Properties of thiols required for sulfur dioxygenase activity at acidic pH

Abstract

In mesoacidophilic sulfur-oxidizing bacteria, such as Acidithiobacillus and Acidiphilium spp., elemental sulfur is oxidized by a thiol-dependent dioxygenase to sulfite. The thiols form with elemental sulfur highly reactive sulfanes, which are the actual substrate of the dioxygenase. Thus far, the identity of the required thiols is unknown. Consequently, the classical in vitro enzyme assay which is generally based on the addition of glutathione (GSH) is unsuitable for characterizing enzymatic sulfur oxidation in these bacteria. In this study, we present data on in vitro pH dependence of sulfur dioxygenase from Acidiphilium acidophilum DSM 700, indicating a limitation at the lower pH range caused by, besides other experimental factors, the quite high pI of the added GSH (which is around 3). It is speculated that thiols with pIs of about 2 could extend dioxygenase activity below the observed pH limit of about 3.5–4. The demonstration of significant sulfur dioxygenase activity at pH values around 2 would support the hypothesis of a periplasmic localization of this enzyme in Gram-negative mesoacidophilic sulfur oxidizers which generally have an acidic periplasm and a neutral cytoplasm. Furthermore, designing an enzymatic assay more adapted to the intracellular conditions would help to identify and characterize the components (thiols and proteins) which are in vivo responsible for the transport, activation, and oxidation of elemental sulfur.