NADH-quinone 1 Bovine heart Complex I contains only ubiquinone-10. Quinones in bacterial membranes differ depending on strains, for example, ubiquinone-10 in R. capsulatus; ubiquinone-8 in P. denitrificans; menaquinone-8 in T. thermophilus; both ubiquinone-8 and menaquinone-8 in aerobically grown E. coli cells (the ratio of UQ and MQ is controlled by oxygen tension). Therefore, in this mini-review, quinone (Q), quinol (QH 2), and semiquinone (SQ) were used for simplicity. 1 oxidoreductase (Complex I) isolated from bovine heart mitochondria was, until recently, the major source for the study of this most complicated energy transducing device in the mitochondrial respiratory chain. Complex I has been shown to contain 43 subunits and possesses a molecular mass of about 1 million. Recently, Complex I genes have been cloned and sequenced from several bacterial sources including Escherichia coli, Paracoccus denitrificans, Rhodobacter capsulatus and Thermus thermophilus HB-8. These enzymes are less complicated than the bovine enzyme, containing a core of 13 or 14 subunits homologous to the bovine heart Complex I. From this data, important clues concerning the subunit location of both the substrate binding site and intrinsic redox centers have been gleaned. Powerful molecular genetic approaches used in these bacterial systems can identify structure/function relationships concerning the redox components of Complex I. Site-directed mutants at the level of bacterial chromosomes and over-expression and purification of single subunits have allowed detailed analysis of the amino acid residues involved in ligand binding to several iron–sulfur clusters. Therefore, it has become possible to examine which subunits contain individual iron–sulfur clusters, their location within the enzyme and what their ligand residues are. The discovery of g=2.00 EPR signals arising from two distinct species of semiquinone (SQ) in the activated bovine heart submitochondrial particles (SMP) is another line of recent progress. The intensity of semiquinone signals is sensitive to Δ μ H + and is diminished by specific inhibitors of Complex I. To date, semiquinones similar to those reported for the bovine heart mitochondrial Complex I have not yet been discovered in the bacterial systems. This mini-review describes three aspects of the recent progress in the study of the redox components of Complex I: (A) the location of the substrate (NADH) binding site, flavin, and most of the iron–sulfur clusters, which have been identified in the hydrophilic electron entry domain of Complex I; (B) experimental evidence indicating that the cluster N2 is located in the amphipathic domain of Complex I, connecting the promontory and membrane parts. Very recent data is also presented suggesting that the cluster N2 may have a unique ligand structure with an atypical cluster-ligation sequence motif located in the NuoB (NQO6/PSST) subunit rather than in the long advocated NuoI (NQO9/TYKY) subunit. The latter subunit contains the most primordial sequence motif for two tetranuclear clusters; (C) the discovery of spin–spin interactions between cluster N2 and two distinct Complex I-associated species of semiquinone. Based on the splitting of the g signal of the cluster N2 and concomitant strong enhancement of the semiquinone spin relaxation, one semiquinone species was localized 8–11 Å from the cluster N2 within the inner membrane on the matrix side (N-side). Spin relaxation of the other semiquinone species is much less enhanced, and thus it was proposed to have a longer distance from the cluster N2, perhaps located closer to the other side (P-side) surface of the membrane. A brief introduction of EPR technique was also described in Appendix Aof this mini-review.
Sulfur is an essential biological element, yet its biochemistry is only partially understood because there are so few tools for studying this element in biological systems. X-ray absorption spectroscopy provides a unique approach to determining the chemical speciation of sulfur in intact biological samples. Different biologically relevant sulfur compounds show distinctly different sulfur K-edge X-ray absorption spectra, and we show here, as an example, that this allows the deconvolution of the sulfur species in equine blood.
X-ray absorption spectroscopy at the sulfur K-edge at ∼2470 eV has been applied to a series of mononuclear iron−sulfur complexes to determine the covalency and its distribution over the ligand field split d-orbitals. A comparison is made between the S K-edges of a model and three different rubredoxin proteins to define the changes in covalency upon incorporation of the site into the protein. It is found that the covalency decreases in the proteins relative to the model. The thiolate−Fe(III) bond in these systems is highly covalent, and a modulation of this covalency in the proteins can contribute to the redox properties of the active site. It is determined that, while the hydrogen bonding effects seem to influence covalency, there is not a direct correlation between the change in covalency, the number of hydrogen bonds, and the redox potentials of these sites.
The objectives of this research are to establish the fundamental kinetics and mechanism of sulfur dioxide oxidation over supported vanadia catalysts and use these insights to facilitate the design of SCR DeNO x catalysts with minimal sulfur dioxide oxidation activity. A series of supported vanadia catalysts were prepared on various metal-oxide supports: ceria, zirconia, titania, alumina and silica. Raman spectroscopy was used to determine the coordination of surface species. At low vanadia loadings, vanadia preferentially exists on oxide support surfaces as isolated tetrahedrally coordinated (M–O) 3V +5=O species. At higher vanadia loadings, the isolated (M–O) 3V +5=O species polymerize on the oxide support surface breaking two V–O–M bonds and forming two V–O–V bridging bonds. The turnover frequency for sulfur dioxide oxidation was very low, 10 −4 to 10 −6 s −1 at 400°C, and was independent of vanadia coverage suggesting that only one vanadia site is required for the oxidation reaction. As the support was varied, sulfur dioxide oxidation activity of the supported vanadia catalysts varied by one order of magnitude (Ce>Zr, Ti>Al>Si). The basicity of the bridging V–O–M oxygen appears to be responsible for influencing the adsorption and subsequent oxidation of the acidic sulfur dioxide molecule. Over the range of conditions studied, the rate of sulfur dioxide oxidation is zero-order in oxygen, first-order in sulfur dioxide and inhibited by sulfur trioxide. The turnover frequency for sulfur dioxide oxidation over WO 3/TiO 2 was an order of magnitude lower than that found for V 2O 5/TiO 2, and no redox synergism between the surface vanadia and tungsten oxide species was evident for a ternary V 2O 5/WO 3/TiO 2 catalyst. This suggests that WO 3 promoted catalysts may be suitable for low-temperature SCR where minimal sulfur dioxide oxidation activity is required.
A continuous seawater sulfate sulfur isotope curve for the Cenozoic with a resolution of ∼1 million years was generated using marine barite. The sulfur isotopic composition decreased from 19 to 17 per mil between 65 and 55 million years ago, increased abruptly from 17 to 22 per mil between 55 and 45 million years ago, remained nearly constant from 35 to ∼2 million years ago, and has decreased by 0.8 per mil during the past 2 million years. A comparison between seawater sulfate and marine carbonate carbon isotope records reveals no clear systematic coupling between the sulfur and carbon cycles over one to several millions of years, indicating that changes in the burial rate of pyrite sulfur and organic carbon did not singularly control the atmospheric oxygen content over short time intervals in the Cenozoic. This finding has implications for the modeling of controls on atmospheric oxygen concentration.
We have analyzed crystal structures of cytochrome bc1 complexes with electron transfer inhibitors bound to the ubiquinone binding pockets Q and/or Q in the cytochrome b subunit. The presence or absence of the Q inhibitor antimycin A did not affect the binding of the Q inhibitors. Different subtypes of Q inhibitors had dramatically different effects on the mobility of the extramembrane domain of the ironsulfur protein (ISP): Binding of 5-undecyl-6-hydroxy-4,7-dioxobenzothiazol and stigmatellin (subtype Q -II and Q -III, respectively) led to a fixation of the ISP domain on the surface of cytochrome b, whereas binding of myxothiazol and methoxyacrylate-stilbene (subtype Q -I) favored release of this domain. The native structure has an empty Q pocket and is intermediate between these extremes. On the basis of these observations we propose a model of quinone oxidation in the bc1 complex, which incorporates fixed and loose states of the ISP as features important for electron transfer and, possibly, also proton transport.
Palladium thiolato complexes [(L)Pd(R)(SR‘)], within which L is a chelating ligand such as DPPE, DPPP, DPPBz, DPPF, or TRANSPHOS, R is a methyl, alkenyl, aryl, or alkynyl ligand, and R‘ is an aryl or alkyl group, were synthesized by substitution or proton-transfer reactions. All of these thiolato complexes were found to undergo carbon−sulfur bond-forming reductive elimination in high yields to form dialkyl sulfides, diaryl sulfides, alkyl aryl sulfides, alkyl alkenyl sulfides, and alkyl alkynyl sulfides. Reductive eliminations forming alkenyl alkyl sulfides and aryl alkyl sulfides were the fastest. Eliminations of alkynyl alkyl sulfides were slower, and elimination of dialkyl sulfide was the slowest. Thus the relative rates for sulfide elimination as a function of the hybridization of the palladium-bound carbon follow the trend sp2 > sp ≫ sp3. Rates of reductive elimination were faster for cis-chelating phosphine ligands with larger bite angles. Kinetic studies, along with results from radical trapping reactions, analysis of solvent effects, and analysis of complexes with chelating phosphines of varying rigidity, were conducted with [Pd(L)(S-tert-butyl)(Ar)] and [Pd(L)(S-tert-butyl)(Me)]. Carbon−sulfur bond-forming reductive eliminations involving both saturated and unsaturated hydrocarbyl groups proceed by an intramolecular, concerted mechanism. Systematic changes in the electronic properties of the thiolate and aryl groups showed that reductive elimination is the fastest for electron deficient aryl groups and electron rich arenethiolates, suggesting that the reaction follows a mechanism in which the thiolate acts as a nucleophile and the aryl group an electrophile. Studies with thiolate ligands and hydrocarbyl ligands of varying steric demands favor a migration mechanism involving coordination of the hydrocarbyl ligand in the transition state.