties of these two TsdA prototypes. Molecular genetic work with the source organisms and tsdA null as well as complementation mutants in combination with detailed biophysical, electrochemical and structural characterization and comparison of the two TsdA prototypes will answer the following questions[1] Which are the molecular details that underlie the adaptation of TsdA proteins to function preferentially either in thiosulfate oxidation or tetrathionate reduction?[2] Which are the physiological functions and advantages of thiosulfate dehydrogenase/tetrathionate reductase? Answers to these questions will contribute to a thorough understanding of this novel and unusual type of cytochromes. Furthermore, the results of this project will shed light on the relevance of thiosulfate not only in bacteria dedicated to energy-generating sulfur metabolism but also in thiosulfate/tetrathionate metabolizing organoheterotrophs relevant to human health.