Thiocyanate Hydrolase Is a Cobalt-Containing Metalloenzyme with a Cysteine-Sulfinic Acid Ligand

Exogenous thiocyanate inhibits sulfurtransferase pathway and induces β-cyanoalanine synthase pathway to enhance exogenous cyanide assimilation in rice plants

Cyanide (CN-) assimilation in plants takes place by β-cyanoalanine synthase (β-CAS) and sulfurtransferase (ST), in which the ST pathway converts CN- into thiocyanate (SCN-). Both chemicals (CN- and SCN-) are frequently detected in the effluent of gold mining operations. In this connection, exogenous SCN- was applied to rice plants with CN- and compared with CN- alone to investigate its effects on CN- assimilation and degradation pathways. Interestingly, the CN- and SCN- content in both roots and shoots were increased with the increase in "CN-" treatments, but surprisingly their content under "SCN-+CN-" treatments did not show the similar trend. The increasing trend remained the same for CN- but the SCN- content was constant with increasing CN- concentrations in comparison with the control (SCN- alone). Additionally, the assimilation rates of CN- in rice plants under "SCN-+CN-" treatments were significantly higher than "CN-" treatments. The application of SCN- with CN- mostly alters the expression of both β-CAS and ST-associated genes. On one side, the application of SCN- significantly repressed the expression of genes encoded with ST in rice plants, but on the other side, it significantly up-regulated the expression of the β-CAS gene located in mitochondria. These results reveal that the application of exogenous SCN- increases CN- assimilation rates by inhibiting the ST pathway and stimulating the β-CAS pathway. This study would provide new insight into the positive effects of exogenous SCN- in increasing CN- assimilation by altering the degradation pathways in rice plants.

HMS-S-S: a tool for the identification of sulfur metabolism-related genes and analysis of operon structures in genome and metagenome assemblies

Sulfur compounds are used in a variety of biological processes including respiration and photosynthesis. Sulfide and sulfur compounds of intermediary oxidation state can serve as electron donors for lithotrophic growth while sulfate, thiosulfate and sulfur are used as electron acceptors in anaerobic respiration. The biochemistry underlying the manifold transformations of inorganic sulfur compounds occurring in sulfur metabolizing prokaryotes is astonishingly complex and knowledge about it has immensely increased over the last years. The advent of next-generation sequencing approaches as well as the significant increase of data availability in public databases has driven focus of environmental microbiology to probing the metabolic capacity of microbial communities by analysis of this sequence data. To facilitate these analyses, we created HMS-S-S, a comprehensive equivalogous hidden Markov model (HMM)-supported tool. Protein sequences related to sulfur compound oxidation, reduction, transport and intracellular transfer are efficiently detected and related enzymes involved in dissimilatory sulfur oxidation as opposed to sulfur compound reduction can be confidently distinguished. HMM search results are coupled to corresponding genes, which allows analysis of co-occurrence, synteny and genomic neighborhood. The HMMs were validated on an annotated test dataset and by cross-validation. We also proved its performance by exploring meta-assembled genomes isolated from samples from environments with active sulfur cycling, including members of the cable bacteria, novel Acidobacteria and assemblies from a sulfur-rich glacier, and were able to replicate and extend previous reports.

Cysteine oxidation to the sulfinic acid induces oxoform-specific lanthanide binding and fluorescence in a designed peptide

Cysteine sulfinic acid (Cys-SO₂^-) is a protein post-translational modification that is formed reversibly under oxidative conditions. A short, encodable peptide was developed whose metal binding and terbium luminescence are dependent on cysteine (Cys) oxidation to the sulfinic acid. The protein design is based on the modification of a key metal-binding aspartate (Asp) in a canonical EF-Hand motif (DKDADGWISPAEAK) to Cys. In this design, Cys in the thiol oxidation state does not mimic the native Asp, and thus the peptide binds terbium(III) (Tb³⁺) poorly and exhibits weak terbium luminescence (fluorescence). In contrast, when Cys is oxidized to the Cys sulfinic acid oxoform, the Cys sulfinate effectively mimics Asp, resulting in a significant increase in terbium affinity and luminescence. Asp residues at positions 1, 3, and 5 of the EF-Hand motif were examined as potential sites for Cys oxidation-responsive metal binding. The peptide with Cys at residue 1 exhibited the highest Tb³⁺ affinity in both oxidation states. The peptide with Cys at residue 3 exhibited a 4.2-fold distinction in affinity between the oxidation states. Most significantly, the peptide with Cys at residue 5 had only modest Tb³⁺ affinity as the Cys thiol, but exhibited a 30-fold increase in Tb³⁺ affinity and an 18-fold increase in Tb³⁺ luminescence on Cys oxidation to the sulfinic acid. This peptide (Ac-DKDACGWISPAEAK-NH₂) exhibited selective Tb³⁺ binding via Cys-SO₂^- over the thiol, S-glutathionyl, S-nitrosyl, and sulfonic acid oxoforms, indicating substantially greater Lewis basicity of the sulfinate than the sulfonate. NMR spectroscopy and quantum homology modeling indicated that the designed peptide binds metal with an overall geometry similar to that of an EF-Hand motif, with the Cys sulfinate effectively replacing Asp as a metal-binding ligand. This peptide was applied to detect Cys oxidation to the sulfinic acid by fluorescence spectroscopy, suggesting its broader application in understanding Cys sulfinic acid biology.

Inhibition of the mitochondrial respiratory components (Complex I and Complex III) as stimuli to induce oxidative damage in Oryza sativa L. under thiocyanate exposure

Repression of the electron transport in mitochondria can result in an increase of reactive oxygen species (ROS) in plant cells. This study was to clarify inhibition of the mitochondrial respiratory components (Complex I and Complex III) as stimuli to induce oxidative damage in Oryza sativa L. under exogenous SCN^- exposure with special emphasis on lipid peroxidation, protein modification, and DNA damage at the biochemical and molecular levels. Our results showed that enzymatic activity and gene expression of cytochrome c reductase (Complex III) in roots and shoots of rice seedlings were significantly repressed by SCN^- exposure, where significant inhibition of NADH dehydrogenase (Complex I) was only detected in shoots, suggesting that Complex III was the main target attacked by SCN^- ligand in rice roots, and both components were arrested in shoots. ROS analysis in tissues indicated that SCN^- exposure caused significant accumulation of H₂O₂ and O₂^-•, increased malondialdehyde (MDA) and carbonyl content in rice materials in a dose-dependent manner. Similarly, a remarkable elevation of electrolyte leakage was observed in rice tissue samples. The comet assay indicated a positive correlation between DNA damage and external SCN^- exposure. In conclusion, oxidative burst generated from the inhibitions of the electron transport in mitochondria in rice seedlings under SCN^- exposure can cause lipid peroxidation, protein modification and DNA damage, eventually decreasing fresh weight of rice seedlings.

Trinuclear copper biocatalytic center forms an active site of thiocyanate dehydrogenase

Biocatalytic copper centers are generally involved in the activation and reduction of dioxygen, with only few exceptions known. Here we report the discovery and characterization of a previously undescribed copper center that forms the active site of a copper-containing enzyme thiocyanate dehydrogenase (suggested EC 1.8.2.7) that was purified from the haloalkaliphilic sulfur-oxidizing bacterium of the genus Thioalkalivibrio ubiquitous in saline alkaline soda lakes. The copper cluster is formed by three copper ions located at the corners of a near-isosceles triangle and facilitates a direct thiocyanate conversion into cyanate, elemental sulfur, and two reducing equivalents without involvement of molecular oxygen. A molecular mechanism of catalysis is suggested based on high-resolution three-dimensional structures, electron paramagnetic resonance (EPR) spectroscopy, quantum mechanics/molecular mechanics (QM/MM) simulations, kinetic studies, and the results of site-directed mutagenesis.

Structural preferences of cysteine sulfinic acid: The sulfinate engages in multiple local interactions with the peptide backbone

Cysteine sulfinic acid (Cys-SO₂^-) is a non-enzymatic oxidative post-translational modification (PTM) that has been identified in hundreds of proteins. However, the effects of cysteine sulfination are in most cases poorly understood. Cys-SO₂^- is structurally distinctive, with long sulfur-carbon and sulfur-oxygen bonds, and with tetrahedral geometry around sulfur due to its lone pair. Cys-SO₂^- thus has a unique range of potential interactions with the protein backbone which could facilitate protein structural changes. Herein, the structural effects of cysteine oxidation to the sulfinic acid were investigated in model peptides and folded proteins using NMR spectroscopy, circular dichroism, bioinformatics, and computational studies. In the PDB, Cys-SO₂^- shows a greater preference for α-helix than Cys. In addition, Cys-SO₂^- is more commonly found in structures with φ > 0, including in multiple types of β-turn. Sulfinate oxygens engage in hydrogen bonds with adjacent (i or i + 1) amide hydrogens. Over half of sulfinates have at least one hydrogen bond with an adjacent amide, and several structures have hydrogen bonds with both adjacent amides. Alternately, sulfur or either oxygen can act as an electron donor for n→π* interactions with the backbone carbonyl of the same residue, as indicated by frequent S⋯CO or O⋯CO distances below the sums of their van der Waals radii in protein structures. In peptides, Cys-SO₂^- favored α-helical structure at the N-terminus, consistent with helix dipole effects and backbone hydrogen bonds with the sulfinate promoting α-helix. Cys-SO₂^- has only modestly greater polyproline II helix propensity than Cys-SH, likely due to competition from multiple side chain-backbone interactions. Cys-SO₂^- stabilizes the i+1 position of a β-turn relative to Cys-SH. Within proteins, the range of side chain-main chain interactions available to Cys-SO₂^- compared to Cys-SH provides a basis for potential changes in protein structure and function due to cysteine oxidation to the sulfinic acid.

Transcriptome analysis of Oryza sativa in responses to different concentrations of thiocyanate

Effective concentrations of potassium thiocyanate (KSCN) to rice seedlings were experimentally determined using relative growth rate as a sensitive endpoint. Agilent 44-K rice microarray was used to profile the molecular responses of rice seedlings exposed to thiocyanate ion (SCN^-) at three different effective concentrations (EC₁₀, EC₂₀, and EC₅₀). A total of 18,498 known genes were collected from SCN-treated rice microarray analysis. Out of all, 1603, 1882, and 5085 differentially expressed genes (DEGs) were observed at EC₁₀, EC₂₀, and EC₅₀ concentrations, respectively. More upregulated/downregulated DEGs were detected in shoots than in roots after SCN^- exposure. Gene functions and pathway enrichment analysis of DEGs indicated that different effective concentrations of SCN^- resulted in multiple enriched GO categories and KEGG pathways and outcomes were quite tissue-specific. Different regulations and adaptations of gene expression in molecular function (MF), biological process (BP), and cellular components (CC) were observed in rice tissues at different effective concentrations of SCN^-, suggesting their different responsive and adaptive strategies. Information collected here presents a detailed description of SCN-induced alternations of gene expression in rice seedlings and provide valuable information for further searching specific genes participating in transportation, phytotoxic responses, and detoxification of SCN^- in rice seedlings.

Thermodynamics of the Cu(II) μ-thiolate and Cu(I) disulfide equilibrium: a combined experimental and theoretical study

The redox equilibrium between dinuclear Cu(II) μ-thiolate and Cu(I) disulfide structures has been analyzed experimentally and via DFT calculations. Two new ligands, L(2)SSL(2) and L(4)SSL(4), and their Cu(II) μ-thiolate and Cu(I) disulfide complexes were synthesized. For L(2)SSL(2), these two redox-isomeric copper species are shown to be in equilibrium, which depends on both temperature and solvent. For L(4)SSL(4) the μ-thiolate species forms as the kinetic product and further evolves into the disulfide complex under thermodynamic control, which creates the unprecedented possibility to compare both species under the same reaction conditions. The energies of the μ-thiolate and disulfide complexes for two series of related ligands have been calculated with DFT; the results rationalize the experimentally observed structures, and emphasize the important role that steric requirements play in the formation of the Cu(II) thiolate structure.

Cloning and expression of a gene encoding a novel thermostable thiocyanate-degrading enzyme from a mesophilic alphaproteobacteria strain THI201

Strain THI201, a member of the alphaproteobacteria, is a novel thiocyanate (SCN(-))-degrading bacterium isolated from lake water enriched with potassium thiocyanate (KSCN). This bacterium carries the enzyme thiocyanate hydrolase (SCNase) that hydrolyses thiocyanate to carbonyl sulfide and ammonia. Characterization of both native and recombinant SCNase revealed properties different from known SCNases regarding subunit structure and thermostability: SCNase of strain THI201 was composed of a single protein and thermostable. We cloned and sequenced the corresponding gene and determined a protein of 457 amino acids of molecular mass 50 267 Da. Presence of a twin-arginine (Tat) signal sequence of 32 amino acids was found upstream of SCNase. The deduced amino acid sequence of SCNase showed 83% identity to that of a putative uncharacterized protein of Thiobacillus denitrificans ATCC 25259, but no significant identity to those of three subunits of SCNase from Thiobacillus thioparus strain THI115. The specific activities of native and recombinant enzyme were 0.32 and 4-15 µmol min(-1) (mg protein)(-1), respectively. The maximum activity of SCNase was found in the temperature range 30-70 °C. The thiocyanate-hydrolysing activity in both enzymes was decreased by freeze-thawing, although 25-100% of the activity of recombinant protein could be retrieved by treating the enzyme at 60 °C for 15 min. Furthermore, both native and recombinant enzymes retained the activity after pre-treatment of the protein solution at temperatures up to 70 °C.

Sequential oxidations of thiolates and the cobalt metallocenter in a synthetic metallopeptide: implications for the biosynthesis of nitrile hydratase

Cobalt nitrile hydratases (Co-NHase) contain a catalytic cobalt(III) ion coordinated in an N2S3 first coordination sphere composed of two amidate nitrogens and three cysteine-derived sulfur donors: a thiolate (-SR), a sulfenate (-S(R)O(-)), and a sulfinate (-S(R)O2(-)). The sequence of biosynthetic reactions that leads to the post-translational oxidations of the metal and the sulfur ligands is unknown, but the process is believed to be initiated directly by oxygen. Herein we utilize cobalt bound in an N2S2 first coordination sphere by a seven amino acid peptide known as SODA (ACDLPCG) to model this oxidation process. Upon exposure to oxygen, Co-SODA is oxidized in two steps. In the first fast step (seconds), magnetic susceptibility measurements demonstrated that the metallocenter remains paramagnetic, that is, Co(2+), and sulfur K-edge X-ray absorption spectroscopy (XAS) is used to show that one of the thiolates is oxidized to sulfinate. In a second process on a longer time scale (hours), magnetic susceptibility measurements and Co K-edge XAS show that the metal is oxidized to Co(3+). Unlike other model complexes, additional slow oxidation of the second thiolate in Co-SODA is not observed, and a catalytically active complex is never formed. The likely reason is the absence of the axial thiolate ligand. In essence, the reactivity of Co-SODA can be described as between previously described models which either quickly convert to final product or are stable in air, and it offers a first glimpse into a possible oxidation pathway for nitrile hydratase biosynthesis.

Effects of exogenous thiocyanate on mineral nutrients, antioxidative responses and free amino acids in rice seedlings

The effects of exogenous thiocyanate (SCN(-)) on amino acids composition, content of mineral nutrients and antioxidative systems in plants were investigated. Young rice seedlings (Oryza sativa L. cv. XZX 45) were grown in nutrient solutions amended with potassium thiocyanate (KSCN). Activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) in plant materials were analyzed in vivo. Mineral nutrients and free amino acids in rice seedlings were also measured to determine metabolic responses to SCN(-) exposure. A significant reduction in transpiration and relative growth was recorded with all treatments (p < 0.05), while changes of total chlorophyll content in leaves was negligible (p > 0.05). SCN-induced toxicity appeared to be more sensitive to activities of POD in shoots and APX activities in roots than the others. The content of nutrient elements in rice seedlings exposed to exogenous SCN(-) was variable, while the effects were more evident at the highest SCN-treatment (p < 0.05). Although the change of total free amino acids in shoots of SCN-exposed seedlings was negligible (p > 0.05), responses of different amino acids to SCN(-) application were quite different. Among fifteen free amino acids detected, serine (Ser), proline (Pro), and methionine (Met) increased, while asparagine (Asp) decreased with an increase of the doses of SCN(-) supplied. Phyto-transport of SCN(-) was apparent and the removal rates were positively correlated to the doses, suggesting that phyto-assimilation of SCN(-) is an enzymatic process through a potentially un-identified degradation pathway.

Self-subunit swapping occurs in another gene type of cobalt nitrile hydratase

Self-subunit swapping is one of the post-translational maturation of the cobalt-containing nitrile hydratase (Co-NHase) family of enzymes. All of these NHases possess a gene organization of <β-subunit> <α-subunit> <activator protein>, which allows the activator protein to easily form a mediatory complex with the α-subunit of the NHase after translation. Here, we discovered that the incorporation of cobalt into another type of Co-NHase, with a gene organization of <α-subunit> <β-subunit> <activator protein>, was also dependent on self-subunit swapping. We successfully isolated a recombinant NHase activator protein (P14K) of Pseudomonas putida NRRL-18668 by adding a Strep-tag N-terminal to the P14K gene. P14K was found to form a complex [α(StrepP14K)₂] with the α-subunit of the NHase. The incorporation of cobalt into the NHase of P. putida was confirmed to be dependent on the α-subunit substitution between the cobalt-containing α(StrepP14K)₂ and the cobalt-free NHase. Cobalt was inserted into cobalt-free α(StrepP14K)₂ but not into cobalt-free NHase, suggesting that P14K functions not only as a self-subunit swapping chaperone but also as a metallochaperone. In addition, NHase from P. putida was also expressed by a mutant gene that was designed with a <β-subunit> <α-subunit> <P14K> order. Our findings expand the general features of self-subunit swapping maturation.

Influence of sequential thiolate oxidation on a nitrile hydratase mimic probed by multiedge X-ray absorption spectroscopy

Nitrile hydratases (NHases) are Fe(III)- and Co(III)-containing hydrolytic enzymes that convert nitriles into amides. The metal-center is contained within an N(2)S(3) coordination motif with two post-translationally modified cysteinates contained in a cis arrangement, which have been converted into a sulfinate (R-SO(2)(-)) and a sulfenate (R-SO(-)) group. Herein, we utilize Ru L-edge and ligand (N-, S-, and P-) K-edge X-ray absorption spectroscopies to probe the influence that these modifications have on the electronic structure of a series of sequentially oxidized thiolate-coordinated Ru(II) complexes ((bmmp-TASN)RuPPh(3), (bmmp-O(2)-TASN)RuPPh(3), and (bmmp-O(3)-TASN)RuPPh(3)). Included is the use of N K-edge spectroscopy, which was used for the first time to extract N-metal covalency parameters. We find that upon oxygenation of the bis-thiolate compound (bmmp-TASN)RuPPh(3) to the sulfenato species (bmmp-O(2)-TASN)RuPPh(3) and then to the mixed sulfenato/sulfinato speices (bmmp-O(3)-TASN)RuPPh(3) the complexes become progressively more ionic, and hence the Ru(II) center becomes a harder Lewis acid. These findings are reinforced by hybrid DFT calculations (B(38HF)P86) using a large quadruple-ζ basis set. The biological implications of these findings in relation to the NHase catalytic cycle are discussed in terms of the creation of a harder Lewis acid, which aids in nitrile hydrolysis.

Phytotoxicity of thiocyanate to rice seedlings

The acute toxicity of potassium thiocyanate (KSCN) and ammonium thiocyanate (NH(4)SCN) to rice seedlings was tested. Hydroponically-grown plants showed different responses to the two species of thiocyanate. NH(4)SCN caused more severe stress to rice seedlings than KSCN. A significant reduction in transpiration and relative growth was observed with all NH(4)SCN treatments (p < 0.01), while the effect of KSCN on rice seedlings was more evident at greater than 100 mg SCN/L (p < 0.01). Both chemicals had a negligible effect on total chlorophyll content in shoots of rice seedlings (p > 0.05). Although phyto-transport of thiocyanate was apparent, rice seedlings showed significantly higher removal potential for NH(4)SCN than KSCN.

Use of metallopeptide based mimics demonstrates that the metalloprotein nitrile hydratase requires two oxidized cysteinates for catalytic activity

Nitrile hydratases (NHases) are non-heme Fe(III) or non-corrin Co(III) containing metalloenzymes that possess an N(2)S(3) ligand environment with nitrogen donors derived from amidates and sulfur donors derived from cysteinates. A closely related enzyme is thiocyanate hydrolase (SCNase), which possesses a nearly identical active-site coordination environment as CoNHase. These enzymes are redox inactive and perform hydrolytic reactions; SCNase hydrolyzes thiocyanate anions while NHase converts nitriles into amides. Herein an active CoNHase metallopeptide mimic, [Co(III)NHase-m1] (NHase-m1 = AcNH-CCDLP-CGVYD-PA-COOH), that contains Co(III) in a similar N(2)S(3) coordination environment as CoNHase is reported. [Co(III)NHase-m1] was characterized by electrospray ionization-mass spectrometry (ESI-MS), gel-permeation chromatography (GPC), Co K-edge X-ray absorption spectroscopy (Co-S: 2.21 Å; Co-N: 1.93 Å), vibrational, and optical spectroscopies. We find that [Co(III)NHase-m1] will perform the catalytic conversion of acrylonitrile into acrylamide with up to 58 turnovers observed after 18 h at 25 °C (pH 8.0). FTIR data used in concert with calculated vibrational data (mPWPW91/aug-cc-TZVPP) demonstrates that the active form of [Co(III)NHase-m1] has a ligated SO(2) (ν = 1091 cm(-1)) moiety and a ligated protonated SO(H) (ν = 928 cm(-1)) moiety; when only one oxygenated cysteinate ligand (i.e., a mono-SO(2) coordination motif) or the bis-SO(2) coordination motif are found within [Co(III)NHase-m1] no catalytic activity is observed. Calculations of the thermodynamics of ligand exchange (B3LYP/aug-cc-TZVPP) suggest that the reason for this is that the SO(2)/SO(H) equatorial ligand motif promotes both water dissociation from the Co(III)-center and nitrile coordination to the Co(III)-center. In contrast, the under- or overoxidized motifs will either strongly favor a five coordinate Co(III)-center or strongly favor water binding to the Co(III)-center over nitrile binding.

Structural basis for catalytic activation of thiocyanate hydrolase involving metal-ligated cysteine modification

Thiocyanate hydrolase (SCNase) is a member of a family of nitrile hydratase proteins, each of which contains a unique noncorrin cobalt center with two post-translationally modified cysteine ligands, cysteine-sulfenic acid or -sulfenate (Cys-SO(H)), and cysteine-sulfininate (Cys-SO(2)(-)), respectively. We have found that a partially matured recombinant SCNase was activated during storage. The crystal structures of SCNase before and after storage demonstrated that Cys-SO(2)(-) modification of gammaCys131 proceeded to completion prior to storage, while Cys-SO(H) modification of gammaCys133 occurred during storage. SCNase activity was suppressed when gammaCys133 was further oxidized to Cys-SO(2)(-). The correlation between the catalytic activity and the extent of the gammaCys133 modification indicates that the cysteine sulfenic acid modification of gammaCys133 is of primary importance in determining the activity of SCNase.

Self-subunit swapping chaperone needed for the maturation of multimeric metalloenzyme nitrile hydratase by a subunit exchange mechanism also carries out the oxidation of the metal ligand cysteine residues and insertion of cobalt

The incorporation of cobalt into low molecular mass nitrile hydratase (L-NHase) of Rhodococcus rhodochrous J1 has been found to depend on the alpha-subunit exchange between cobalt-free L-NHase (apo-L-NHase lacking oxidized cysteine residues) and its cobalt-containing mediator (holo-NhlAE containing Cys-SO(2)(-) and Cys-SO(-) metal ligands), this novel mode of post-translational maturation having been named self-subunit swapping, and NhlE having been recognized as a self-subunit swapping chaperone (Zhou, Z., Hashimoto, Y., Shiraki, K., and Kobayashi, M. (2008) Proc. Natl. Acad. Sci. U. S. A. 105, 14849-14854). We discovered here that cobalt was inserted into both the cobalt-free NhlAE (apo-NhlAE) and the cobalt-free alpha-subunit (apo-alpha-subunit) in an NhlE-dependent manner in the presence of cobalt and dithiothreitol in vitro. Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy analysis revealed that the non-oxidized cysteine residues in apo-NhlAE were post-translationally oxidized after cobalt insertion. These findings suggested that NhlE has two activities, i.e. cobalt insertion and cysteine oxidation. NhlE not only functions as a self-subunit swapping chaperone but also a metallochaperone that includes a redox function. Cobalt insertion and cysteine oxidation occurred under both aerobic and anaerobic conditions when Co(3+) was used as a cobalt donor, suggesting that the oxygen atoms in the oxidized cysteines were derived from water molecules but not from dissolved oxygen. Additionally, we isolated apo-NhlAE after the self-subunit swapping event and found that it was recycled for cobalt transfer into L-NHase.

Spin-state-dependent oxygen sensitivity of iron dithiolates: sulfur oxygenation or disulfide formation

The oxygen sensitivity of two related iron(III) dithiolate complexes of the ligand [4,7-bis-(2'-methyl-2'-mercatopropyl)-1-thia-4,7-diazacyclononane], (bmmp-TASN)FeCN (1) and (bmmp-TASN)FeCl (2), has been examined. Oxygen exposure of the low-spin complex 1 yields the disulfonate complex (bmmp-O6-TASN)FeCN (3) as an olive-green solid with characteristic peaks in the IR spectrum at 1262, 1221, 1111, 1021, 947, 800, and 477 cm(-1). The corresponding nickel dithiolate, (bmmp-TASN)Ni (4), yields the related disulfonato derivative, (bmmp-O6-TASN)Ni (5) upon addition of H2O2 (IR bands at 1258, 1143, 1106, 1012, 800, and 694 cm(-1). Oxygen exposure of the high-spin complex 2 results in disulfide formation and decomplexation of the metal with subsequent iron-oxo cluster formation. Complexes 1 and 2 were examined using density functional theory calculations. A natural bond order/natural localized molecular orbital covalency analysis reveals that the low-spin complex 1 contains Fe-Sthiolate bonds with calculated covalencies of 75 and 86%, while the high-spin complex 2 contains Fe-Sthiolate bonds with calculated covalencies of 11 and 40%. The results indicate the degree of covalency of the Fe-S bonds plays a major role in determining the reaction pathway associated with oxygen exposure of iron thiolates. The X-ray structures of 1, 4, and 5 are reported.

Thiol-copper(I) and disulfide–dicopper(I) complex O2-reactivity leading to sulfonate–copper(II) complex or the formation of a cross-linked thioether–phenol product with phenol addition

In order to better understand copper mediated oxidative chemistry via ligand-Cu(I)/O(2) reactivity employing S-donor ligands for copper, O(2)-reactivity studies of the copper(I) complexes (1 and 2, Chart 2) have been carried out with a tridentate N(2)S thiol ligand (1-(N-methyl-N-(2-(pyridin-2-yl)ethyl)amino)propane-2-thiol; L(SH)) or its oxidized disulfide form (L(SS)). Reactions of [L(SH)Cu(I)](+) (1) and [L(SS)(Cu(I))(2)(X)(2)](2+) (2) with O(2) give approximately 90% and approximately 70% yields of [L(SO3)Cu(II)(MeOH)(2)](+) (3), respectively, where L(SO3) is S-oxygenated sulfonate; 3 was characterized by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. Mimicking TyrCys galactose oxidase cofactor biogenesis, a new C-S bond is formed (within new thioether moiety L(SPhOH)) from cuprous complex (both 1 and 2) dioxygen reactivity in the presence of 2,4-tBu(2)-phenolate. In addition, the disulfide ligand (L(SS)) reacts with 2equiv. cupric ion salts and the phenolate to efficiently give the cross-linked product L(SPhOH) in high yield (>90%) under anaerobic conditions. Separately, complex [L(SPhO)Cu(II)(ClO(4))] (4), possessing the cross-linked L(SPhOH), was characterized by ESI mass spectrometry and X-ray crystallography.

Thiocyanate hydrolase, the primary enzyme initiating thiocyanate degradation in the novel obligately chemolithoautotrophic halophilic sulfur-oxidizing bacterium Thiohalophilus thiocyanoxidans

Thiohalophilus thiocyanoxidans is a first halophilic sulfur-oxidizing chemolithoautotrophic bacterium capable of growth with thiocyanate as an electron donor at salinity up to 4 M NaCl. The cells, grown with thiocyanate, but not with thiosulfate, contained an enzyme complex hydrolyzing thiocyanate to sulfide and ammonia under anaerobic conditions with carbonyl sulfide as an intermediate. Despite the fact of utilization of the <<COS pathway>>, high cyanase activity was also detected in thiocyanate-induced cells. Three-stage column chromotography resulted in a highly purified thiocyanate-hydrolyzing protein with an apparent molecular mass of 140 kDa that consists of three subunits with masses 17, 19 and 29 kDa. The enzyme is a Co,Fe-containing protein resembling on its function and subunit composition the enzyme thiocyanate hydrolase from the Betaproteobacterium Thiobacillus thioparus. Cyanase, copurified with thiocyanate hydrolase, is a bisubstrate multisubunit enzyme with an apparent subunit molecular mass of 14 kDa. A possible role of cyanase in thiocyanate degradation by T. thiocyanoxidans is discussed.

Structure of Thiocyanate Hydrolase: A New Nitrile Hydratase Family Protein with a Novel Five-coordinate Cobalt(III) Center

Thiocyanate hydrolase (SCNase) of Thiobacillus thioparus THI115 is a cobalt(III)-containing enzyme catalyzing the degradation of thiocyanate to carbonyl sulfide and ammonia. We determined the crystal structures of the apo- and native SCNases at a resolution of 2.0 A. SCNases in both forms had a conserved hetero-dodecameric structure, (alphabetagamma)(4). Four alphabetagamma hetero-trimers were structurally equivalent. One alphabetagamma hetero-trimer was composed of the core domain and the betaN domain, which was located at the center of the molecule and linked the hetero-trimers with novel quaternary interfaces. In both the apo- and native SCNases, the core domain was structurally conserved between those of iron and cobalt-types of nitrile hydratase (NHase). Native SCNase possessed the post-translationally modified cysteine ligands, gammaCys131-SO(2)H and gammaCys133-SOH like NHases. However, the low-spin cobalt(III) was found to be in the distorted square-pyramidal geometry, which had not been reported before in any protein. The size as well as the electrostatic properties of the substrate-binding pocket was totally different from NHases with respect to the charge distribution and the substrate accessibility, which rationally explains the differences in the substrate preference between SCNase and NHase.

Denitrification in a binary culture and thiocyanate metabolism in Thiohalophilus thiocyanoxidans gen. nov. sp. nov. – a moderately halophilic chemolithoautotrophic sulfur-oxidizing Gammaproteobacterium from hypersaline lakes

Anaerobic enrichment culture with thiocyanate as electron donor and nitrate as electron acceptor at 2 M NaCl inoculated with a mixture of sediments from hypersaline lakes in Kulunda Steppe (Altai, Russia) resulted in a selection of a binary consortium of moderately halophilic, obligately chemolithoautotrophic sulfur-oxidizing bacteria (SOB) capable of complete denitrification of nitrate with thiosulfate as the electron donor. One consortium member, strain HRhD 3sp, was isolated into pure culture with nitrate and thiosulfate using a density gradient. This strain was responsible for the reduction of nitrate to nitrite in the consortium, while a second strain, HRhD 2, isolated under microoxic conditions with thiosulfate as substrate, was capable of anaerobic growth with nitrite and thiosulfate. Nitrite, either as substrate or as product, was already toxic at very low concentrations for both strains. As a result, optimal growth under anaerobic conditions could only be achieved within the consortium. On the basis of phylogenetic analysis, both organisms were identified as new lineages within the Gammaproteobacteria. As well as thiosulfate, strain HRhD 2 can also use thiocyanate as electron donor, representing a first halophilic SOB capable of growth with thiocyanate at 2-4 M NaCl. Product and enzymatic analysis identified the "carbonyl sulfide (COS) pathway" of primary thiocyanate degradation in this new species. On the basis of phenotypic and genetic analysis, strain HRhD 2 is proposed to be assigned to a new genus and species Thiohalophilus thiocyanoxidans.

Crystallographic and mutational studies of Mycobacterium tuberculosis recA mini-inteins suggest a pivotal role for a highly conserved aspartate residue

The 440 amino acid Mtu recA intein consists of independent protein-splicing and endonuclease domains. Previously, removal of the central endonuclease domain of the intein, and selection for function, generated a 168 residue mini-intein, DeltaI-SM, that had splicing activity similar to that of the full-length, wild-type protein. A D422G mutation (DeltaI-CM) increased C-terminal cleavage activity. Using the DeltaI-SM mini-intein structure (presented here) as a guide, we previously generated a highly active 139 residue mini-intein, DeltaDeltaI(hh)-SM, by replacing 36 amino acid residues in the residual endonuclease loop with a seven-residue beta-turn from the autoprocessing domain of Hedgehog protein. The three-dimensional structures of DeltaI-SM, DeltaDeltaI(hh)-SM, and two variants, DeltaDeltaI(hh)-CM and DeltaDeltaI(hh), have been determined to evaluate the effects of the minimization on intein integrity and to investigate the structural and functional consequences of the D422G mutation. These structural studies show that Asp422 is capable of interacting with both the N and C termini. These interactions are lacking in the CM variant, but are replaced by contacts with water molecules. Accordingly, additional mutagenesis of residue 422, combined with mutations that isolate N-terminal and C-terminal cleavage, showed that the side-chain of Asp422 plays a role in both N and C-terminal cleavage, thereby suggesting that this highly conserved residue regulates the balance between the two reactions.

Functional expression of thiocyanate hydrolase is promoted by its activator protein, P15K

Thiocyanate hydrolase (SCNase) is a cobalt-containing enzyme with a post-translationally modified cysteine ligand, gammaCys131-SO(2)H. When the SCNase alpha, beta and gamma subunits were expressed in Escherichia coli, the subunits assembled to form a hetero-dodecamer, (alphabetagamma)(4), like native SCNase but exhibited no catalytic activity. Metal analysis indicated that SCNase was expressed as an apo-form irrespective of the presence of cobalt in the medium. On the contrary, SCNase co-expressed with P15K, encoded just downstream of SCNase genes, in cobalt-enriched medium under the optimized condition (SCNase((+P15K))) possessed 0.86 Co atom/alphabetagamma trimer and exhibited 78% of the activity of native SCNase. SCNase((+P15K)) showed a UV-Vis absorption peak characteristic of the SCNase cobalt center. About 70% of SCNase((+P15K)) had the gammaCys131-SO(2)H modification. These results indicate that SCNase((+P15K)) is the active holo-SCNase. P15K is likely to promote the functional expression of SCNase probably by assisting the incorporation of cobalt ion.