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Substrate pathways and mechanisms of inhibition in the sulfur oxygenase reductase of acidianus ambivalens.

ABSTRACT: BACKGROUND:The sulfur oxygenase reductase (SOR) is the initial enzyme of the sulfur oxidation pathway in the thermoacidophilic Archaeon Acidianus ambivalens. The SOR catalyzes an oxygen-dependent sulfur disproportionation to H(2)S, sulfite and thiosulfate. The spherical, hollow, cytoplasmic enzyme is composed of 24 identical subunits with an active site pocket each comprising a mononuclear non-heme iron site and a cysteine persulfide. Substrate access and product exit occur via apolar chimney-like protrusions at the fourfold symmetry axes, via narrow polar pores at the threefold symmetry axes and via narrow apolar pores within in each subunit. In order to investigate the function of the pores we performed site-directed mutagenesis and inhibitor studies. RESULTS:Truncation of the chimney-like protrusions resulted in an up to sevenfold increase in specific enzyme activity compared to the wild type. Replacement of the salt bridge-forming Arg(99) residue by Ala at the threefold symmetry axes doubled the activity and introduced a bias toward reduced reaction products. Replacement of Met(296) and Met(297), which form the active site pore, lowered the specific activities by 25-55% with the exception of an M(296)V mutant. X-ray crystallography of SOR wild type crystals soaked with inhibitors showed that Hg(2+) and iodoacetamide (IAA) bind to cysteines within the active site, whereas Zn(2+) binds to a histidine in a side channel of the enzyme. The Zn(2+) inhibition was partially alleviated by mutation of the His residue. CONCLUSIONS:The expansion of the pores in the outer shell led to an increased enzyme activity while the integrity of the active site pore seems to be important. Hg(2+) and IAA block cysteines in the active site pocket, while Zn(2+) interferes over a distance, possibly by restriction of protein flexibility or substrate access or product exit.

SUBMITTER: Veith A

PROVIDER: S-EPMC3128934 | biostudies-literature | 2011

REPOSITORIES: biostudies-literature

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Substrate pathways and mechanisms of inhibition in the sulfur oxygenase reductase of acidianus ambivalens.

Veith Andreas A Urich Tim T Seyfarth Kerstin K Protze Jonas J Frazão Carlos C Kletzin Arnulf A

Frontiers in microbiology 20110307

<h4>Background</h4>The sulfur oxygenase reductase (SOR) is the initial enzyme of the sulfur oxidation pathway in the thermoacidophilic Archaeon Acidianus ambivalens. The SOR catalyzes an oxygen-dependent sulfur disproportionation to H(2)S, sulfite and thiosulfate. The spherical, hollow, cytoplasmic enzyme is composed of 24 identical subunits with an active site pocket each comprising a mononuclear non-heme iron site and a cysteine persulfide. Substrate access and product exit occur via apolar ch ...[more]

PMID: 21747782

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Project description:BackgroundThe thermoacidophilic and chemolithotrophic archaeon Acidianus ambivalens is routinely grown with sulfur and CO(2)-enriched air. We had described a membrane-bound, tetrathionate (TT) forming thiosulfate:quinone oxidoreductase. Here we describe the first TT hydrolase (TTH) from Archaea.ResultsA. ambivalens cells grown aerobically with TT as sole sulfur source showed doubling times of 9 h and final cell densities of up to 8 × 10(8)/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 (tth1) was identified following N-terminal sequencing of the protein. Northern hybridization showed that tth1 was transcribed in TT-grown cells in contrast to a second paralogous tth2 gene. The deduced amino acid sequences showed similarity to the TTH from Acidithiobacillus 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.ConclusionThe soluble, extracellular, and acidophilic TTH identified in TT-grown A. ambivalens 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.

Project description:Sequence comparisons showed that the sulfur oxygenase reductase (SOR) of the haloalkaliphilic bacterium Thioalkalivibrio paradoxus Arh 1 (TpSOR) is branching deeply within dendrograms of these proteins (29 to 34% identity). A synthetic gene encoding TpSOR expressed in Escherichia coli resulted in a protein 14.7 ± 0.9 nm in diameter and an apparent molecular mass of 556 kDa. Sulfite and thiosulfate were formed from elemental sulfur in a temperature range of 10 to 98°C (optimum temperature ? 80°C) and a pH range of 6 to 11.5 (optimum pH ? 9; 308 ± 78 U/mg of protein). Sulfide formation had a maximum specific activity of 0.03 U/mg, or <1% of the corresponding activity of other SORs. Hence, reductase activity seems not to be an integral part of the reaction mechanism. TpSOR was most active at NaCl or glycine betaine concentrations of 0 to 1 M, although 0.2% of the maximal activity was detected even at 5 M NaCl and 4 M betaine. The melting point of TpSOR was close to 80°C, when monitored by circular dichroism spectroscopy or differential scanning fluorimetry; however, the denaturation kinetics were slow: 55% of the residual activity remained after 25 min of incubation at 80°C. Site-directed mutagenesis showed that the active-site residue Cys44 is essential for activity, whereas alanine mutants of the two other conserved cysteines retained about 0.5% residual activity. A model of the sulfur metabolism in T. paradoxus is discussed. IMPORTANCE:Sulfur oxygenase reductases (SORs) are the only enzymes catalyzing an oxygen-dependent disproportionation of elemental sulfur and/or polysulfides to sulfite, thiosulfate, and hydrogen sulfide. SORs are known from mesophilic and extremophilic archaea and bacteria. All SORs seem to form highly thermostable 24-subunit hollow spheres. They carry a low-potential mononuclear nonheme iron in the active site and an indispensable cysteine; however, their exact reaction mechanisms are unknown. Typically, the reductase activity of SORs is in the range of 5 to 50% of the oxygenase activity, but mutagenesis studies had so far failed to identify residues crucial for the reductase reaction. We describe here the first SOR, which is almost devoid of the reductase reaction and which comes from a haloalkaliphilic bacterium.

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Substrate pathways and mechanisms of inhibition in the sulfur oxygenase reductase of acidianus ambivalens.

Publications

Substrate pathways and mechanisms of inhibition in the sulfur oxygenase reductase of acidianus ambivalens.

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