Guanylyl Cyclase C Receptor: Regulation of Catalytic Activity by ATP

Guanylyl cyclase C (GCC), a member of the family of membrane bound guanylyl cyclases is the receptor for the heat-stable enterotoxin (ST) peptides and the guanylin family of endogenous peptides. GCC is activated upon ligand binding to increase intracellular cGMP levels, which in turn activates other downstream signalling events in the cell. GCC is also activated in vitro by nonionic detergents. We have used the T84 cell line as a model system to investigate the regulation of GCC activity by ATP. Ligand-stimulated GCC activity is potentiated in the presence of ATP, whereas detergent-stimulated activity is inhibited. The potentiation of GCC activity by ATP is dependent on the presence of Mg2+ ions, and is probably brought about by a direct binding of Mg-ATP to GCC. The protein kinase-like domain of GCC, which has earlier been shown to play a critical role in the regulation of GCC activity, may be a possible site for the binding of Mg-ATP to GCC.

Interaction Between Two Residues in the Inter-Domain Interface of Escherichia coli Peptidase N Modulates Catalytic Activity

The role of interaction between Asn259 (catalytic domain) with Gln821 (C-terminal domain) in PeptidaseN was investigated. The k(cat) of PeptidaseN containing Asn259Asp or Gln821Glu is enhanced whereas it is suppressed in Asn259AspGln821Glu. Structural analysis shows this interaction to change the relative disposition of active site residues, which modulates catalytic activity.

Mechanism of catalytic activity of metal oxides and chromites on ammonium perchlorate pyrolysis

This study investigates the mechanism of action of transition metal chromites on the decomposition of ammonium perchlorate.

Ultrafine powders of SrTiO3 from the hydrothermal preparation and their catalytic activity in the photolysis of water

Ultrafine powders of SrTiO3 are prepared at 100–150°C by the hydrothermal method, starting from TiO2·xH2O gel and Sr(OH)2 and H2O-isopropanol mixed solvent as the medium, The X-ray diffractograms of the powder show line broadening. The minimum crystallite size obtained ranges from 5 to 20nm with 20% H2O-80% C3H7OH as the reaction medium, as estimated from X-ray half-peak widths and TEM studies. The electron diffraction results indicate high concentration of lattice defects in these crystallites. The optical spectra of the particle suspensions in water show that the absorption around the band gap is considerably broadened, together with the appearance of maxima in the far ultraviolet. Aqueous suspensions of SrTiO3 powders, as such, do not produce H2 or O2 on UV irradiation. After coating with rhodium, H2 and O2 are evolved on illumination. However, the turn over number of O2 is lower than the stoichiometrically expected values from the corresponding values of H2. No correlation of the photocatalytic activity with surface area is observed. The activity of Rh-SrTiO3 slowly deteriorates with extended period of irradiation.

Structural effects of a dimer interface mutation on catalytic activity of triosephosphate isomerase.The role of conserved residues and complementary mutations

The active site of triosephosphate isomerase (TIM, EC: 5.3.1.1), a dimeric enzyme, lies very close to the subunit interface. Attempts to engineer monomeric enzymes have yielded well-folded proteins with dramatically reduced activity. The role of dimer interface residues in the stability and activity of the Plasmodium falciparum enzyme, PfTIM, has been probed by analysis of mutational effects at residue 74. The PfTIM triple mutant W11F/W168F/Y74W (Y74W*) has been shown to dissociate at low protein concentrations, and exhibits considerably reduced stability in the presence of denaturants, urea and guanidinium chloride. The Y74W* mutant exhibits concentration-dependent activity, with an approximately 22-fold enhancement of kcat over a concentration range of 2.5–40 μm, suggesting that dimerization is obligatory for enzyme activity. The Y74W* mutant shows an approximately 20-fold reduction in activity compared to the control enzyme (PfTIM WT*, W11F/W168F). Careful inspection of the available crystal structures of the enzyme, together with 412 unique protein sequences, revealed the importance of conserved residues in the vicinity of the active site that serve to position the functional K12 residue. The network of key interactions spans the interacting subunits. The Y74W* mutation can perturb orientations of the active site residues, due to steric clashes with proximal aromatic residues in PfTIM. The available crystal structures of the enzyme from Giardia lamblia, which contains a Trp residue at the structurally equivalent position, establishes the need for complementary mutations and maintenance of weak interactions in order to accommodate the bulky side chain and preserve active site integrity.

Allosteric serine hydroxymethyltransferase from monkey liver: Correlation of conformational changes caused by denaturants with the alterations in catalytic activity

The far-ultraviolet region circular dichroic spectrumof serine hydroxymethyltransferase from monkey liver showed that the protein is in an α-helical conformation. The near ultraviolet circular dichoric spectrum revealed two negative bands originating from the tertiary conformational environment of the aromatic amino acid residues. Addition of urea or guanidinium chloride perturbed the characteristic fluorescence and far ultraviolet circular dichroic spectrum of the enzyme. The decrease in (θ)222 and enzyme activity followed identical patterns with increasing concentrations of urea, whereas with guanidinium chloride, the loss of enzyme activity preceded the loss of secondary structure. 2-Chloroethanol, trifluoroethanol and sodium dodecyl sulphate enhanced the mean residue ellipticity values. In addition, sodium dodecyl sulphate also caused a perturbation of the fluorescence emission spectrum of the enzyme. Extremes of pH decreased the – (θ)222 value. Plots of –(θ)222and enzyme activity as a function of pH showed maximal values at pH 7.4-7.5. These results suggested the prevalence of "conformational flexibility" in the structure of serine hydroxymethyltransferase.

Deciphering the Distinct Role for the Metal Coordination Motif in the Catalytic Activity of Mycobacterium smegmatis Topoisomerase I

Mycobacterium smegmatis topoisomerase I (Mstopol) is distinct from typical type IA topoisomerases. The enzyme binds to both single- and double-stranded DNA with high affinity, making specific contacts. The enzyme comprises conserved regions similar to type IA topoisomerases from Escherichia coli and other eubacteria but lacks the typically found zinc fingers in the carboxy-terminal domain. The enzyme can perform DNA cleavage m the absence of Mg2+ but religation needs exogenously added Mg2+. One molecule of Mg2+ tightly bound to the enzyme has no role in DNA cleavage but is needed only for the religation reaction. The toprim. (topoisomerase-primase) domain in MstopoI comprising the Mg2+ binding pocket, conserved in both type IA and type II topoisomerases, was subjected to mutagenesis to understand the role of Mg2+, in different steps of the reaction. The residues D108, D110, and E112 of the enzyme, which form the acidic triad in the DXDXE motif, were changed to alanines. D108A mutation resulted in an enzyme that is Mg2+ dependent for DNA cleavage unlike Mstopol and exhibited enhanced DNA cleavage property and reduced religation activity. The mutant was toxic for cell growth, most likely due to the imbalance in cleavage-religation equilibrium. In contrast, the E112A mutant behaved like wild-type enzyme, cleaving DNA in a Mg2+-independent fashion, albeit to a reduced extent. Intra- and intermolecular religation assays indicated specific roles for D108 and E112 residues during the reaction. Together, these results indicate that the D108 residue has a major role during cleavage and religation, while E112 is important for enhancing the efficiency of cleavage. Thus, although architecturally and mechanistically similar to topoisomerase I from E. coli, the metal coordination pattern of the mycobacterial enzyme is distinct, opening up avenues to exploit the enzyme to develop inhibitors.

Hydrocarbon oxidation and three-way catalytic activity on a single step directly coated cordierite monolith: High catalytic activity of Ce0.98Pd0.02O2−δ

Catalytic activity of cordierite honeycomb by a completely new coating method for the oxidation of major hydrocarbons in exhaust gas is reported here. The new coating process consists of (a) dipping and growing γ-Al2O3 on cordierite by combustion of monolith dipped in the aqueous solution of Al(NO3)3 and oxalyldihydrazide (ODH) (or glycine) at 600 °C and active catalyst phase Ce0.98Pd0.02O2−δ on γ-Al2O3-coated cordierite again by combustion of monolith dipped in the aqueous solution of ceric ammonium nitrate, ODH and 1.2 × 10−3 M PdCl2 solution at 500 °C. Weight of active catalyst can be varied from 0.02 wt% to 2 wt% which is sufficient but can be loaded even up to 12 wt% by repeating dip dry combustion. Adhesion of catalyst to cordierite surface is via oxide growth, which is very strong. ‘HC’ oxidation over the monolith catalyst is carried out with a mixture having the composition, 470 ppm of both propene and propane and 870 ppm of both ethylene and acetylene with the varying amount of O2. Three-way catalytic test is done by putting hydrocarbon mixture along with CO (10 000 ppm), NO (2000 ppm) and O2 (15 000 ppm). Below 350 °C full conversion is achieved. In this method, handling of nano-material powder is avoided.

A Unique Nickel System having Versatile Catalytic Activity of Biological Significance

A new dinuclear nickel(II) complex, [Ni-2(LH2)(H2O)(2)(OH)(NO3)](NO3)(3) (1), of an ``end-off'' compartmental ligand 2,6-bis(N-ethylpiperazine-iminomethyl)-4-methyl-phenolato, has been synthesized and structurally characterized. The X-ray single crystal structure analysis shows that the piperazine moieties assume the expected chair conformation and are protonated. The complex 1 exhibits versatile catalytic activities of biological significance, viz. catecholase, phosphatase, and DNA cleavage activities, etc. The catecholase activity of the complex observed is very dependent on the nature of the solvent. In acetonitrile medium, the complex is inactive to exhibit catecholase activity. On the other hand, in methanol, it catalyzes not only the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a catechol which is very difficult to oxidize, under aerobic conditions. UV vis spectroscopic investigation shows that TCC oxidation proceeds through the formation of an intermediate. The intermediate has been characterized by an electron spray ionizaton-mass spectrometry study, which suggests a bidentate rather than a monodentate mode of TCC coordination in that intermediate, and this proposition have been verified by density functional theory calculation. The complex also exhibits phosphatase (with substrate p-nitrophenylphosphate) and DNA cleavage activities. The DNA cleavage activity exhibited by complex 1 most probably proceeds through a hydroxyl radical pathway. The bioactivity study suggests the possible applications of complex 1 as a site specific recognition of DNA and/or as an anticancer agent.

A Unique Nickel System having Versatile Catalytic Activity of Biological Significance

A new dinuclear nickel(II) complex, [Ni-2(LH2)(H2O)(2)(OH)(NO3)](NO3)(3) (1), of an "end-off" compartmental ligand 2,6-bis(N-ethylpiperazine-iminomethyl)-4-methyl-phenolato, has been synthesized and structurally characterized. The X-ray single crystal structure analysis shows that the piperazine moieties assume the expected chair conformation and are protonated. The complex 1 exhibits versatile catalytic activities of biological significance, viz. catecholase, phosphatase, and DNA cleavage activities, etc. The catecholase activity of the complex observed is very dependent on the nature of the solvent. In acetonitrile medium, the complex is inactive to exhibit catecholase activity. On the other hand, in methanol, it catalyzes not only the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a catechol which is very difficult to oxidize, under aerobic conditions. UV vis spectroscopic investigation shows that TCC oxidation proceeds through the formation of an intermediate. The intermediate has been characterized by an electron spray ionizaton-mass spectrometry study, which suggests a bidentate rather than a monodentate mode of TCC coordination in that intermediate, and this proposition have been verified by density functional theory calculation. The complex also exhibits phosphatase (with substrate p-nitrophenylphosphate) and DNA cleavage activities. The DNA cleavage activity exhibited by complex 1 most probably proceeds through a hydroxyl radical pathway. The bioactivity study suggests the possible applications of complex 1 as a site specific recognition of DNA and/or as an anticancer agent.

Enhanced reducibility of Ce1-xTixO2 compared to that of CeO2 and higher redox catalytic activity of Ce1-x-yTixPtyO2-delta compared to that of Ce1-xPtxO2-delta

Nanocrystalline Ce1-xTixO2 (0 <= x <= 0.4) and Ce1-xTixPtyO2-delta (x = 0.15, gamma = 0.01, 0.02) solid solutions crystallizing in fluorite structure have been prepared by a single step solution combustion method. Temperature programmed reduction and XPS study of Ce1-xTixO2 (x = 0.0-04) show complete reduction of Ti4+ to Ti3+ and reduction of similar to 20% Ce4+ to Ce3+ state compared to 8% Ce4+ to Ce3+ in the case of pure CeO2 below 675 degrees C. The substitution of Ti ions in CeO2 enhances the reducibility of CeO2. Ce0.84Ti0.15Pt0.01O2-delta crystallizes in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. The H/Pt atomic ratio at 30 degrees C over Ce0.84Ti0.15Pt0.01O2-delta is 5 and that over Ce0.99Pt0.01O2-delta is 4 against just 0.078 for 8 nm Pt metal particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce1-x-yTixPtyO2 (x = 0.15, 0.01, 0.02) compared to Ce1-xPtxO2 (x = 0.01, 0.02). Synergistic involvement of Pt2+/Pt degrees and Ti4+/Ti3+ redox couples in addition to Ce4+/Ce3+ due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near E-F is shown to be responsible for improved redox property and higher catalytic activity.

High-Surface Step Density on Dendritic Pd Leads to Exceptional Catalytic Activity for Formic Acid Oxidation

Dendrite Pd with corrugated surfaces, obtained by a novel AC technique, exhibits an exceptionally high catalytic activity for the oxidation of formic acid because of the presence of a high density of surface steps. The formation of twinned dendrites leads to a predominance of exposed 111 facets with a high density of surface steps as evident from high resolution electron microscopy investigations. These surface sites provide active sites for the absorption of the formic acid molecules, thereby enhancing the reaction rate. Control experiments by varying the time of deposition reveal the formation of partially grown dendrites at shorter times indicating that the dendrites were formed by growth rather than particle attachment. Our deposition method opens up interesting possibilities to produce artisotropic nanostructures with corrugated surfaces by exploiting the perturbations involved in the growth process.

Reactivity and catalytic activity of layered YBA2CU3O7-delta (123) type defect perovskites

The chemical modifications of structure, reactivity and catalytic properties of layered triple perovskite oxides, related to the YBa2Cu3O7-delta (123) system, have been briefly reviewed. These oxides form a versatile family of materials with wide-ranging chemical and physical properties. The multiple sites available for chemical doping, and the ability to reversibly intercalate oxygen at the defect sites have rendered these oxides important model systems in the area of oxide catalysis. An attempt has been made to comprehend the hitherto known catalytic reactions and correlate them to various factors like structure, oxygen diffusional limitations, different geometries adopted by various substituents, oxidative non-stoichiometry and activation energy for oxygen desorption. In particular, results on the enhanced catalytic activity of cobalt-substituted 123 oxide systems towards the selective catalytic oxidation of ammonia to nitric oxide and carbon monoxide to carbon dioxide are presented.

Ultrafast Microwave-Assisted Route to Surfactant-Free Ultrafine Pt Nanoparticles on Graphene: Synergistic Co-reduction Mechanism and High Catalytic Activity

We demonstrate an ultrafast method for the formation of, graphene supported Pt catalysts by the co-reduction of graphene oxide and Pt salt using ethylene glycol under microwave irradiation conditions. Detailed analysis of the mechanism of formation of the hybrids indicates a synergistic co-reduction mechanism whereby the presence of the Pt ions leads to a faster reduction of GO and the presence of the defect sites on the reduced GO serves as anchor points for the heterogeneous nucleation of Pt. The resulting hybrid consists of ultrafine nanoparticles of Pt uniformly distributed on the reduced GO susbtrate. We have shown that the hybrid exhibits good catalytic activity for methanol oxidation and hydrogen conversion reactions. The mechanism is general and applicable for the synthesis of other multifunctional hybrids based on graphene.