Some sulfur-oxidizing bacteria playing an important role in global geochemical cycles utilize thiocyanate as the sole source of energy and nitrogen. In these bacteria the process of thiocyanate into cyanate conversion is mediated by thiocyanate dehydrogenases — a recently discovered family of copper-containing enzymes with the three‑copper active site unique among the other copper proteins. To get a deeper insight into the structure and molecular mechanism of action of thiocyanate dehydrogenases we isolated, purified, and comprehensively characterized an enzyme from the bacterium Pelomicrobium methylotrophicum. High-resolution crystal structures of the thiocyanate dehydrogenase in the free state and in the complexes with the transition state analog, thiourea, and the closest substrate analog, selenocyanate, unveiled the fine details of molecular events occurring at the enzyme active site. During the reaction thiocyanate dehydrogenase undergoes profound conformational change that affects the position of the constituent copper ions and results in the activation of the attacking water molecule. The structure of the enzyme complex with the selenium atom bridged in-between two copper ions was obtained representing an important transient intermediate. Structures of the complexes with inhibitors supplemented with quantum chemical calculations clarify the role of copper ions and refine molecular mechanism of catalysis by thiocyanate dehydrogenase.

During determination of the thiocyanate dehydrogenase (TcDH) structure difficulties have occurred, related to the fact that enzyme crystals have been either twinned or strongly anisotropic. The diffraction quality of crystals can be improved by using mutant forms as objects of a study or by studying the structure of a related enzyme from another organism. Based on the analysis of the oligomeric structure of TcDH, the mutant forms of the enzyme that are promising for improving the diffraction properties have been proposed. The crystals have been obtained and the structures of the TcDH mutant forms with the substitutions T169A and K281A have been solved. The structure of the mutant form with the substitution T169A is found to be similar to the previously solved structures. In the structure of the mutant form with the substitution K281A, a change in the tetramer structure that made twinning impossible has been detected.

The thermophilic anaerobic Gram-positive bacterium Carboxydothermus ferrireducens utilizes insoluble Fe(III) oxides as electron acceptors in respiratory processes using an extracellular 11-heme cytochrome c OmhA as a terminal reductase. OmhA is able to transfer electrons to soluble and insoluble Fe(III) compounds, substrates of multiheme oxidoreductases, and soluble electron shuttles. The crystal structure of OmhA at 2.5 Å resolution shows that it consists of two functionally distinct parts: the cytochrome с electron transfer and the S-layer binding domains. Nonaheme C-terminal subdomain of the cytochrome с domain is structurally similar to the extracellular multiheme cytochrome OcwA from the metal-reducing Gram-positive bacterium "Thermincola potens." S-layer binding domain of OmhA is responsible for interaction with the S-layer that surrounds the Carboxydothermus ferrireducens cell envelope. The structural foundations enabling the embedding of extracellular multiheme cytochromes to the S-layer of a Gram-positive-type cell wall and putative electron transfer pathways to insoluble minerals are discussed.

Dissimilatory nitrate reduction to ammonia (DNRA) is a common biochemical process in the nitrogen cycle in natural and man-made habitats, but its significance in wastewater treatment plants is not well understood. Several ammonifying Trichlorobacter strains (former Geobacter) were previously enriched from activated sludge in nitrate-limited chemostats with acetate as electron (e) donor, demonstrating their presence in these systems. Here, we isolated and characterized the new species Trichlorobacter ammonificans strain G1 using a combination of low redox potential and copper-depleted conditions. This allowed purification of this DNRA organism from competing denitrifiers. T. ammonificans is an extremely specialized ammonifier, actively growing only with acetate as e-donor and carbon source and nitrate as e-acceptor, but H2 can be used as an additional e-donor. The genome of G1 does not encode the classical ammonifying modules NrfAH/NrfABCD. Instead, we identified a locus encoding a periplasmic nitrate reductase immediately followed by an octaheme cytochrome c that is conserved in many Geobacteraceae species. We purified this octaheme cytochrome c protein (TaNiR), which is a highly active dissimilatory ammonifying nitrite reductase loosely associated with the cytoplasmic membrane. It presumably interacts with two ferredoxin subunits (NapGH) that donate electrons from the menaquinol pool to the periplasmic nitrate reductase (NapAB) and TaNiR. Thus, the Nap-TaNiR complex represents a novel type of highly functional DNRA module. Our results indicate that DNRA catalyzed by octaheme nitrite reductases is a metabolic feature of many Geobacteraceae, representing important community members in various anaerobic systems, such as rice paddy soil and wastewater treatment facilities.

The search of a putative physiological electron acceptor for thiocyanate dehydrogenase (TcDH) newly discovered in the thiocyanate-oxidizing bacteria Thioalkalivibrio paradoxus revealed an unusually large, single-heme cytochrome c (CytC552), which was co-purified with TcDH from the periplasm. Recombinant CytC552, produced in Escherichia coli as a mature protein without a signal peptide, has spectral properties similar to the endogenous protein and serves as an in vitro electron acceptor in the TcDH-catalyzed reaction. The CytC552 structure determined by NMR spectroscopy reveals significant differences compared to those of the typical class I bacterial cytochromes c: a high solvent accessible surface area for the heme group and so-called “intrinsically disordered” nature of the histidine-rich N- and C-terminal regions. Comparison of the signal splitting in the heteronuclear NMR spectra of oxidized, reduced, and TcDH-bound CytC552 reveals the heme axial methionine fluxionality. The TcDH binding site on the CytC552 surface was mapped using NMR chemical shift perturbations. Putative TcDH-CytC552 complexes were reconstructed by the information-driven docking approach and used for the analysis of effective electron transfer pathways. The best pathway includes the electron hopping through His528 and Tyr164 of TcDH, and His83 of CytC552 to the heme group in accordance with pH-dependence of TcDH activity with CytC552.

Copper-containing enzymes catalyze a broad spectrum of redox reactions. Thiocyanate dehydrogenase (TcDH) from Thioalkalivibrio paradoxus Arh1 enables the bacterium to use thiocyanate as a unique source of energy and nitrogen. Oxidation of thiocyanate takes place in the trinuclear copper center of TcDH with peculiar organization. Despite the TcDH crystal structure being established, a role of some residues in the enzyme active site has yet to be obscured. F436 residue is located in the enzyme active site and conserved among a number of TcDH homologs, however, its role in the copper center formation or the catalytic process is still not clear. To address this question, a mutant form of the enzyme with F436Q substitution (TcDHF436Q) was obtained, biochemically characterized, and its crystal structure was determined. The TcDHF436Q had an unaltered protein fold but did not possess enzymatic activity, whereas it contained all three copper ions, according to ICP-MS data. The structural data showed that the F436Q substitution resulted in a disturbance of hydrophobic interactions within the active site crucial for a correct transition between open/closed forms of the enzyme–substrate channel. Thus, we demonstrated that F436 does not participate in copper ion binding, but rather possesses a structural role in the TcDH active site.

Flavocytochrome c sulfide dehydrogenase (FCC) is one of the central enzymes of the respiratory chain in sulfur-oxidizing bacteria. FCC catalyzes oxidation of sulfide and polysulfide ions to elemental sulfur accompanied by electron transfer to cytochrome c . The catalytically active form of the enzyme is a non-covalently linked heterodimer composed of flavin- and heme-binding subunits. The Thioalkalivibrio paradoxus ARh1 genome contains five copies of genes encoding homologous FCCs with an amino acid sequence identity from 36 to 54%. When growing on thiocyanate or thiosulfate as the main energy source, the bacterium synthesizes products of different copies of FCC genes. In this work, we isolated and characterized FCC synthesized during the growth of Tv. paradoxus on thiocyanate. FCC was shown to oxidize exclusively sulfide but not other reduced sulfur compounds, such as thiosulfate, sulfite, tetrathionate, and sulfur, and it also does not catalyze the reverse reaction of sulfur reduction to sulfide. Kinetic parameters of the sulfide oxidation reaction are characterized.

Detailed impedance and voltammetric studies of hexameric octaheme nitrite reductase immobilized on carbon-based nanomaterials, specifically nanotubes and nanoparticles, were performed. Well-pronounced bioelectrocatalytic reduction of nitrite on enzyme-modified electrodes was obtained. Analysis of the impedance data indicated the absence of long-lived intermediates involved in the nitrite reduction. Cyclic voltammograms of biomodified electrodes had a bi-sigmoidal shape, which pointed to the presence of two enzyme orientations on carbon supports. The maximum (limiting) catalytic currents were determined and, by applying the correction by the mixed kinetics equation, the Tafel dependences were plotted for each catalytic wave/each enzyme orientation. Finally, two schemes for the rate-limiting processes during bioelectrocatalysis were proposed, viz. for low- and high-potential orientations.

According to steady-state kinetic experiments, thiourea inhibits thiocyanate dehydrogenase TcDH, whereas urea does not. The QM/MM modeling combined with electron density analysis reveals the molecular mechanism of this process. For both compounds, interactions with bulk water molecules are similar, but upon binding to the active site of thiocyanate dehydrogenase a sulfur atom forms stronger coordination bonds with copper ions than an oxygen.