AUTHOR=Protze Jonas , Müller Fabian , Lauber Karin , Naß Bastian , Mentele Reinhard , Lottspeich Friedrich , Kletzin Arnulf 

TITLE=An Extracellular Tetrathionate Hydrolase from the Thermoacidophilic Archaeon Acidianus Ambivalens with an Activity Optimum at pH 1

JOURNAL=Frontiers in Microbiology

VOLUME=2

YEAR=2011

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2011.00068

DOI=10.3389/fmicb.2011.00068

ISSN=1664-302X

ABSTRACT=<p><bold>Background</bold>: The thermoacidophilic and chemolithotrophic archaeon <italic>Acidianus ambivalens</italic> is routinely grown with sulfur and CO<sub>2</sub>-enriched air. We had described a membrane-bound, tetrathionate (TT) forming thiosulfate:quinone oxidoreductase. Here we describe the first TT hydrolase (TTH) from Archaea. <bold>Results</bold>: <italic>A. ambivalens</italic> cells grown aerobically with TT as sole sulfur source showed doubling times of 9 h and final cell densities of up to 8 × 10<sup>8</sup>/ml. TTH activity (≈0.28 U/mg protein) was found in cell-free extracts of TT-grown but not of sulfur-grown cells. Differential fractionation of freshly harvested cells involving a pH shock showed that about 92% of the TTH activity was located in the pseudo-periplasmic fraction associated with the surface layer, while 7.3% and 0.3% were present in the soluble and membrane fractions, respectively. The enzyme was enriched 54-fold from the cytoplasmic fraction and 2.1-fold from the pseudo-periplasmic fraction. The molecular mass of the single subunit was 54 kDa. The optimal activity was at or above 95°C at pH 1. Neither PQQ nor divalent cations had a significant effect on activity. The gene (<italic>tth1</italic>) was identified following N-terminal sequencing of the protein. Northern hybridization showed that <italic>tth1</italic> was transcribed in TT-grown cells in contrast to a second paralogous <italic>tth2</italic> gene. The deduced amino acid sequences showed similarity to the TTH from <italic>Acidithiobacillus</italic> and other proteins from the PQQ dehydrogenase superfamily. It displayed a β-propeller structure when being modeled, however, important residues from the PQQ-binding site were absent. <bold>Conclusion</bold>: The soluble, extracellular, and acidophilic TTH identified in TT-grown <italic>A. ambivalens</italic> cells is essential for TT metabolism during growth but not for the downstream processing of the TQO reaction products in S°-grown cells. The liberation of TTH by pH shock from otherwise intact cells strongly supports the pseudo-periplasm hypothesis of the S-layer of Archaea.</p>