Interaction between the catalytic and modifier subunits of glutamate-cysteine ligase
Yi YangYing ChenElisabet JohanssonScott N. SchneiderHoward G. ShertzerDaniel W. NebertTimothy P. Dalton
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Catalytic activities of oxidized Pt-Pd/Al2O3 and reduced Pt-Pd/Al2O3 catalysts,oxidized Pt-Rh-Pd/Al2O3 and reduced Pt-Rh-Pd/Al2O3 catalysts were studied with flow-system micro reactor.All these catalysts were characterized by TPD,XRD and TPR and their catalytic mechanisms were discussed.The results showed that oxidation activity of oxidized Pt-Pd/Al2O3 catalyst is superior to that of reduced Pt-Pd/Al2O3 catalyst,and they have the same catalytic mechanism.The oxidation activity of oxidized Pt-Rh-Pd/Al2O3 catalyst is not as good as that of reduced Pt-Rh-Pd/Al2O3 catalyst,which is attributed to their different catalytic mechanisms.Noble metals combined with oxygen are harmful to the reduction activities.
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Embryos of Drosophila melanogaster contain two distinct DNA ligases (DNA ligase I and II). DNA ligase I was eluted at 0.2 M KCl and DNA ligase II at 0.6 M KCl on phosphocellulose column chromatography. The former was rich in early developing embryos and its activity decreased during embryonic development. The latter was found constantly throughout the developing stages of embryos. DNA ligase I existed in a cytoplasmic fraction and DNA ligase II is concentrated in nuclei. Both enzymes ligate 5'‐phosphoryl and 3'‐hydroxyl groups in oligo(dT) in the presence of poly(dA). DNA ligase II is also able to join oligo(dT)(poly(rA). Both enzymes require ATP and Mg 2+ for activity. The K m for ATP is 2.7 × 10 −6 M for DNA ligase I, and 3.0 × 10 −5 M for DNA ligase II. DNA ligase I requires dithiothreitol and polyvinyl alcohol, but DNA ligase II does not. Both enzymes are inhibited in the presence of N ‐ethylmaleimide. DNA ligase I is active at a low salt concentration (0–30 mM KCl), but DNA ligase II is active at high salt concentrations (50–100 mM). DNA ligase I is more labile than DNA ligase II. The molecular masses of DNA ligase‐AMP adducts were determined as 86 and 75 kDa for DNA ligase I, and as 70 (major protein) and 90 kDa (minor protein) for DNA ligase II under denaturing conditions. A sedimentation coefficient of 4.2 S was observed for DNA ligase II. Consequently, Drosophila DNA ligase I and II are quite similar to mammalian DNA ligase I and II. Drosophila DNA ligase I and a DNA ligase by B.A. Rabin et al. [(1986) J. Biol. Chem. 261, 10637–10645] seem to be the same enzyme.
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Three Mo/ZSM-5 catalysts from different HZSM-5 were prepared by wet impregnation method and their catalytic activity has also been tested. The results showed that the catalytic performance for NO selective catalytic reduction over 1# catalyst which has the lowest value of Si/Al ratio is much better than 2# and 3# catalysts at the same experiment condition, its NO conversion reaches 94% at 530 ℃, while only 48% and 41% was given by 2# and 3# catalyst. To understand the effect of different Si/Al ratio on the activity of Mo/ZSM-5, the bulk-phase and surface compositions of the catalysts were determined by XRD, XPS and NO-TPD respectively. The results showed that Mo/ZSM-5 catalysts with different Si/Al ratio not only exhibits an obvious difference of catalytic activity for selective catalytic reduction of NO, but also results in the distinction of bulk phase structure and surface properties. Especially the different surface Mo contents may cause the difference in activity of three Mo/ZSM-5 catalysts.
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