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Catalytic Role (catalytic + role)
Selected AbstractsCatalytic role of manganese in autoclave oxidation of germanium-rich sphalerite concentratesTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2009Liang Duoqiang Abstract An attempt was made to investigate the catalytic role of manganese in enhancing the reaction rate with respect to autoclave oxidation of germanium-rich sphalerite concentrates. A series of batch experiments was performed in different conditions to investigate the variables such as temperature and oxygen pressure. Experimental results obtained show that as a catalyst, aqueous divalent manganese can accelerate the leaching of sphalerite concentrates significantly. On a tenté d'étudier le rôle catalytique du manganèse pour améliorer la vitesse de réaction de l'oxydation en autoclave de concentrés de sphalérite riche en germanium. Une série d'expériences discontinues ont été menées dans différentes conditions afin d'étudier les variables telles que la température et la pression d'oxygène. Les résultats expérimentaux obtenus montrent qu'en tant que catalyseur, le manganèse divalent aqueux peut significativement accélérer la lixiviation des concentrés de sphalérite. [source] The catalytic role of the distal site asparagine-histidine couple in catalase-peroxidasesFEBS JOURNAL, Issue 5 2003Christa Jakopitsch Catalase-peroxidases (KatGs) are unique in exhibiting an overwhelming catalase activity and a peroxidase activity of broad specificity. Similar to other peroxidases the distal histidine in KatGs forms a hydrogen bond with an adjacent conserved asparagine. To investigate the catalytic role(s) of this potential hydrogen bond in the bifunctional activity of KatGs, Asn153 in Synechocystis KatG was replaced with either Ala (Asn153,Ala) or Asp (Asn153,Asp). Both variants exhibit an overall peroxidase activity similar with wild-type KatG. Cyanide binding is monophasic, however, the second-order binding rates are reduced to 5.4% (Asn153,Ala) and 9.5% (Asn153,Asp) of the value of native KatG [(4.8 ± 0.4) × 105 m,1·s,1 at pH 7 and 15 °C]. The turnover number of catalase activity of Asn153,Ala is 6% and that of Asn153,Asp is 16.5% of wild-type activity. Stopped-flow analysis of the reaction of the ferric forms with H2O2 suggest that exchange of Asn did not shift significantly the ratio of rates of H2O2 -mediated compound I formation and reduction. Both rates seem to be reduced most probably because (a) the lower basicity of His123 hampers its function as acid-base catalyst and (b) Asn153 is part of an extended KatG-typical H-bond network, the integrity of which seems to be essential to provide optimal conditions for binding and oxidation of the second H2O2 molecule necessary in the catalase reaction. [source] Clay mineral-organic matter relationships in the early solar systemMETEORITICS & PLANETARY SCIENCE, Issue 12 2002Victoria K. Pearson These organic-rich meteorites provide a valuable and tangible record of the chemical steps taken towards the origin of life in the early solar system. Chondritic organic matter is present in the inorganic meteorite matrix which, in the CM and CI chondrites, contains evidence of alteration by liquid water on the parent asteroid. An unanswered and fundamental question is to what extent did the organic matter and inorganic products of aqueous alteration interact or display interdependence? We have used an organic labelling technique to reveal that the meteoritic organic matter is strongly associated with clay minerals. This association suggests that clay minerals may have had an important trapping and possibly catalytic role in chemical evolution in the early solar system prior to the origin of life on the early Earth. [source] Re-examining the role of Lys67 in class C ,-lactamase catalysisPROTEIN SCIENCE, Issue 3 2009Yu Chen Abstract Lys67 is essential for the hydrolysis reaction mediated by class C ,-lactamases. Its exact catalytic role lies at the center of several different proposed reaction mechanisms, particularly for the deacylation step, and has been intensely debated. Whereas a conjugate base hypothesis postulates that a neutral Lys67 and Tyr150 act together to deprotonate the deacylating water, previous experiments on the K67R mutants of class C ,-lactamases suggested that the role of Lys67 in deacylation is mainly electrostatic, with only a 2- to 3-fold decrease in the rate of the mutant vs the wild type enzyme. Using the Class C ,-lactamase AmpC, we have reinvestigated the activity of this K67R mutant enzyme, using biochemical and structural studies. Both the rates of acylation and deacylation were affected in the AmpC K67R mutant, with a 61-fold decrease in kcat, the deacylation rate. We have determined the structure of the K67R mutant by X-ray crystallography both in apo and transition state-analog complexed forms, and observed only minimal conformational changes in the catalytic residues relative to the wild type. These results suggest that the arginine side chain is unable to play the same catalytic role as Lys67 in either the acylation or deacylation reactions catalyzed by AmpC. Therefore, the activity of this mutant can not be used to discredit the conjugate base hypothesis as previously concluded, although the reaction catalyzed by the K67R mutant itself likely proceeds by an alternative mechanism. Indeed, a manifold of mechanisms may contribute to hydrolysis in class C ,-lactamases, depending on the enzyme (wt or mutant) and the substrate, explaining why different mutants and substrates seem to support different pathways. For the WT enzyme itself, the conjugate base mechanism may be well favored. [source] Catalytic role of manganese in autoclave oxidation of germanium-rich sphalerite concentratesTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2009Liang Duoqiang Abstract An attempt was made to investigate the catalytic role of manganese in enhancing the reaction rate with respect to autoclave oxidation of germanium-rich sphalerite concentrates. A series of batch experiments was performed in different conditions to investigate the variables such as temperature and oxygen pressure. Experimental results obtained show that as a catalyst, aqueous divalent manganese can accelerate the leaching of sphalerite concentrates significantly. On a tenté d'étudier le rôle catalytique du manganèse pour améliorer la vitesse de réaction de l'oxydation en autoclave de concentrés de sphalérite riche en germanium. Une série d'expériences discontinues ont été menées dans différentes conditions afin d'étudier les variables telles que la température et la pression d'oxygène. Les résultats expérimentaux obtenus montrent qu'en tant que catalyseur, le manganèse divalent aqueux peut significativement accélérer la lixiviation des concentrés de sphalérite. [source] Nanostructure Copper-exchanged ZSM-5 Catalytic Activity for Conversion of Volatile Organic Compounds (Toluene and Ethyl Acetate)CHINESE JOURNAL OF CHEMISTRY, Issue 2 2010S. Ali Hosseini Abstract Gas phase catalytic oxidation of ethyl acetate and toluene was examined over copper modified ZSM-5 catalysts under atmospheric pressure. Nanostructure of ZSM-5 was characterized by XRD, SEM and TEM techniques. Elemental composition of ZSM-5 was determined using EDX, ICP-AES and XPS techniques. Results of catalytic studies showed better catalytic activity of Cu-ZSM-5 catalysts than those of parent ZSM-5 and HZSM-5, which revealed catalytic role of copper ions in the Cu-ZSM-5 catalysts. Effects of some parameters over catalytic conversion of these compounds were also studied. Ethyl acetate showed more reactivity than toluene over the Cu-ZSM-5 catalysts. Furthermore, the catalytic activity of Cu-ZSM-5 catalysts increased with increasing the copper loadings. The conversion behavior of a binary mixture of ethyl acetate and toluene was different from that of a single form. A promotive and inhibitive behavior was observed for conversion of ethyl acetate and toluene in the binary mixture, respectively. Water vapor as co-feed had an inhibitive effect on conversion of organic compounds over the Cu-ZSM-5 catalysts. [source] Structure of d -ribulose 5-phosphate 3-epimerase from Synechocystis to 1.6,Å resolutionACTA CRYSTALLOGRAPHICA SECTION D, Issue 9 2004Eric L. Wise The crystal structure of d -ribulose 5-phosphate 3-epimerase (RPE) from the cyanobacterium Synechocystis was determined by X-ray crystallography to 1.6,Å resolution. The enzyme, which catalyzes the epimerization of d -ribulose 5-phosphate and d -xylulose 5-phosphate, assembles as a hexamer of (,/,)8 -barrels in the crystallographic asymmetric unit. The active site is highly similar to those of two previously reported RPEs and provides further evidence for essential catalytic roles for several active-site residues. [source] | ||||||||||